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Chernick, Cedric L.The Chemistry of the Noble oases, Understanding theAtom Series.Atomic Energy Commission, Oak Ridge, Tenn. Div. ofTechnical Information.67!3p.U. S. Atomic Energy Commission, P. O. Box 62, OakRidge, Tenn., 3/830 (free)

?DRS Price ME-$0.25 HC Not Available from ?.DRS.*Chemical Ronding, Chemical Reactions, *Chemistry,*Resource Materials, *Science History, *ScientificResearchAtomic Energy Commission

ABSTRACTThe history of the discovery, isolation,

characterization, production and use of argon, kryptor, xenon,helium, and radon is followed by an account of early attempts toreact them with other elements. The use of the electron shell theoryof valence to explain their inertness and the reactions of chemiststo the production of xenon compounds is described. The presently known compounds of xenon and krypton are listed, and the use of molecularshapes of these compounds as determined by x-ray cr7stallaaraphy andelectron diffraction to test theories of chemical bonding isdiscussed. Illustrations, a short bibliography, and a film list areincluded. (AL)

The Cliemietry ofthe noble gases

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U. S. ATOMIC ENERGY COMMISSION/Division of Technical Information

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The Understanding the Atom Series

Nuclear Energy is playing a vital role in the life of everyman, woman, and child in the United"States today. In theyears ahead it will affect increasingly, all the peoples of theearth. I t is essential that all Americans gain an understandingof this vital force if they are to discharge thoughtfully theirresponsibilities as citizens and if they are to realiw fully themyriad benefits that nuclear energy offers them.

The United States Atomic Energy Commission providesthis booklet to help you achieve such understanding.

etwaa

Edward J Brunenkant, DirectorDivision of Technical Information

UNITED STATES ATOMIC ENERGY COMMISSION

Dr. Olen.. T. Seaborg, ChairmanJunes T. RameyWilfrid E. JohnsonFrancesco Costagliola

The Chemistry ofthe noble gasesby Cedric L. Chernick

CO CONTENTS

1.f1Pr\

THE GASES THEMSELVESDiscovery

Occurrence and Production2

4

Uses 7

O EARLY HISTORY 10

Attempts To Form Compounds 10

Why the Gases Are Inert . . . . . ........ 11

PREPARATION OF THE FIRST XENON COMPOUNDS . . 18

COMPOUNDS OF XENON 23Fluorine-Containing Compounds 23Oxygeittontaining Compounds 28More Complex Compounds 31

COMPOUNDS OF OTHER NOBLE GASES 32Radon 32Krypton 32Helium, Neon, and Argon 33

SHAPES OF MOLECULES 33Solid State 33Gas Phase 35Predicted Shapes and Chemical Banding 37

POSSIBLE USES 43SUGGESTED REFERENCES 45

United States Atomic Energy CommissionDivision of Technical Information

Lite my of Congress Catalog Card Number: 67 -629721967

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These luminous Geister tube script signs weremade by 13.0. Sperling, a glassblower at the Na-tional Bureau of Standards, for the 1901 Louisi-ana Purclose Exposition, Sl. Lonls, Missouri.They are believed to have been the first exam-ples of the use of the noble doses (and hydrogen.)for display purposes. Each titre was filled byP. 0. Nulling, an NBS scientist, with a sam-ple lf the appropriate gas obtained directly fromSir William Ramsay (sec page 31, About 1930,the commercial use of neon tube signs began (seepage 71, and since there neon signs have becomecommonplace Me world over. Meanwhile, until1%2, at least, Me noble gases remained amongthe most fascinating, moat palling, and leastknown of all elements.

The Chemistry ofthe noble gases

By CEDRIC L. CHERNICK

THE GASES THEMSELVES

If you've made up your mind that chemistry is a dullsubject, and want to continue to think so, you should notread this booklet. It will only upset your comfortable con-viction. If that should happen, it will be quite traditional.by the way, because information about tho "noble gases"has been shattering cherished beliefs with remarkableconsistency for some years now.

For over 60 years the 6 gases helium, neon, argon,krypton, xenon, and radon were the confirmed bachelorsamong the known elements. MI the other elements wouldenter into chemical combination with one or another of theirkind, irrespective of whether they were solids, gases, orllgslds in their normal state. Not so helium, neon, argon,krypton, xenon, and radon. They were chemically aloof andvtould have nothing to do with other elements, or even withone another.

This behavior earned them a unique position in the Pe-riodic Table of the Elements and they were called nameslike the "inert gases" or the "noble gases".` They werealso labeled the "rare gases", although helium and argonare not really "rare".t

The inability of these gases to form chemical compoundswas, until 1962, one of the most accepted ftintilmentals In

"Noble" by reason of their apparent reluctance to mingle withthe common herd of elements.

tXenon, hoverer, Is the rarest of all stable elements on earth.

chemistry. Then along came some scientists with whatPhilip Abelson, editor of the magazine Science, later called"a germ of skepticism". In the space of only a couple ofmonths all the dogma relating to the inertness of xenon wasoverthrown--it had definitely become a "joiner". Radonand krypton began "mingling" chemically soon thereafterand, although the other three gases are still holding out, thedamage to a firmly cherished belief was done.

Table 1ABUNDANCE OF NOBLE GASES 1N AIR AT SEA LEVEL

Element Symbol Paris per Million (by volume)

Helium He 5Neon Ne 18Argon Ar 9930Krypton Kr 1

Xenon Xe 0.1Radon lin 6 x lo-14

Some idea of the excitement these discoveries causedamong scientists can be gleaned from the fact that lessthan a yenr after the first discovery of a xenon compoundwas announced, a conference on "Noble (las Compounds"was held at Argonne National Laboratory near Chicago.Some 100 scientists discussed work they had done in thefield, and almost 60 made formal reports! The proceedingsof that meeting filled a 400page book entitled Noble GasCompounds.* Not bad, considering that just a short timebefore not even one noble gas compound was known.

This booklet will attempt to show how these gases losttheir bachelorhood, and why today they are called "heliumgroup gases" or "noble gases" instead of "inert gases".

Discovery

The first Indication of the existence of an inert constitu-ent in the atmosphere cante in 1785 when Henry Cavendishtfound that he could not convert atmospheric nitrogen corn-

'Edited by II. H. Hyman. See Suggested References. page 45.IThe great English chemist and physicist who also discovered

hydrogen.

2

pletely to nitrous acid. He concluded that, "If there is anypart of our atmosphere which differs from the rest it isnot more than 1/120 part of the whole". This result wasapparently forgotten or neglected, and the problem aroseagain in r:,iittles on the density of nitrogen in the early1890s. At that time Lord Ray lege discovered that nitrogenobtained by removal of the then known gases from an airsample, or "atmospheric nitrogen", was denser than nitro-

Sir William Ramsay

gen prepared by chemical meansthat is,"chernical nitro-gen". A number of theories were advanced for the discrep-ancy in the densities of the nitrogen samples from the twosources. Either the "chemical" nitrogen was too light, orthe "atmospheric" nitrogen too heavy, because of the pres-ence of other gases. In 1894, however, Lord Rayleigh andWilliam Ramsayt showed that the "atmospheric" nitrogenwas a mixture of nitrogen and a heavier, previously undis-covered, gas. This gas turned out to be a new element tLatwas given the name "argon", on account of its chemicalinactivity (from the Greek word, argon, meaning inactive,idle).

'John W. Sinai. who inherited the title Lord Rayleigh, was di-rector of the Cavendish Laboratory at Cambridge VniversIty inEngland when he c7. 1 this Important work. He is almost alwaysreferred to by his title.

Itiam say was a Scots chemist who was knighted in 1902. Ile re-ceived the 1904 Nobel Pete in chemistry for his discoveries ofnoble gases. 'Lord Rayleigh received the 1904 Nobel Prite inphysics in recognition of his nitrogen s:orlies with Ramsay.

3

The discovery of the other 5 gases followed rapidly; by1900 they had all been isolated and identified. Ramsay andhis assistant, Morris Travers, in continuing their researchon argon made use of newly developed methods for liquefy-ing gases. The earth's atmosphere consists mainly of nitro-gen (78%), oxygen (21%), and argon (1%), which have boilingpoints sufficiently different (-195.8°C, 182,98°C, and185.7°C, respectively) that they can readily be sepa-rated by fractional distillation of liquid air. As Ramsayand Travers improved their techniques, they found thatthey could obtain several more fractions when distillingliquid air. Three of these fractions contained elementsnever before isolated, namely, neon (Greek, neos, new),krypton (Greek, krytdos, hidden), and xenon (Greek, xenon,stranger).

Ramsay was also instrumental in discovering the exis-tence of helium (Greek, hetios, the sun). This element hadbeen noted In the :yin's spectrum as early as 1868, but wasonly isolated as a terrestrial element in 1895 when Ramsayobtained it by heating the uranium-rontainingmineralcleve-ite. (The helium in this mineral was physically trappedand was not chemically combined.)

The final noble gas to be discovered was radon. In 1900Frtedrich Dorn, a German physicist, found that radiumevolved a gas that he called "radium emanation'', This gaswas later given the name niton, but since 1923 it has beenknown as radon. MI isotopes of radon are radioactive.

Occurrence and ProductionThe atmosphere is our major source for neon, argon,

krypton, anti xenon, and these gases are now producedcommercially as a by -proOict during fractional distillationof liquid air to produce liquid oxygen and nitrogen. Lique-faction of thousands of tons of air per day makes these 4gases available in sufficient quantities for present needs.

