Mileposts on the road to Mileposts on the road to the atomthe atom

All matter is made from elementsAll matter is made from elementsWhat makes elements unique?What makes elements unique?

A component unique to each elementA component unique to each elementA common component but assembled in a different wayA common component but assembled in a different way

Learning ObjectivesLearning Objectives

Describe what is meant by “atom”Describe what is meant by “atom” Describe Law of Conservation of Mass and Describe Law of Conservation of Mass and

Definite ProportionsDefinite Proportions Identify main features of Dalton’s atomic Identify main features of Dalton’s atomic

theorytheory Describe Avogadro’s HypothesisDescribe Avogadro’s Hypothesis Identify main features of the periodic tableIdentify main features of the periodic table

Glimpse into the interiorGlimpse into the interior

What we “see” now What we “see” now with scanning tunneling with scanning tunneling microscope could only microscope could only be speculated on 2,000 be speculated on 2,000 years agoyears ago

400 BC400 BCProposal that matter is particulate Proposal that matter is particulate

Democritus posed the question: could matter be Democritus posed the question: could matter be subdivided forever? He answered no: there is a limit subdivided forever? He answered no: there is a limit to the extent to which matter can be subdivided, and to the extent to which matter can be subdivided, and he coined the term atom from the Greek for he coined the term atom from the Greek for uncuttable uncuttable a-tomos.a-tomos.

Democritus’ legacyDemocritus’ legacy

No indication about the size of these No indication about the size of these atomsatoms. In . In fact Democritus atoms could be extremely fact Democritus atoms could be extremely largelarge

Democritus’ idea has endured with some Democritus’ idea has endured with some modifications:modifications: Atoms are not uncuttableAtoms are not uncuttable Atoms are all very smallAtoms are all very small Atoms are themselves made of particlesAtoms are themselves made of particles

Science in the dark agesScience in the dark ages The decline of Greek civilization saw a concomitant The decline of Greek civilization saw a concomitant

decline of intellectual activity in Europedecline of intellectual activity in Europe The major “scientific” activity was alchemy, largely The major “scientific” activity was alchemy, largely

the pursuit of the transformation of matter into gold the pursuit of the transformation of matter into gold Uncritical acceptance of Greek thinking about matter Uncritical acceptance of Greek thinking about matter

lingered until the Age of Enlightenmentlingered until the Age of Enlightenment Matter is made of the four elements (earth, wind and Matter is made of the four elements (earth, wind and

fire...and water)fire...and water) The modern scientific era began in the 17The modern scientific era began in the 17 thth century, century,

pioneered by a few inspired individuals who broke pioneered by a few inspired individuals who broke free from long-established conventionsfree from long-established conventions

17801780Re-emergence of the atom:Re-emergence of the atom:

Daniel Bernoulli’s kinetic theory of gasesDaniel Bernoulli’s kinetic theory of gases

Atoms were assumed to be hard round spheres Atoms were assumed to be hard round spheres which behaved much like billiard balls. which behaved much like billiard balls.

Success of kinetic theory in describing gases Success of kinetic theory in describing gases lends support to atomic description of matter. lends support to atomic description of matter.

It is still used, with scarcely any modificationIt is still used, with scarcely any modification

17901790Order out of chaosOrder out of chaosAntoine Lavoisier defines elements: substances that Antoine Lavoisier defines elements: substances that could not be broken down into simpler substances.could not be broken down into simpler substances. Beginnings of periodic table Beginnings of periodic table

and definition of and definition of elementselements Introduced concept of Introduced concept of

compoundscompounds (combination of (combination of elements)elements)

Demonstrated Law of Demonstrated Law of Conservation of MassConservation of Mass

No additional insights into the No additional insights into the atom at this stage.atom at this stage.