Helium is the second most abundant element in the uni-verse. About 76% of the nu..ss of tie universe, It is esti-

'This mineral is also known as uranthite; one variety cf uran-inite. pitcl.birnde, is an important source et uranium for produc-tion of atomic energy.

4

mated, is hydrogen; NEliUM makes up about 23%, and all theother elements together compose the retraining i% of themass. Helium is so light that it is continually escaping fromthe earth's atmosphere into interstellar space. The presentconcentration o' helium in the atmosphere therefore prob-ably represents a steady-state concentration, that is, theamount being released from the earth's crust is equal tothe amount escaping from the atmosphere into space. Theconstant escape explains why there is so little to be foundin our air. Helium can be obtained from the atmosphere inthe same way neon, argon, krypton, and xenon are, but ismore readily obtained from ..ceumulations that have builtup in the earth's crust.

This helium in the earth is continually being formed byradioactive decay.* All radioactive materials that decay byemitting alpha particles produce helium, since an alphaparticle is nothing more than a helium nucleus with a posi-tive charge. Most of the helium in the earth's crust comesfrom the decay of uranium and thorium.

The helium is obte4ned by tapping natural gas wells,which yield an average helium content of about 2%. Mostof these helium wells are in an area within 250 miles ofAmarillo, Texas, although small amounts have been foundin natural gas elsewhere in the U. S. Since the earl!. 195eshelium - containing gases also have been found in SouthAfrica, Russia, end Canada. In other parts of the world thehelium content of natural gases and mineral springs is toolow to make separation commercially attractive.

The helium is recovered from the natural gas by an ini-tial liquefaction that leaves only helium and nitrogen ingaseous form. Further liquefaction, this Um; -wider pres-sure, causes most of the nitrogen to condense and leaverhelium of about 98% purity In the gas phase. This can oe.further pitrified by passing it through a liquidnitrogen-cooled trap containing charcoal, which absorbs the re-maining impurities.

The final one of our noble gase.-, radon, is obtained fromthe radioactive decay of radium. One gram of radium pro-duces about 0.0001 milliliter of radon per day. (We should

For tiore about radioactivity see Omr Atomic World. and otherZY)0!:e in this series.

5

Figure 1 A U. S. Bureau of Mines helium plant in Keyes, Okla-homadtli the "cold boxes", or refrigerating units, in the fore-ground.

keep in mind, however, that 1 gram of radium is a verylarge amount in terms of the total available. *) Radon has ashort half-life (the commonest isotope,T coming from ra-dium, is radon-222 whose half-life is 3.8 days), whichmeans that about half the ra,ion atoms will disintegratein a little under 4 days. Since radium has a much longerhalf-life than that, about 1620 years, the amount of daughterradon in contact with the parent radium reaches a constantconcentration. In other words the amount of radon beingproduced is balanced 13:, the amount disintegrating, and as

From the discovery of radium by Marie and Pierre Curie in1898 until 1940 only about 1000 grams were isolated, and althoughproduction increased during World War II, it is doubtful whetherthere are more than 100 grams of pure radium available in theWe World today.

t Isotopes are the various forms of :` Le same element. For a fulldefinition of this and other unfatniliai words, see Nuclear Terms,A Brief Glossary, a companion bot...det In this series.

6

soon as the primary source ,(the radium) is removed, theradon concentration begins to 0-crease because of its con-tinuing disintegration. After 1 half-life (3.8 days) only halfthe radon remains; after a second half-life, 1/2 of that willhave disintegrated, that is 1/2 of 1/2 or 1/4; in a month therewill be less than 1% left; and after n half-lives the fractionremaining will be (1/2)n. The amount of radon one canisolate at any given time is, therefore, dependent on theamount of radium originally available.

A number of isotopes of the noble gases can be producedartificially, either directly by bombardment in a particleaccelerator, or as the product of decay of an artificiallyexcited atom, or by nuclear fission. The latter method isused for production of krypton and xenon in atomic reactors.Fission is a process in which a heavy atom splits to form2 lighter atoms of approximately equal mass*; one ormore neutrons and a large amount of energy also arereleasedt simultaneously.

Uses

Many of the uses of these gases are outgrowths of theirinertness. The greater abundances, and hence lower costs,of helium and argon result in their use as inert atmo-spheres in which to weld and fabricate metals. The elec-trical and other properties of the noble gases make mostof them ideal gases for filling numerous types of electronictubas and in lasers. For this, the gases may be used singlyor mixed with one or more of the others. Perhaps the bestknown use is in the familiar "neon" advertising signs. Theglow produced by neon alone is red. The other gases pro-duce less brilliant colors: helium (pale pink), argon (blue),krypton (pale blue), and xenon, (blue-green).

Helium, because of its lightness, finds use as a liftinggas for balloons and airships, although it is heavier thanhydrogen. This weight disadvantage, however, is far over-balanced by the fact that helium is nonflammable. Recently,

*For example, if uranium-235 fissions, krypton-90 and barium-144, or xenon-140 and strontium-94 might be formed.

tFor a full explanation of fission, see Our Atomic World, acompanion booklet in this series.

7

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Figure 2 A mobile helium liquefier fills this U. S. Navy blimp.

c;411 411

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Figure 3 A technician checks a liquid-helium refrigerator priorto shipment. This unit is designed to cool masers and supercon-ducting magnets used for space communication.

helium has been used as a cooling medium in nuclear reac-tors, and it is also a diluent for oxygen in breathing sys-tems for deep-sea divers. Helium being less soluble in theblood than nitrogen, the helium-oxygen mixture is prefer-able to normal air for persons working under pressure,since its use tends to prevent "the bends", a serious con-dition caused by gas bubbles in the body fluids and tissues.Liquid helium, which is the only substance that will remainliquid at temperatures close to absolute zero (-273°C), isfinding increasing use in low-temperature physicscryo-genics.* Radon has been used as a source of gamma raysfor treatment of cancer, but more convenient gamma-raysources produced in nuclear reactors now are more fre-quently chosen for medical therapy.

*See Cryogenics, The Uncommon Cold, another booklet in thisseries, for an explanation of this branch of science.

9

EARLY HISTORY

Attempts To Form CompoundsAs in the case of other elements, the discovery of the

noble gases was followed by an examination of their chem-ical properties. It soon became obvious that these elementswere different they would not enter into chemical com-bination with any other elements or with one another. Manyattempts were made to induce chemical reactions betweennoble gases and both metals and nonmetals. A great manytechniques were used but none proved successful. Althoughmany claims were made that compounds had been formedcontaining noble gas atoms chemically bound to otheratoms, most of these either were unconvincing or shownto be incorrect. The scientists who came closest to successwere the American chemists, Don Yost and Albert Kaye.In 1933 they set out to test tte prediction, made that yearby another American, Linus Pauling, that krypton and xenonmight react with fluorine. Yost and Kaye passed electricdischarges through mixtures of xenon and fluorine and ofkrypton and fluLzine. Their results were inconclusive andthey stated in a communication to the Journal of the Ameri-can Chemical Society, "It cannot be said that definite evi-dence for compound formation was found. It does not follow,of course, that xenon fluoride is incapable of existing".

Very soon after the discovery of the noble gases it wasshown that argon, ki,-`'Nn, and xenon will form hydratescompounds in which the gases are associated with watermolecules. At first the hydrates were thought to be truechemical compounds, but they were later shown to beclathrate compounds; in this type of compound the inert gasis trapped in holes in a crystalline "cage" formed by thewater molecules. The host molecule in hydrates is water,but several other clathrate hosts have also been used, suchas the organic compounds phenol and quinol. For a com-pound to act as a host the cavities in its crystalline struc-ture must be large enough to provide room for the inertgas atom, but small enough to keep it trapped in the cage.So far no host molecules have been found whose cages aresmall enough to keep helium or neon atoms trapped, so no

10

clathratc compounds of these gases are known. Incidentally,the t/henomenon of clathrate formation provides a methodof separating neon from argon by trapping; the argon in aclathrate cage and pumping off the neon.

Clathrate compounds are not true chemical compounds,because they do not contain real chemical bonds. The onlyforces between the inert gas and the host molecule arerelatively weak electrostatic interactions. The inert gas isreadily released by destroying the crystalline cage, eitherby dissolving the host in a suitable solvent or by heating itto its melting point.

Why the Gases Are Inert

Before discussing the reasons for the inertness of thenoble gases it is interesting to look at the relationshipsbetween elements, and how they combine chemically withone another. The theory that each element has a fixed com-bining capacity was proposed by the English chemist SirEdward Frank land in 1852. This capacity was called thevalence of an atom. As most of the elements then knownwould combine with either oxygen or hydrogen, the valencevalues were related to the number of atoms of oxygen orhydrogen with which one atom of each element would com-bine. Two atoms of hydrogen combine with 1 atom of oxygento form H2O, so hydrogen was givena valence of 1, and oxy-gen a valence of 2. The valence of any other element wasthen the number of atoms of hydrogen (or twice the numberof oxygen atoms) that combined with 1 atom of that element.In ammonia we have the formula NH3, so nitrogen has avalence of 3; in carbon dioxide, CO2, the carbon valence is4. Valences are always whole numbers. Some elementsexhibit more than one valence, and the maximum valenceappears to be 8.

In the late 1860s the Russian chemist Dmitri Mendeleevmade an intriguing observation when listing the elementsin the order of increasing atomic weights. He found that thefirst element after hydrogen was lithium with a valence of1, the second heaviest was beryllium with a valence of 2,the third, boron with a valence of 3, and so on. As he con-tinued he found a sequence of valences that went 1, 2, 3, 4,

11

3, 2, 1, and then repeated itself. If he arranged the elementsin vertical columns next to one another, in the order ofincreasing atomic weights, he found the elements in eachhorizontal row across the page had the same valence andstrikingly similar chemical properties.

This kind of periodicity, or regular recurrence, had beennoted by other scientists, but Mendeleev made a great step

Dmitri Mendeleev

forward by leaving gaps in his table where the next knownelement, in order of weight, did not fit because it had thewrong valence or the wrong properties. He predicted thatthese gaps would be filled by yet-to-be-discovered ele-ments, and he even went as far as to predict the propertiesof some of these elements from the position they wouldoccupy in his table. A reproduction of an early version ofMendeleev's Periodic Table of the Elements is shown inFigure 4. As can be seen, thiswas based on the 63 elementsthen known. In later versions o the Table the elements arearranged in order across the horizontal rows, and thosewith similar properties fall in the same vertical column.

At the time of the setting up of the Periodic Table thenoble gases were still undiscovered. There were no gapsleft for them, as spaces could be left only where at least 1element in a group was already known. When argon wasdiscovered some problem therefore arose as to its place

12

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Figure 4 Above is an early {1869) version of Mendeleev's PeriodicTable. The heading reads, "Tentative .1steni of the elements".The subheading reads, "Based on atomic weights and chemicalsimilarities". This table is reproduced from Dmitri IvanovlchMendeleev, N. A. Figurovskii, Russian Academy of Science, Mos-cow, 1961.

in the periodic system. Its atomic weight suggested it mightbelong somewhere near potassium. When its lack of chem-ical reactivity was discovered, Mendeleev proposed that ithad zero valence and should come between chlorine andpotassium. He suggested that a group of such gases mightbe found. The valence periodicity then would be 0, 1, 2, 3,4, 3, 2, 1. This new group led to a complete periodicityof 8, which we shall see is a very significant number.

Both Frankland and Mendeleev based their ideas on theirknowledge of chemical properties. The theoretical supportfor both proposals caw with the development of a theoryof atomic structure and the electronic theory of valence.Theories stating that matter is composed of small, indi-visible particles, called atoms, had been proposed as

13.

early as 400 B.C., but were of a philosophic rather thanscientific nature. The scientific atomic theory reallystarted with the English scientist John Dalton in the 19thcentury. In his theory small, indivisible, and indestructibleparticles also were called atoms, but he gave them prop-erties that had phy3ical significance. More important,Dalton's theory not only would explain observed experi-mental results, but also could predict the results of newexperiments.

Toward the end of the 19th century the discovery of theelectron demonstrated that atoms themselves were divisibleand led to the proposal of the orbital atom. The atom cameto be considered as being made up of a nucleus, containingmost of the mass, and electrons revolving around the nu-cleus rather like the planets revolve around the sim.* Eachelectron has a single of unit negative charge and the entireatom is electrically neutral, or uncharged, because in thenucleus there are a number of protons (equal to the numberof electrons), each of which has a unit positive charge.

Atomic Number The number of protons in a given atomof an element is called the atomic number. In addition tothe protons, the nucleus contains uncharged particles calledneutrons. The neutrons and protons have about the samemass, and the electrons, by comparison, have negligiblemass. An element of atomic mass (A) and atomic number(Z) will have a nucleus consisting of Z protons and (AZ)neutrons, and this will be surrounded by Z electrons. Forexample, an atom of lith.ium with mass (A) of 7 and atomicnumber (Z) of 3 will have a nucleus consisting of 3 pro-cons and 4 neutrons (AZ), surrounded by 3 electrons.

The lightest element, hydrogen, has Z equal to 1, andeach successively heavier element differs from the onepreceding it by an increase of 1 in Z, and has one moreproton and one more electron than the next lighter one.Thus, the second heaviest element, helium, has Z equalto 2, and so on. For the heavier elements, such as uranium(Z = 92), one might imagine a chaotic situation with many

This theoretical "model" of the atom has since been modifiedto explain additional experimental results more fully. Now an atomoften is considered as a nucleus with electrons moving rapidly andrandomly around ity and having no definite boundary surface.

14

electrons buzzing all around the nucleus. Fortunately, theelectrons are restricted to movement in certain fixed or-bits or shells.* The number of electrons in each shell, andthe order in which additional electrons build up the shellsof heavier elements, is governed by quantum mechanicalconsiderations.t The first shell may contain 2 electrons,the oecond one 8, the third 18 and so on. However, themaximum number of electrons possible in any outermostshell is 8.

Subshells The shells themselves arc actually split intosubshells, which are designated by the letters s, p, d, and f,successively moving outward from the nucleus, The numberof electrons in a given sublevel is restricted, being amaximum of 2 for s, 6 for 10 for d, and 14 for f. Thevarious shells are distinguished from one another by num-bers from 1 to 7, where 1 indicates the innermost shelland 7 the outermost. A further restriction is that there isonly an s sublevel for the first shell, only s and p for thesecond, and only s, p, and d for the third. Beyond the thirdlevel s, p, d, and f sublevels are all permitted. These re-strictions are actually the same as those indicated in thepreceding paragraph; namely, the first shell contains 2electrons, which we write 1s2, the second shell has 8,written 2s22p6, the third 18, written 3s23p630.

The ele( trons do not necessarily fill the shells and sub-shells in consecutive order. The first (lightest) 18 ele-ments' electrons are added regularly, the electrons fillingthe ls, 2s, 2p, 3s, and 3p subshells in sequence. However,in the nineteenth element, the new electron does not go intothe 3d subshell, as might be expected, but into the 4s sub-shell. (Questions of this sort are decided on the basis ofenergy considerations. It is energetically more favorableto put the 19th electron into the 4s Pubshell.) From thispoint on we can write down the electronic configurationsof the succeeding (heavier) elements only if we know the

*A shell is also referred to in other theories as an energy level.tQuantum mechanics is a form of mathematical analysis Involv-

ing quanta, or definite units of energy in which radiation is emittedor absorbed. The different orbits, or energy levels, of planetaryelectrons are separated from each other by whole numbers ofquanta.

15

order in which the subshells are filled. We should note thatwhen the electrons do go into the 3d subshell this is con-sidered to be inside the 4s level. Consequently, there canbe 10 electrons in this subshell without violating the ruleof having a maximum of 8 electrons in the outermost shell.

Table HELECTRONIC STRUCTURES OF SELECTED ELEMENTS

ElementAtomicNumber Electronic Structure

HydrogenHeliumLithiumBeryllium

1

234

1s11s21s2, 2s11s2, 2s2

Boron 5 1s2, 2s2, 2p1Neon 10 1s2, 2s2, 2p6Sodium 11 1s2, 2s2, 2p8, 3s1Argon 18 1S2, 2S2, 20, 3s2, 3p6Potassium 19 1s2, 2s2, 2p6, 3s2, 3p6, 4s!Calcium 20 1s2, 2s2, 2p6, 3s2, 3p6, 4s2Scandium 21 1s2, 20. 20, 3s2, 30, 3d1, 4s2Titanium 22 1s2, 2s2, 2p11, 3s2, 30, 3d2, 4s2

This is always the case for d and f subshells: they arealways inside the next or next-but-one s subshell whenbeing filled. Table II gives the electronic structures forseveral elements.*

Now we are ready to look at the electronic theory ofvalence and some of its consequences. About 1920 a num-ber of chemists, most notably the American G. N. Lewis,suggested that the electrons in the outermost shells wereresponsible for elements' chemical reactions. Compounds(that is, molecules) are formed by the transfer or sharingof electrons, and the number of such electrons provided orobtained by an atom of any element during the combiningprocess is its valence. However, there is a kind of regu-lation of the number of electrons that can participate in thisbonding. It was suggested that the elements were alwaysbeing prodded to attain the maximum number of electronsin their outer shell, namely 8. An electronic structure with

*For a discussion of the electronic configuration of anotherinteresting family of the elements see Rare Earths, The FraternalFifteen, a companion booklet in this series.

16

8 electrons in the outer shell is cor3idered to be morestable and is called a closed-shell arrangement. Atoms,then, tend to adjust their electronic structure to that of thenearest element with a completed outer shell. The adjust-ment is made by losing, gaining, or sharing electrons withother atoms.

The closed-shell arrangement of electrons happens tobe the electronic structure of atoms of the noble gases.Moreover, only the 6 noble gases have this arrangementof maximum stability. This fact is the basis for the short-hand notation for writing electronic structures. FromTable II we can see the electronic structure of sodium is1s2, 2s2, 2p6, 3s1; sodium has 1 electron more than theclosed-shell arrangement 1s2, 2s2, 2P6, which is the elec-tronic structure of neon. The sodium electronic configura-tion can therefore be written (Ne), 3s1. Similarly potas-sium can be written (Ar), 4s1, scandium can be indicatedby (Ar), 3d1, 4s2, etc. The closed-shell arrangements arealto called cores,

Two atoms with the same number of electrons outsidea stable core would tend strongly to adjust their elec-tronic configuration in a similar manner; that is, theywould have the same val ,nce and therefore the same chem-ical properties. This fact is borne out by the fact that ele-ments in the same group in the Periodic Table have thesame outer electronic structures. Table III on pages 24-25is a modern version of the Periodic Table, showing theelectronic structures. Note that different elements some-times appear to have identical electronic structures; forexample, the outer shells of calcium and zinc are both 4s2.However, calcium is (Ar), 4s2 while zinc is (Ar), 3di0, 4s2.The presence of the complete d subshell causes zinc tohave somewhat different properties. Those elements inwhich the d and / subshells are being filled are called tran-sition elements, as opposed to the nontransition elementsin which the electrons are going into s and p subshells.