Chemical composition laws and Chemical composition laws and arguments for an atomic worldarguments for an atomic world

Law of Conservation of Mass:Law of Conservation of Mass:Matter is neither created nor destroyed in the course of a Matter is neither created nor destroyed in the course of a

chemical reaction. chemical reaction. Does not apply to nuclear changesDoes not apply to nuclear changes

Law of Definite Proportions:Law of Definite Proportions:In forming compounds, elements combine together in In forming compounds, elements combine together in

definite mass ratiosdefinite mass ratios No knowledge of actual atom ratios at this stage, but how No knowledge of actual atom ratios at this stage, but how

else to explain fixed quantities unless the elements were else to explain fixed quantities unless the elements were present as discrete bodiespresent as discrete bodies

Strong indication for combination of the atoms in simple Strong indication for combination of the atoms in simple ratiosratios

1803 - musing on atoms:1803 - musing on atoms:John Dalton’s atomic theory John Dalton’s atomic theory

Sample of any element contains Sample of any element contains tiny particles called atomstiny particles called atoms

Atoms cannot be subdivided, Atoms cannot be subdivided, created or destroyedcreated or destroyed LawLaw ofof ConservationConservation ofof MatterMatter

All atoms of the same element are All atoms of the same element are the samethe same

All atoms of different elements are All atoms of different elements are differentdifferent

Atoms combine together in simple Atoms combine together in simple whole number ratioswhole number ratios Law of Multiple Proportions: The ratio of Law of Multiple Proportions: The ratio of

the masses of one element combined with the masses of one element combined with the same mass of another element is a the same mass of another element is a simple whole numbersimple whole number

Significance of Law of Multiple Significance of Law of Multiple Proportions for atomic worldProportions for atomic world

Illustration using carbon dioxide (COIllustration using carbon dioxide (CO22) and ) and

carbon monoxide (CO)carbon monoxide (CO)

Combination of C and O as atoms in simple Combination of C and O as atoms in simple number ratios is most sensible explanationnumber ratios is most sensible explanation

22 CO

CO

MassO

MassO

18081808Gay-Lussac law of combining Gay-Lussac law of combining volumes: atomic implications volumes: atomic implications

Gases react with other gases to give products, in Gases react with other gases to give products, in volumes which have simple whole number ratios. volumes which have simple whole number ratios.

““Mystery” of the reaction of hydrogen and oxygen to Mystery” of the reaction of hydrogen and oxygen to provide water.provide water.

2 vols of hydrogen + 1 vol of oxygen gives 2 vols of 2 vols of hydrogen + 1 vol of oxygen gives 2 vols of water (Why not 1 vol?)water (Why not 1 vol?)

The ratio of 2:1 was inconsistent with Dalton’s belief The ratio of 2:1 was inconsistent with Dalton’s belief that formula of water was HO (principle of that formula of water was HO (principle of simplicity)simplicity)

Mystery of the gas volumesMystery of the gas volumes

Combining H and O in 1:1 ratio (Dalton’s Combining H and O in 1:1 ratio (Dalton’s proposed ratio) does not satisfy Conservation proposed ratio) does not satisfy Conservation of Matter…of Matter…

H:O = 2:1H:O = 2:1

However, 2:1 ratio (Gay Lussac’s expt) However, 2:1 ratio (Gay Lussac’s expt) predicts only 1 vol of Hpredicts only 1 vol of H22O, not 2 vols as O, not 2 vols as

observed. Hmmm...observed. Hmmm...

Enter Avogadro:Enter Avogadro:What if gases are What if gases are diatomicdiatomic molecules? molecules?

Matter conserved, Matter conserved, andand each volume contains each volume contains same number of particlessame number of particles

18111811Amadeo Avogadro’s hypothesis Amadeo Avogadro’s hypothesis

Solved riddle of gas volumes by positing Solved riddle of gas volumes by positing that the molecules in the gas contained that the molecules in the gas contained twotwo atoms. atoms. Most elemental gases are diatomicMost elemental gases are diatomic

Second part of hypothesis was that the Second part of hypothesis was that the same volumes of all gases contain the same volumes of all gases contain the same number of particles.same number of particles.Results conflicted with Dalton’s views and were Results conflicted with Dalton’s views and were

not recognized for nearly 50 yearsnot recognized for nearly 50 years

Key result: Able to calculate the relative Key result: Able to calculate the relative atomic masses of the elements: atomic masses of the elements: development of a scale of atomic weight.development of a scale of atomic weight.