The fact that the noble gases have completed outershells means that they have nothing to gain by losing,gaining, or sharing Electrons. They already have the stableelectronic structures that other elements are striving toattain. This meant; that they should have zero valence and

17

should not form chemical compounds. Thus, the observedexperimental fact that the vases were inert was supportedby theory. This startling agreement between experiment andtheory was successful in discouraging attempts to makechemical compounds with the noble gases for a period ofalmost 90 years.

PREPARATION OF THE FIRST

XENON COMPOUNDSUntil 1962 all the accepted evidence pointed to the fact

that the noble gases were chemically inert. A few bravesouls had predicted that compounds of them might exist,but textbooks and teachers stressed the inertness of thegases and these statements went unchallenged.

As we have seen, the discovery of the first noble gas wasan outcome of an investigation of the density of nitrogen.The discovery of the first chemical compound of a noblegas was also a by-product of an unrelated investigation.the beginning really goes back to the Manhattan Project'and the production of the first atomic bomb. An importantingredient for the bomb was the uranium isotope 235U. Thiswas separated from natural uranium ,which is a mixturecontaining mostly another isotope, "IU) by gaseous diffu-sion, the "gas" fo; this process being a volatile uraniumcompound, uranium hexafluoride, This wartime inter-est in OF 6 created an interest in other metallic hexa-fluorides, compounds containing 6 fluorine atoms bound to1 metal ato-n. The study of the properties of these com-pounds. and the search for new hexafluoricies, was under-taken after the war in many lrboratories, especially thoseof the U. S. Atomic Energy Commission, which had workersexperienced in handling such chemically reactive materials.A group of scientists at the AEC's Argonne National Lab-oratory was particularly active in this field. They dis-covered hexafloorides of platinum, technetium, ruthenium,and rhodium, and investigated the properties of these andother hexailuoride molecules.

'The World War 11 code name for the program of the WarDeparter.vnt unit that predated the present Atomic Energy Corn-

The next step in the story took place at the University ofBritish Columbia in Vancouver, where Neil Bartlett, a youngBritish chemist, was doing research on fluorides of plati-inum. He and one of his colleagues discovered a compoundcontaining platinum, oxygen, and fluorine, which they for-mulated as 02+13tFc". In order to form this type of com-pound, an electron must be removed from the 02 part of themolecule, leaving it with a net positive charge. This electronbecomes associated with the PtFc part, giving this part anet negative charge. The surprising thing about this reac-tion is that the energy required to remove an electronfrom an oxygen molecule, the ionization 1)otentfal, is quitehigh. As a matter of fact, no compound containing 02+ hadever bceu known before the discovery of 02+PtFr. Althoughthe 02+13tFr they first synthesized was not made directlyfrom Pt Fe, Bartlett soon found that PtFs and molecular oxy-gen will react to give this compound. This suggested to himthat Pt Fc (platinum hexafluoride) must have a strong af-finity for electrons.

Soon after the discovery of 02+PtFr. Bartlett realizedthat the ionization potential of xenon Is almost exactly thesame as that of molecular oxygen. This led him to wonderif the platinum hexafluoride, with its powerful electron-attracting properties, could pull an electron away fromxenon to form a chemical compound. He decided to try anexperiment to confirm this idea, He filled a glass containerwith a known amount of the deep red platinum hexafluoridevapor awl separated it by a glass diaphragm from a similarcontainer filled with a known amount of the colorlessxenon gas. When the diaphragm between them was brekenthere was an immediate and spectacular reaction: The kgases combined to produce a yellow solidi Initial mea-surements of the amounts of gases reacting indicatedthat Le combining ratio was 1-to-I. In the June 1982Proceedings of the Cheinical Society of London. Bartlettreported preparation of the world's first compound inwhich a noble gas was chemically boundthe yellow solid,Xe4Pt

The announcement was greeted with surprise and insome places disbelief. This is not surprising since one ofthe accepted and revered dogmas of chemistry had just

IS

been shattered by his one experiment. More surprises wereyet to come.

The scientists at Argonne, where Pt Fs had been firstmade, confirmed Bartlett's results almost as soon as theylearned of his experiments. They went on to extend hiswork, and showed that xenon would also combineamaz-ingly with the hexafluorides of plutonium, ruthenium, andrhodium.

However, things were not as straightforward as they hadat first seemed. The combining ratios, which had been1-to-1 in the first experiments, were found to vary ir-regularly from one hexafluoride to another, and some-times even varied for the same hexafluoride. Even withPtF1 It appeared that there might be at least 2 compoundsformed, XePtF1 and Xe(PtFs)2. This threw some doubt onthe idea that XePtFA and 02413tFi might be completelyanalogous. The group at Argonne began to wonder if theattraction between xenon and the hexafluorides was due,not to the strong attraction of the htxafluorides for elec-trons, but instead to the hexafluorides' ability to providefluorine, that is, to act as fluorinating agents. If this wereso, it was reasoned, the xenon might actually react withfluorine itself.

Howard H. Claassen, then an Argonne consultant fromWheaton College, and Henry Selig and John G. Maim of theArgonne Chemistry Division next decided to test this idea.A known amount of xenon was condensed in a nickel con-tainer and a fivefold excess of fluorine was added. Thecontainer was sealed and heated to 400°C for 1 hour. Aftercooling the container to the temperature of dry ice (-78°C),the experimenters pumped the unreacted gas away. Uxenon were really an inert gas, the container should havebeen empty at this stage. To everyone's surprise it wasnot empty when weighed, Furthermore, the gain in weightcould be accounted for exactly by assuming that all thexenon initially present had reacted with fluorine 'o form acompound with the formula XeFi. The contents of the canwas sublimed into a glass tube as brilliant, colorlesscrystals (Figure 5). Within weeks of the time the originalannouncement of the preparation of XePtF1 reached Ar-gonne, a simple compound containing a noble gas and one

20

. r

101,

I

0

Figure 5 Crystals of xcaort letafluorldc. (Also see cover Photo-VW Ph

other element had been prepared. The date was August 2,1962.

One might wonder why the expression "more surpriseswere yet to come" was used a couple of paragraphs ago.Those who had objected to the "violation" of the idea ofabsolute inertness of the noble gases could still rationalizethat a compound as exotic as one between Xe and rip,might not contain true chemical bonding, and that It mighteven be a new type of clathrate compound. The preparationof XeF4 removed all such possible explanations, and thechemical world was faced with the naked truth that at leastone "inert" gas was not inert. Chemical textbooks becameobsolete overnight in this respect, and professors andteachers had to rewrite their lecture notes.

21

,,,frg 11 ) %41 i.

""1079.16-__

4

nave* 6 John G. Maim (left) and lioxard ti. Claassen adjnstingapparaks similar to that 11/sed for the first preparation of XeF4 atArgonne National Laboratory.

22

COMPOUNDS OF XENON

FluorineContaining Compounds

As we have already seen, the first noble gas compoundscontained the element fluorine.* Of the many compoundsdiscovered since then, it turns out that they all either con-tain fluorine or are made from fluorine-containing com-pounds. Let us consider first the 3 known binary fluorides,that is, compounds containing only xenon and fluorine. 13yheating together a mixture of xenon and fluorine underappropriate conditions, chemists can produce xenon di-fluoride, XeFt, xenon tetrafluoride, XeF4, and xenon hexa-fluori le, XeFt. Which of these fluorides is produced de-pends on the ratio of fluorine to xenon, the temperatureof the reaction, and the pressure in the reaction vessel.These may be adjusted to form any one of the 3 fluoridesin a reasonably pure state. If care is not taken, however,mixtures of the fluorides result and these are difficult toseparate. Table IV on page 26 shows the conditions thathave been used to prepare several-gram quantities 00U Ft,XeF4, and XeFs.

In order to prepare a fluoride of xenon it is only neces-sary to have a source of fluorine atoms, which then reactwith the xenon. Heating fluorine gas is one way to producesuch atoms; they have also .i.:een produced by subjectingfluorine, or fluorine- containing compounds, to electricdischarges or ionizing radiations, such as the gamma raysfrom a cobalt-60 source or a beam of electrons, a beam ofultraviolet light, or a beam of neltrons from a reactor.

The fact that xenon fluorides can be formed answereda puzzling question that had been plaguing scientists andengineers who were studying reactor fuels.t In experi-ments to test the fuels and fuel assemblies for a molten-salt reactor, a mixture of lithium fluoride, berylliumfluoride, zirconium fluoride, and uranium fluoride was

'Fluorine Is the most active nonmetallic element, and combineswith all other elements (disregarding the noble gases) so stronglythat It cannot be prepared from any of its natural compound* byany purely chemical reduction.

tFor more about reactors, see Nactear Reactors and AtomicFad, companion booklets in this series.

23

Tab

le II

I PE

RIO

DIC

TA

BLE

OF

TH

E E

LEM

EN

TS

GROUP

IsB

oM

aIV

.V

sV

I.V

II.V

IIIb

libM

bIV

bV

bV

II)V

ilb0

rom

p1

110

079!

H -'

26.

9311

Li 2.

4*0

127

Be

2.2

510

.111

B2.

22,1

612

.011

1

C2.

27,2

1600

67

N2E

243

15,9

994

020

20

916

9904

F20

45

= ?

,) )

261

1111

1

1Ne

'2.2

9...-

31

3

1122

9011

112

Na

3.

3631

2

Mg .2

1326

.961

5

AI

3.23

.1

1421

1.06

6

Si

3.23

.2

1530

.973

6

P32

3,3

1622

.064

S30

3,4

1735

.423

Cl

3.23

.2

1.8 .

3694

6

Ar

4

-19

79.v

at

K a.

204R

OS

Ca

4.7

2146

956

Sc

3414

67

.90

Ti

3046

7

30.

V30

4.2

231

.916

Cr

304.

1

2554

.930

Mn

304.

2

2656

.047

Fe

3444

.2

2736

900

Co

304.

2

3671

Ni

3040

2963

54

Cu

3144

.1

30

Zn

3404

.2

3169

.73

Ga

4.14

,1

3272

.59

Ge

4824

0

3ns

ztA

s4.

4,3

26-9

.

Se

4.24

0

3579

.909

Br

4,24

0

36-

-13

410

, _ g K

r-:

i.W

V16

.47

RI)

3.

38

Sr

sw

3911

1.90

5

Y40

5.2

4011

11.2

2

Zr

403.

3

196

900

NI)

41e5

.2

4295

.94

Mo

&Pu

s

43(9

9)

Tc

405.

2

101.

07

Ru

405.

1

107.

9115

Rh

405.

3

4610

6.4

Pd

&es

1624

70

Ag

4005

.1

1124

0

Cd

4125

,2

4911

4.12

In5.

25.1

110.

69

Sn

W50

5112

1.75

Sb

5.25

.3

5217

7.60

Te

5.25

0

126.

904

I5.

75r2

54,

' 131

14/3

Xet

'°'4

0

6

5513

2.96

1

Cs 6.

5612

7.36

Ba

667

57r3

6

Le

Wiw

i

717

1A9

Hf

5.06

67

7310

3.94

0

Ta

5d11

64.3

Tax

a,

W 541.

'

7510

62

Re

SO

W

76rr

o.z

Os

7in

.2

I r30

641.

78in

toP

t5,

661

7919

6.96

7

Au

5110

661

8020

0-59

Hg

5411

6.7

8120

4.37

TI

5410

6.26

,1

8220

7.19

Pb

6.26

,2

8320

1.90

0

Bi

6.26

,3

(210

)

Po

6471

p4

85(2

10)

At

6425

03

.N (

2ZI)

7

87(2

22

Fr

6A47

.1

(226

1

Ra

11.)

607.

2

8(2

V)

Adm

.12

1016

17.3

24

*Lov

erha

nide

s

..Act

ivvi

dos

*58

140.

12

Ce

OW

90=

CM

Tit

ger.

,

5916

1907

Pr

406w

7

91(2

32)

Pa

5421

1441

7, 2

6014

24

Nd

W6.

292

mos

U5P

6/I7

.2

61(1

223

Pm

406.

193

(233

)

Np

sord

a2.7

6215

6.36

Sm

404.

71I

pmP

u51

2.7

6315

1.96

Eu

406.

295

(243

)

Am

sow

641.

..V.2

5

Gd

4175

4168

7

96(2

42)

Cm

5eed

12..

6515

6.96

1

Tb

404.

7

97(2

47)

Bk

sp6.

122.

=

6616

650

Dy

4066

298

(z5i

)

Cf

5#6.

02.,

164.

930

Ho

4044

799

(254

Es

ti4.2

4442

,oso

4.4,

47.2

w4.

34,4

7

167.

26

Er

4076

67

100

(1:3

)

Fm

6916

6.93

4

Tm

4036

62

101

(256

:f

Md

7017

3.01

Yb

4046

E7

02(2

543

No

7117

4.97

Lu41

1451

16.7

103

(257

)

Lw

25

Table IVCONDITIONS USED FOR PREPARING Tilt: XENON FLUORIDES

Temperature Time PressureCompound Ratio Xe/F2 ('C) (hours) (atmospheres)

XeF2 7.5 : 1 900 16 75Xe F4 1 : 5 900 1 6Xe11 1 : 20 250 16 50

used as the fuel. This was sealed in a container and sub-jected to neutron irradiation. Under these conditions the235U in the uranium fluoride, UF4, undergoes fission. Thefission results in the "5U atoms' breaking up into newfission-product atoms of nearly equal mass, and somefree neutrons, and the release of a large amount of energy.Among the expected fission products there always aresome xenon isotopes, and the amount of xenon so producedis sometimes used as a measure of the amount of fissionthat has taken place. You can imagine the surprise of thescientists when no xenon could be found in the gases fromthe molten-salt reactor experiments, although other prod-ucts showed that fission had undoubtedly taken place.

Puzzle Explained With the discovery of xenon tetrafluoridethe puzzle was explained. It turned out that free fluorine isgenerated in the reactor-fuel mixture by the neutron irra-diation. Under certain conditions this fluorine can reactwith the fission product xenon to form a xenon fluoride. Inthose cases where no xenon was found, the conditions hadbeen right for xenon fluoride formation. This was anothercase in which a discovery in one field of science answereda problem in another.

Perhaps the most startling experiment with xenon andfluorine was reported towards the end of 1965. Xenon andfluorine when mixed in a dry glass flask will react if themixture is exposed to sunlight I In this case the energy pro-vided by the sunlight is enough to produce the needed fluo-rine atoms. This being the case, one may wunder why ittook so many years to prepare the first noble gas com-pounds. Several explanations have been offered, such asthe difficulty h getting thoroughly dried glassware, andlack of knowledge of the techniques for handling fhorine

26

Table VPHYSICAL PROPERTIES OF THE XENON FLUORIDES

VaporMelting Pressure Density

Color of Color of Point at 25'C gm/ccCompound Solid Vapor ('C) (mm) at 25'C

XeF2 Colorless Colorless 129 4.6 4.32XeF1 Colorless Colorless 117 2.5 4.04XeFi Colorless Greenish- 49.5 27 3.41

Yellow

and reactive fluorides. These undoubtedly played a part,but the major factor was probably the lack of an adequateamount of xenon. (Until recently xenon was not generallyavailable in most laboratories because of its high cost.)

The xenon fluorides are colorless crystalline materialsat room temperature, but they react readily with moisture.For this reason they must be handled in thoroughly driedequipment and are usually manipulated in metal vacuumsystems. A typical experimental setup is shown on page 22.The necessity of avoiding a reaction with water (hydrolysis)is extremely Important, as we shall see later. Providingthis precaution is observed, the fluorides are stable atroom temperature and can be stored for prolonged periodsin nickel containers.

Some of the physical properties of the fluorides aregiven in Table V. Each of the fluorides will react withhydrogen, forming hydrogen fluoride and liberating ele-mental xenon; for example,

XeF4 + 2112 Xe + 4111'

The relative ease of this reaction with hydrogen establishesXeF1 as the most reactive of the xenon fluorides, and XeFlas the least reactive. This order of reactivity has beenconfirmed by other experiments, in which the xenon fluo-rides act as fluorinating agents. In addition, it has beenfound that both Xen and XeF4 can be stored in thoroughlydried glass containers, but XeF1 reacts even with dry glassor quartz. Note that xenon, in forming the three fluorides,exhibits valences of 2, 4, and 8.

27

Oxygen-Containing Compounds

Under normal conditions it does not appear to be pos-sible to obtain a chemical reaction between oxygen andxenon or between oxygen and a xenon fluoride. In thosecases where oxygen has been introduced into a xenon-containing compound the introduction has been achievedby the replacement of fluorine. One of the first oxygen-containing compounds to `.)e discovered was xenon oxidetetrafluoride, XeOF4. Chemists attempting to store XeF1in glass found that a clear, colorless liquid was formedby reaction of the XeF1 with the glass. The liquid wasanalyzed and found to have the formula XeOF1. The oxygennad been obtained from the glass, which maybe regarded assilicon dioxide, Ri Oz. The reactive fluorine in the XeF1replaced the oxygen in the Si Oz, converting it to Si Fz:

2XeFl + Si Oz = 2Xe0F4 + SIF4

Since fluorine has a valence of 1 and oxygen a valence of 2,2 fluorine atoms had to be removed to allow the insertionof 1 oxygen atom.

This oxygen-containing compound is also formed whenXeF1 reacts with just enough water to provide for the re-placement of 2 of the fluorine atoms. This reaction may bewritten:

XeF1 + 1120 = XeOF4 + 2HF

Xenon oxide tetrafluoride is somewhat less reactive thanXeF1, but is more reactive than XeF4. It may be kept un-changed in dried nickel containers, but it slowly attacksglass or quartz.