18151815William Prout: weights of atoms are simple William Prout: weights of atoms are simple

multiples of hydrogen atom.multiples of hydrogen atom. Coincidence or significance?Coincidence or significance?

Proposed that atoms of heavier elements were made Proposed that atoms of heavier elements were made from hydrogen atomsfrom hydrogen atoms

Implication that larger atoms comprise smaller unitsImplication that larger atoms comprise smaller units Partial truth: there are common factors between atoms Partial truth: there are common factors between atoms

of different elementsof different elements But they are not H atoms…But they are not H atoms…

Atomic weight scale – the link Atomic weight scale – the link between mass and number of atomsbetween mass and number of atoms

Experimentally mass could be measuredExperimentally mass could be measured Without knowledge of atomic mass, impossible to Without knowledge of atomic mass, impossible to

know how many atoms of one element combines with know how many atoms of one element combines with anotheranother

Essential to know number of atoms to understand Essential to know number of atoms to understand chemistrychemistry

Atomic weight scale, enabled by Avogadro’s Atomic weight scale, enabled by Avogadro’s hypothesis, provides link between experimental hypothesis, provides link between experimental observable (mass) and numbers of atomsobservable (mass) and numbers of atoms

1869 - 1869 - elements orderedelements orderedDimitri Mendeleev and the periodic table Dimitri Mendeleev and the periodic table The most important document in The most important document in

chemistry chemistry Ordered the elements according to Ordered the elements according to

their atomic weights and their atomic weights and properties. properties.

Only 60 elements identified at the Only 60 elements identified at the timetime

Predicted existence of Predicted existence of undiscovered elements and their undiscovered elements and their properties which were often properties which were often proved startlingly accurate.proved startlingly accurate.

Facts of periodic tableFacts of periodic tableThere are 91 naturally occurring elements: 2 liquids, 11 There are 91 naturally occurring elements: 2 liquids, 11

gases, 23-25 nonmetalsgases, 23-25 nonmetals

The Periodic Table: Groups and PeriodsThe Periodic Table: Groups and Periods

Groups: columns of elements

Periods: rows of elements

Group (family) behaviourGroup (family) behaviour

1A ALKALI METALS: Reactive. Reactivity 1A ALKALI METALS: Reactive. Reactivity increases down group. Physical properties vary in a increases down group. Physical properties vary in a gradual fashion.gradual fashion.

2A ALKALINE EARTH METALS : Similar to 2A ALKALINE EARTH METALS : Similar to alkalis but less reactive. Lots of common calcium alkalis but less reactive. Lots of common calcium compounds: calcium carbonate (limestone, marble), compounds: calcium carbonate (limestone, marble), calcium oxide (lime), calcium sulphate (gypsum, calcium oxide (lime), calcium sulphate (gypsum, plaster of Paris)plaster of Paris)

Important groups on nonmetal sideImportant groups on nonmetal side

7A HALOGENS: Very reactive lots of compounds 7A HALOGENS: Very reactive lots of compounds with most elements. Chlorides in particular are with most elements. Chlorides in particular are common: NaCl. KCl. Reactivity common: NaCl. KCl. Reactivity decreasesdecreases down down groupgroup

8A RARE, INERT, NOBLE GASES: All gases, 8A RARE, INERT, NOBLE GASES: All gases, discovered late because of lack of compounds. Not discovered late because of lack of compounds. Not rare - 1 % argon in air; helium is second most rare - 1 % argon in air; helium is second most abundant element in universe. Very unreactive. abundant element in universe. Very unreactive.

Unanswered questionsUnanswered questions

What features of atoms are responsible for What features of atoms are responsible for differences in element properties?differences in element properties?

How can atoms actually bind together in How can atoms actually bind together in compounds; what are the attractive forces?compounds; what are the attractive forces?

What about those other experimental What about those other experimental observations accumulating…? observations accumulating…?