The reaction of XeFs with enough water to provide forthe replacement of all 6 fluorine atoms with oxygen atomsyields Xe03, xenon trioxide:

XeF1 +31130= Xe03 +6HF

Xenon trioxide also results when XeOF4 is allowed to re-main in contact with glass for prolonged periols, or when

26

Xe0F4 reacts with water. The reaction of XeF4 with watercan also result in the formation of Xe03. This is a some-what surprising reaction, however. In Xe03 the xenon hasa valence of 6, the xenon being combined with three oxygenatoms each of valence 2. When Xe03 is formed from XeFsor XeOF4 the valence of the xenon in the original com-pounds is also 6. However, when Xe03 is prepared fromXeF4, the valence of the xenon in the starting material isonly 9. This type of reaction comes about by disproportion-ation of the xenon atoms; some of them end up in a highervalence state and some in a lower one, that is, sorge ofthe xenon atoms are oxidized and others are reduced. Theproduction of xenon trioxide from xenon tetrafluoride andwater may be formulated thus:

3XeF4 + 6H20 = Xe + 2XeO3 + I2HF

Starting off with 3 xenon atoms each having a valence of 4,the procedure ends up with 1 xenon atom of valence zero and2 of valence 6, thus balancing the valences. In alkaline solu-tions, for example caustic soda, the disproportionation cango a step further and yield compounds containing xenonwith a valence of 9, such as sodium perxenate, Na4Xe04.The perxenate salts react with concentrated sulphuric acidto yield the 13-valent xenon tetroxide, Xe04.

EXTREME CARE MUST BE TAKEN WITH BOTH OF THEXENON OXIDES, BECAUSE THEY ARE POWERFUL EX-PLOSIVES UNDER CERTAIN CONDITIONS. Xenon trioxideis relatively safe in solution in water. Whenthe water evap-orates, however, the pure xenon trioxide is left in the formof colorless crystals, which are as powerful as TNT intheir explosive power! Unlike the case with TNT, it is notknown under what conditions the crystals can be handledsafely, nor exactly what causes them to explode. Thismakes working with xenon trioxide extremely hazardous.Moreover, because the xenon fluorides react with moistureto give xenon trioxide, even working with these compoundscan also be dangerous. The metal container shown in Fig-ure 'I was damaged by the explosion of about 100 mg.(0.0035 oz.) of xenon trioxide. Even experienced and care-ful scientists have been injured when working with xenon

29

Figure 7 This nickel can. aboxi 4 inches long and 114 inches aide(photo is approyimalcly actual sire), Nos rmplNrcd by dclonahon of100 milligrams of Xc0j.

compounds. These, then, are not materials to be workedwith in a basement laboratory in a home, but should onlybe handled in well-equipped laboratories by experiencedworkers who give every regard to safety precautions.

30

More Complex CompoundsMention has been made of XePtF6 and similar compounds

in which xenon combines with metal hexafluorides. Theexact nature of these compounds is hard to elucidate andis still being investigated. Both xenon difluoride and xenonhe.vafiuoride will react with a number of other fluorides toform addition compounds. Table VI shows the formulas of

some of the complexes that have been reported. Apart

T::!)le VICOMPLEXES OF XENON AND KRYPTON FLUORIDES

Noble GasCompound XeF2 XeF4 XeF6 XeOF4 KrF2

ComplexingFluoride Ratio of Noble Gas Compound to Complexing Fluoride

NaF * 1:2 * *

la * 1;2 1:31:6

RbF t t 1:2 2:3 t1 :1

CsF t 1:2 1:3 t1:1 2:3

1:1

SbF6 1;2 t 1:2 1:2 1:21:1 t2:1 t

AsF5 * t 1:1 2.14 tBF3 * t 1:1 * tTaF6 1:2 t t t tVF6 * * 2;1 1 t

*No compound formed.alas not been tried.tCompound forms; formula not yet known.§ Unstable above 20°C.

from their chemical composition, and a few physical prop-erties, not much else is known about these complexes.Xenon tetrafluoride does not appear to form a similarseries of addition compounds.

31

COMPOUNDS OF OTHER NOBLE GASES

Radon

The ionization potential of radon is the lowest of any ofthe noble gases, which might lead one to think it would bethe most willing to form compounds. This may in fact bethe case, but experiments with radon are severely ham-pered because of its high radioactivity. Work done withvery small amounts of material (about one billionth ofa gram) has shown that radon gas reacts with fluorineat 400°C to yield a compound that is not gaseous at roomtemperature, as both radon and fluorine are. The courseof the reaction was followed only by monitoring with ra-diation detecting instruments the movement of the radio-activity associated with one of the products of decay of theradon. The formula of the compound produced has not beendetermined, and further investigation will be needed inwhich larger quantities of radon can be used. This willrequire elaborate shielding to protect the experimentersfrom the high radioactivity.

Krypton

After xcnon and radon, krypton should be the most likelyof the remaining noble gases to form compounds. Its ioniza-tion potential is somewhat higher than that of eit.ner oxygenor xenon, and it will not react with platinum, ruthenium,or rhodium hexafluorides (PtF6, RuF6, and RhF6, respec-tively). The simple heating of krypton and fluorine also hasfailed to produce I compound. However, a krypton fluoridecompound can be formed under the more drastic experi-mental conditions of passing an electric discharge or anelectron beam through a mixture of the 2 gases. Thekrypton fluoride will decompose almost as fast as it isformed if it is left in the discharge or beam zones. But ifthe container is immersed in a cold bath the krypton fluo-ride condenses on the container wall, and is thus removedfrom the zone in which the energy is generated. In this waykrypton difluoride also has been produced, and possiblykrypton tetrafluoride. The evidence for the formation ofthe latter is somewhat inconclusive, however.

32

Krypton difluoride is a colorless, crystalline compoundthat decomposes into krypton and fluoride at room temper-ature. At the temperature of dry ice, 78°C, krypton di-fluoride may be stored unchanged for prolonged periodsof time. Chemically, it is a much more reactive compoundthan xenon difluoride, and in fact, its fluorinating proper-ties appear to be even greater than those of xenon hexa-fluoride.

Helium, Neon, and ArgonAll evidence now available points to the fact that these

gases are still inert. If one were to look at the propertiesof the fluorides of krypton we have just discussed, in corn-.p.rison with those of the xenon fluorides, one would im-mediately expect that fluorides of the three lightest noblegases could be prepared only under extreme conditions,and even then would be stable at only low temperatures.Attempts to prepare compounds have so far failed, butwho knows what may be found some day? Only a few yearsago the idea of a xenon fluoride seemed prt.dosterous, too.

SHAPES OF MOLECULES

Solid StateIn solids, the molecules are condensed to form crystals,

and the way in which the atoms are arrayed in the mole-cules may be determined by using beams of X rays orneutrons. When such a beam is directed at a crystal iteither passes through the spaces between atoms undis-turbed, or else it strikes an atom and is scattered or de-flected. The amount of scattering can be detected and mea-sured, giving a pattern that can be related to the location ofthe atoms and therefore to the structure of the crystal.

The determination of the actual array of the atoms inany unknown crystal has to be made in an indirect manner.A guess is made of its probable structure and the patternthat this structure would produce is calculated. This pat-tern is compared with the experimental pattern. When anexact match is obtained, it is apparent the structure isknown. This used to be a long, tedious operation, but mod-ern computer technology has simplified the process.

33

The atom:, of material are spread out in all three di-mensions throughout every crystal and this complexity intheory could lead to very complicated structures. For-tunately, it turns out that there are certain arrays of atoms

0 Xenon atomsFluorine atoms

Figure 8 Crystal structure of XeF2,left, and XeF4.

that repeat themselves throughout the crystal lattice; theseare called "unit cells" and the problem is reduced to oneof finding the locations of the atoms in each of the unitcells.

Both X-ray-diffraction and neutron-diffraction techniqueshave been used to determine the structures of XeF2 andXeF4, and the X-ray method alone has been used for Xe03.Figure 8 shows the crystal structures of XeF2 and XeF4 sodetermined. The high reactivities of Xe F6, XeOF4, andKrF2 produce problems when an attempt is made to exam-ine their solid phase structures. Samples to be examinedby X-ray techniques are usually loaded into long, thin glasscapillaries. Figure 9 shows a scientist positioning onesuch capillary in an X-ray camera. As XeF6, Xe0F4, andKrF2 are incompatible with glass, and also are most easilyhandled below room temperature, they require special

34

tr,

r

111101

Figure 9 Argonne scientist Stanleyconta;ning XeF4 in an X-ray camera,like aiject in the center of the picture.

Siegel positions a capillaryThe capillary is the needle-

techniques, and their solid-phasedetermined.

structures have yet to be

Gas Phase

Whereas in the solid phase the molecules forming thecrystal are quite close together and can influence oneanother, in the gas phase they are relatively far apart andone can virtually look at indiviclual molecules.

The method of electron diffraction has been used toexamine XeF4 and XeF6. A beam of electrons is passedthrough the vapor of the compound in the same way thatX rays or neutrons are passed through crystals. The sametype of trial-and-error analysis of the data is made untilthe experimental and calculated patterns agree. For XeF4the structure is similar to one of the srn-Ller arrays thatmake up the crystal (solid) unit cell. That is, the xenonatom is located at the center of a square with the 4 fluorineatoms at the corners. The XeF6 structure turns out to bemore complicated. The first guess would be that the mole-cule would have the xenon at the center of an octahedron

35

with fluorine atoms at each corner (Figure 10). This guesswould be based on the fact that other hexafluorides, suchas SFs (sulfur hexafluoride), have this type of structure.However, the electron diffraction pattern for XeF6 cannot

Figure 10 "First guess"structure for XeF6. Xe

F

be reconciled with this type of structure. There appearsto be some deviation from the octahedral symmetry, andthis produces a complex pattern that has not so far beenresolved.

Information can also be obtained about the shapes ofmolecules by studying what happens when they interactwith light. Consider the atoms in a molecule as balls, andthe chemical bonds between the atoms as springs. If asmall amount of energy is given to such a ball-and-springmolecule it can begin to vibrate, the balls moving back andforth about an equilibrium position with characteristicresonant frequencies. These frequencies are determinedby the weights of the balls, the length and strength of thesprings, and the geometric arrangement of the balls. Ina real molecule, the frequencies are determined by themasse of the atoms, the shape of the molecule, and thestrengths of the chemical bonds. The number of atoms inthe molecule determines the number of characteristicfrequencies.

In the study of the vibrational activity of molecules, en-ergy in the form of light is passed through the compound tobe identified. The emerging light is then examined to deter-mine whether any particular frequencies of light have beenabsorbed or emitted* during the experiment, and the num-ber of such frequencies. Here again a scientist first has to

The energy of a given amount of light E is related to its fre-quency v by the equation E = hv, h being a constant known asPlanck's constant.

36

gueos at the shape of the molecule and calculate for eachshape how many different, distinguishable ways there arein which the atoms could be set into resonant (vibrational)motion. The experimental results then allow him to chooseamong the possible shapes.

Based on spectroscopic examination of their vapors,XeF2 and KrF2 are found to be Linear and XeF4 is squareplanar, that is, the atoms in XeF2 are in a straight line(FXe --F), and the atoms of XeF4 form a flat square, withXe at the center and four F atoms at the corners. Oncemore the reactivity of XeF6 makes an unequivocal answerdifficult to obtain for this compound.

Predicted Shapes and Chemical BondingBefore starting on this subject we must first clarify one

point. Although thP newly discovered xenon fluorides ap-peared to be a violation of the known rules of valence andchemical bonding, and might therefore requi-e somethingunique and exotic in the way of an explanation, this type ofcompound was not really new. Previously known com-pounds, such as bromine trifluoride, BrF3, have atoms thatmust share more than the 8 electrons of a completed va-lence shell. Before trying to see how this can be explainedwe have to go back and learn a little more about s, p,and f orbitals and electrons.

We saw earlier that the number of electrons in a givensubshell is limited, 2 for s, 6 for p, 10 for d, and 14 for J.These subshells are themselves further broken down intoorbitals, each of which can contain a maximum of 2 elec-trons. These orbitals can be regarded as a pictorial rep-resentation of the probability of finding a given electron ina given place at a given time. For s electrons, the orbitalhas a spherical shape with the nucleus at the center. Theelectrons can be anywhere from directly at the nucleusto a great distance away. However, there is a preferredlocation for them, and the sphere has a definite size. Forthe p orbitals the electrons are most likely to be found intwo regions, one on either side of the nucleus; the result-ing shape is something like a dumbbell. As no two orbitalsmay have the same direction, the 3p orbitals, each contain-ing 2 electrons, are located perpendicular to one another

37

(Figure 11). For the d and f electrons the pictorial repre-sentation becomes more difficult so we will manage withoutit; anyone interested in more detail may consult a book thatspecifically deals with the subject.*

S Orbital

Figure 11 Graphic representation of s and p orbitals.

The orbitals we have just described represent whathappens in individual atoms. When atoms combine to be-come molecules, however, the electrons in the orbitalsare no longer affected only by their own nuclei, but comeunder the influence of all the nuclei in the molecule.Bonding, then, is described as the combination or inter-action of the atomic orbitals to form molecular orbitals.

For the xenon fluorides the molecular-orbital appr"achto the question of bonding is based on the involvement ofthe outer 2p orbitals of the fluorine atoms and the 5p or-bitals of the xenon. The calculations involved in workingout the exact quantitative description of these moleculesare difficult. Scientists know the equations that should be

*Such as Coulson's Valence in the Suggested References, page 45.

38

used, but so far have been able to solve them for only thesimplest molecule, H2. We can also solve them quite wellfor the other light elements by making certain approxima-tions. But for the heavier elements we can obtain onlycrude solutions that allow us to establish trends in proper-

XeF6

Xe

F

Figure 12 Overlapping of xenon 5p orbitals with fluorine 2porbitals

ties. However, a simple approach suggests that we canlook at the formation of the xenon-fluorine bond as beingproduced by the linear combination of the 5p and 2p or-bitals from the xenoi. and fluorine, respectively. Figure 12shows the representations for XeF2, XeF4, and XeF6, in-dicating molecules that are respectively linear, squareplanar, and octahedral.

A second approach that has been proposed for describingthe bonding in xenon compounds is called the valence-shellelectron-pair repulsion theory. This is generally applicable

39

to all molecules. It considers the electrons around a cen-tral atom in pairs. If the 2 electrons come from the centralatom they form an unshared pair, or lone pair; If one comesfrom the central atom and one from another atom theyform a single bond; if 2 come from the central atom and 2from another atom they form a double bond. Fluorine,being univalent, forms single bonds; oxygen, being divalent,forms double bonds. The shape of the resulting moleculedepends on the total number of bonds plus lone pairs.Table VII shows the geometrical shapes associated withgiven totals of bonds plus lone pairs. Table VIII (page 42)shows how this theory applies to some xenon compounds. Inour examinations of the gaseous molecules, we would notsee the lone pairs and so would see XeF2 as linear, XeF4as square planar, and XeF6 as some form of distortedoctahedron. Xe03 would appear as a triangle pyramid,Xe04 as tetrahedral, and XeOF4 as a square pyramid.

The valence-shell electron-pair repulsion theory hasshown us one way to predict shapes of molecules, but itremains to be explained how bonding can take place withan atom of one of the noble gases, which already has acompleted outer shell of 8 electrons. To do this, we mustsuppose there is involvement of the d orbitals of xenon.

Hybrid Orbilais If we remove electrons from the 5s and5p orbitals and put them in the empty 5d orbitals, xenonthen no longer will have the filled outer shell. Once thistype of promotion takes place we no longer can identify ouroriginal orbitals. We now have orbitals with a mixture ofs, p, and d character, which are called hybrid orbitals. ForXeF2 we need 2 electrons from the xenon to "share" withthe fluorines in forming bonds, so that each fluorine has ashare in 8 electrons. To achieve this we promote 1 xenon5p electron to a 5d orbital. Instantaneously we can imaginethat xenon now has a 5p and a 5d orbital, each with only 1electron, and therefore is able to form bonds by pairingwith electrons from °tin atoms. These orbitals are"filled" by sharing the 2p orbital of the fluorine that alsohas only 1 electron. (Remember fluorine's electronic struc-ture is 1s2, 2s2, 2p5, or, alternatively, 1s2, 2s2, 2p2, 2p2,2p.) Having now used one 5s orbital, three 5p orbitals, andone 5d orbital of xenon, we have a hybrid made of 5 or-

40

Table VII

Number of bondsplus lone pairs Shape

2

3

4

5

6

7

1111.--Linear

WI

Triangle Bentplanar

Tetrahedral Trianglepyramid

Bent

Trigonalbipyramid

Distortedtetrahedron

1-shaped Linear

Octahedral Squarepyramid

Squareplanar

Pentagonalbipyramid

Distortedoctahedron

41

Table VIIISHAPES OF XENON COMPOUNDS PREDICTED BY THE

VALENCE-BOND, ELECTRON-PAIR REPULSION THEORY

Total number Number of Number of Number ofCompound of electrons XeF bonds Xe 0 bonds lone-pairs Shape

XeF2 10 2 3 LinearXeF4 12 4 2 Square

planarXe Fe 14 6 1 Distorted

octall.dronXe0 F3 14 4 1 1 Square

pyramidXe 03 14 3 1 Triangle

pyramidXe 04 16 4 Tetrahedral

Remember: Each XeF bond involves 2 electrons and each Xe 0 bond in-volves 4 electrons.

bitals that results in a trigonal bipyramidal shape shown inTable VII. This type of hybrid is designated as an sp3dorbital.

The promotion of the 5p electron to the 5d orbital re-quires the expenditure of energy. This promotion can onlytake place if the energy we get back when the electrons areused in bond formation is greater than the energy requiredfor the promotion; that is, if we have a net gain in energy.In actual fact the two-stage process we have described ispurely fictitious. The formation of the hybrid orbitals andthe bond formation take place simultaneously. For XeF4we have sp3d2 hybridization and for XeF6 it is sp3d3.

This kind of expansion of the valence shell can only takeplace for atoms with unoccupied d orbitals that are close inenergy to the orbitals from which the electrons must bepromoted. This, Lniggests that bonding for helium and neonmay not be possible, because they do not have d orbitals.(There are no ld or 2d orbitals.) The promotional energy3p - 3d is quite high, and makes the possibility of argoncompounds questionable. The 4p 4d promotional energyis just small enough to allow krypton fluorides to be made,and for them to be stable at low temperatures.

For XeF2 and XeF4 Loth our molecular-orbital andvalence-shell approaches pr e d i c t the same molecularshapes, and they both agree with experimental evidence.

42

The valence-shell method also predicts the correct shapesfor Xe03 and Xe04. The difference between the two methodsis apparent in their treatment of XeF6. The molecular-orbital approach predicts an XeF6 molecule with octahedralsymmetry, while the valence-shell approach suggests thatthere will be distortion from this type of symmetry. Theexperimental results obtained so far favor a distortedmolecule. However, the amount of distortion appears to besmall, and may not be as large as would be expected fromthe valence-shell c;nsiderations. As so often is the case,the facts may lie somewhere between the two theories.

In summary, we can conclude that the tendency of anelement to achieve a relatively stable, completed outershell of 8 electrons can still be regarded as a good de-scription of chemical bonding. Most of the chemical bond-ings we know can be related to this. The basis for thePeriodic Table still remains a sound and workable one.Our only change in thinking is that we can no longer callkrypton, xenon, and radon "inert" gases.

POSSIBLE USES

Almost everything that can be said about uses of thenoble gas compounds must be in the nature of speculationor flight of fancy. One practical consideration of importanceis that krypton, xenon, and radon are so scarce and ex-pensive that any use of their compounds on a large scaleis doubtful. Xenon, for example, costs about $150 perounce, and small amounts of XeF1 have been sold at about$2500 per ounce. So actual "uses" will be few.

The first possible consideration Is tie use of xenon fluo-rides as good fluorinating agents. When the fluorinationprocess is complete, easily separable and recoverablexenon is left. They may therefore find some specializedresearch use for adding fluorine to some exotic organicmolecules. They have also been suggested as potentialorldants in rocket propulsion systems, although the highatomic weight of xenon does not make even XeFI see veryattractive for this purpose.

The tact that xenon is a f.a.:on product has been men-tioned Perhaps the xenon compounds will be put to some

43

use in nuclear studies. The volatile xenon gas resultingfrom fission could perhaps be converted to a much lessvolatile xenon fluoride.

Since xenon reacts with fluorine under conditions wherethe other noble gases do not, this may be made the basisfor a method of separating it from the other gases.

If we could tame xenon trioxide to the point where wecould know when and how it would explode, we might havea valuable new explosive. An advantage would be that nosolid residues are left after xenon trioxide blows up.

Radon is occasionally used in cancer therapy. A smallglass tube placed close to a tumor exposes that particulararea to a large dose of radioactivity, which hopefully willdestroy the tumor. However, glass ampoules to hold radongas are fragile and metal ones are hard to seal; moreoverthe release of radon gas is dangerous. There would be adistinct advantage to having a nonvolatile radon compoundfor medicinal uses.

The most likely compounds of practical value are theperxenates, or xenon trioxide In solution. These are power-ful oxidizing agents and may find many uses in analyticalchemistry. The beauty of using such materials is that theyintroduce few additional chemical species into the systemunder investigation.'

Whether or not practical uses for these compounds areever found, they have already served one purpose: Chemistshave been reminded never to take anything for granted.What may seem to be a proven fact now may one day haveto yield its validity to a new experiment or a new theory.Even when thinking about closed shells there is no room forclosed minds.

44

SUGGESTED REFERENCES

Books

Argon, Helium, and the Rare Gases, Gerhard A. Cook (Ed.), Inter-science Publishers, Inc., New York 10016, 1961, 2 volumes,$17.50 each. These volumes cover the discovery, occurrence,properties, and uses of the noble gases. Some of the materialrequi-es a high technical knowledge.

Noble Gas Compounds, Herbert H. Hyman (Ed.), University ofChicago Press, Chicago, Illinois 60637, 1963, 404 pp., $12.50.Collection of papers covering in detail physical and chemicalproperties of compotuds of krypton, xenon, and radon. Severalpapers deal with theoretical aspects of the existence of thesecompounds.

Noble Gases and Their Compounds, G. J. Moody and J. D. R.Thomas, Pergamon Press, Ire., New York 10022, 1964, 62 pp.,$2.00. This short monograph deals mainly with the chemistryof the noble gases. The technical level Is not as advanced aseither of the other two books cited.

The Gases of the Atmosphere: The History of Their Discovery,Sir William Ramsay, Macmillan and Company, London, 1915,306 pp. Discovery of the Rare Gases, M. W. Travers, Longm.ans,Green and Company, New York, 1928, 128 pp., 95.00. (Out ofprint but available through libraries,) These two books, writtenby men who played major roles in the discovery of the noblegases, give a fascinating insight into the beginrings of this story,They ere also Interesting for their description of science andscientists at the end of the 19th century.

A History of Me Concept of Valency to 1930, W. G. Palmer, Cam-bridge University Press, New York 10022, 1965, 178 pp., $8.00.A historical account of the development of the ideas of valence.

Valence, C. A. Coulson, Oxford University Press, Inc., New York10016, 1961, 404 pp., $6.00. A modern and more theoretical ap-proach to the subject than the previous book, The nlolecularwbital and valencebond theories are both considered. Thisbook is recommended mainly for readers with advanced chemi-cal knowledge.

The Noble Gases, Howard H. Claassen, D. C. Heath and Company,13oston. Massachusetts 02116, 1966, 117 pp., $1,95, This bookreviews the ph.sical compoutds of the noble gases that relatemost c!osely to chemistry, and then goes on tc discuss tablegas compounds in detail. There is a wide variation in the levelsof the chapters but, as each Is complete in itself. the book con-tains something for everyone,

Noble Gases, Isaac Asirnov, Basic 13ooks, Inc.. New York 10016,1966, $4.50. This well- written and interesting account of thenoble gases is for persons who have no technical background.

45

Articles

Graduate Level

The Chemistry of Xenon, J. G. Malmo, H. Selig, J. Jortner, andS. A. Rice, Chemical Revieus,65: 199-236 (1965). Deals withthe preparation and properties at xenon compounds. Over halfthe article covers theoretical interpretations of the bonding andphysical properties.

The Nature of the Bonding in Xenon Fluorides and Related Mole-cules, C. A. Coulson, Journal of the American Chemical Society,86: 1442-145i (1964). An excellent article on this aspect of noblegas chemistry.

Undergraduate levelThe Chemistry of the Noble Gases, H. Selig, J. G. Maim, and II. H.

Claassen, Scientific American, 210: 66-77 (May 1964). Review ofwork leading up to preparation of first xenon compounds, somechemistry, and some simple explanations of bonding.

Noble Gas Compounds, Neil Bartlett, International Science andTechnology, 33: 55-66 (September 1964). Review of discovery ofnoble gases and their relation to other elements. Some chem-istry of their compounds is reviewed and speculations are madeconcerning other possible compounds.

General Level

The Noble Gas Compounds, C. L. Chernick, Chemistry, 37: 6-12(January 1964). Brief review of preparation, properties, andbonding in compounds of xenon. Some reference to krypton andradon.

Argonne's Contributions to Xenon Chemistry, Argonne Rerieus,1: 17-19 (October 1964). Although this is somewhat closer to theundergraduate category it is Included here because it containswarnings of the hazards in attempting to work with fluorine andxenon fluorides.

Solid Noble Gases, Gerald 1. Pollack, Scientific American, 215: 64(October 1966).

Motion Pictures

Available for loan without charge from the AEC Headquarters FilmLibrary, Division of Public Information, U. S. Atomic EnergyCommission, Washington, D. C. 20545 and from other AEC filmlibraries.A Chemical Somersault, 29 minutes, black and white, sound, 1964.

Produced by Ross-McElroy Productions for National Educa-tional Television, under a grant from the U. S. Atomic EnergyCommission. This film is suitable for audiences with a minimumscientific The fact that the noble gases were th nightto be chemically inert is detailed and is followed by a descrip-tion of the exp.lrrnents leading to the preparation of the first

.46

noble gas compounds. Subsequent discoveries of other com-pounds and their properties are also included.

Xenon Tetrafluciule, 6 minutes, color, sound, 1962. Produced byArgonne National Laboratory for the U. S. Atomic Energy Com-mission. Semitechnical description of the preparation of xenontetrafluoride. The apparatus and techniques are well presented.

The following film may be rented or purchased from any ModernLearning Aids Film Library or through the headquarters office,1212 Avenue of the Americas, New York 10036.

A Research Problem: Inert (0 Gas Compounds, Film No. 4160, 19minutes, color, sound, 1963. Produced by the CHEM-Study Com-mittee. Shows the preparation of XeF4, its reaction with waterand the detonation of a crystal of Xe03. The preparation of KrF2by photolysis of fluorine in solid krypton at the temperature ofliquid hydrogen is also shown.

CREDITS

Corer courtery Argeaso Hinkel! Laboratory CAM.)Author's ;bob ANL )treethplera cautery Netleial Setae el Steed's*

Pale

Nobel laeuteteAir ?extorts ea Clam wail, 1st Allestows, Preerylvitlit WC/

II AKft AK

12 Wary tetra INeale Dtatiftry bbe Strateats, !ousel et et re** Shoe"21 Ara.

AlitSO ANt . q_fr 4.0Se Oak Mir /hyoid talgoitoey NW, Deocidairei NeSS 12 ANL tn 114=r . - 1"

47

THE COVER

Crystals of xenon let rafluo ride createdin the experiment that first combinedone of the Noble Gases with a singleother element. Formation of this newcompound caused great scientific ex-citement. The colorless crystals areenlarged about 100 times in this photo-graph, which was so striking, estheti-cally as well as scientifically, thatArgonne National Laboratory officialshad it reproduced on the laboratory'sChristmas card in 1962.

THE AUTHOR

CEDRIC L. CHERNICK was born in Manchester, England, andreceived his 13.S.. M.S., and Ph. D. degrees in chemistry fromManchester University. lie spent 2 years as a Research Associateat Indiana University. In 1959 he joined the Argonne National Lab-oratory staff, working as an associate scientist with the Miorinechemistry group, as assistant to the director of the ChemistryDivision and most recently on the Laboratory Director's staff. Hehas authored or coauthored a number of scientific papers in pro-fessional journals as well as several encyclopedia articles andchapters in books. In the photograph the author (third from left)discusses the noble gases with (left to right) Howard H. Claassen,John G. Maim, and Henry 11, Selig. (See page 20.)

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