historical development of aioms

7/30/2019 Historical Development Of aioms

1/5

Historical Development of the Atomic Model

The currently accepted model for the atom (called the Quantum Mechanical Model) has undergone a longevolution that began in 400 B.C. with the Greek philosopher Democritus who first coined the name "atom" for the

proposed smallest particle of matter. Early theoriesof the structure of matter were not based upon experiments.Through a period of many centuries the study of atoms was forgotten until the time of John Dalton. As scientists

began to study the relationship between several physical phenomenon such as electricity, and magnetism they

began to develop different models about atomic structure. The progress from his time to the present is documentedbelow.

Democritus (400 B.C.) - "by convention bitter, by convention sweet, but in reality atoms and void"

Isaac Newton (1704) Newton proposed a mechanical universe with small solid masses in motion.

John Dalton (1808) - Dalton's Atomic Theory began modern atomic theory. He proposed an "atomic theory" withspherical solid atoms based upon measurable properties of mass. The strength of Dalton's model was that it couldexplain known chemical laws: the Laws of Definite Proportions, Multiple Proportions, and Conservation of Mass.Dalton's model was known as the billiard ball model.

Michael Faraday (1832) Faraday studied the effect of electricity on solutions, coined term "electrolysis" as asplitting of molecules with electricity, developed laws of electrolysis. Faraday himself was not a proponent ofatomism.

Plucker (1859) - Plucker built one of the first gas discharge tubes ("cathode ray tube") and studied the conductionof electricity by gases using the cathode ray tubes. He found that the walls of the glass tubes would fluoresce at low

pressures and also that the luminescence was affected by a magnet.

Dmitri Mendeleev (1869) Mendeleev arranged elements into 7 groups with similar properties. He discoveredthat the properties of elements "were periodic functions of the their atomic weights". This became known as thePeriodic Law.

James Clerk Maxwell (1873) Maxwell proposed electric and magnetic fields filled the void.

G.J. Stoney (1879) Stoney proposed that electricity was made of discrete negative particles he called electrons".

Sir William Crookes (1879) Crookes discovered cathode rays had the following properties: travel in straight linesfrom the cathode; cause glass to fluoresce; impart a negative charge to objects they strike; are deflected by electricfields and magnets to suggest a negative charge; cause pinwheels in their path to spin indicating they have mass.

Goldstein (1886) - Goldstein used a cathode ray tube to study "canal rays" which had electrical and magneticproperties opposite of an electron (positive particles were also formed in discharge tubes).Roentgen (1895) - Roentgen observed that nearby chemicals glowed when he was using a cathode ray tube. Furtherexperiments found very penetrating rays coming from the cathode ray tube that were not deflected by a magnetic
http://www.perseus.tufts.edu/GreekScience/index.htmlhttp://www.perseus.tufts.edu/GreekScience/index.htmlhttp://www.perseus.tufts.edu/GreekScience/index.html


2/5

field. He named them "X-rays". X-rays are produced when high speed electrons from the cathode material strikethe anode and are abruptly stopped.

Becquerel (1896) - Becquerel discovered radioactivity when a uranium compound he had thrown into a drawerdarkened a photographic film stored in the drawer. He discovered some chemicals spontaneously decompose andgive off very pentrating rays. This discovery supported the idea of a divisible atom.

J.J. Thomson (1897-1904) - Thomson determined the charge to mass ratio (e/m) of an electron equaled1.759 x 108 C/g through experiments he did with gas discharge tubes. He suggested that cathode rays consist ofnegatively charged particles.

To this point scientists knew that a wide variety of cathode materials in ray tubes emitted electrons and thatelectrons were smaller in mass than hydrogen atoms.

Thomson proposed his "Raisin Bun or Plum Pudding Model" of the atom. He thought the atom was made of auniform sphere or "dough" of positive charge in which were embedded negative electrons or "raisins". Enoughelectrons in the dough would make the atom neutral.

Thomson also studied "canal rays" and found they were associated with the proton H+.

"Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificantfraction of the mass of an atom of hydrogen? --which itself is so small that a crowd of these atoms equal innumber to the population of the whole world would be too small to have been detected by any means then knownto science." J.J. Thomson 1934

Ernest Rutherford (1898) Rutherford studied radiations emitted from uranium and thorium and named themalpha and beta.

Marie Sklodowska Curie (1898) Curie studied uranium and thorium and called their spontaneous decay process"radioactivity". She and her husband Pierre also discovered the radioactive elements polonium and radium.

Soddy (1900) Soddy observed spontaneous disintegration of radioactive elements into variants he called"isotopes" or totally new elements, discovered "half-life", made initial calculations on energy released duringdecay.

Max Planck (1901) - Planck was especially interested in the nature of radiation from hot materials. In 1901 hedevised a theory that perfectly described the experimental evidence, but part of it was a radical new idea: energydid not flow in a steady continuum, but was delivered in discrete packets Planck later called quanta. Thatexplained why, for example, a hot iron poker glows distinctly red and white.Planck studied the relationship between the energy of light and the frequency of light. The relationship that was

eventually stated in the formula, E= h, where E is the energy of a quantum of light, h is Planck's constant, and

is the frequency of light in the quantum. According to his theory each colour of light consists of a stream ofquanta each with a specific frequency and a characteristic energy.

Rutherford and Soddy (1902) With Frederick Soddy at McGill University, Rutherford showed that elements suchas uranium and thorium became different elements (i.e. transmuted) through the process of radioactive decay. Atthe time, such an incredible idea was not to be mentioned in polite company: it belonged to the realm of alchemy,not science. For this work, Rutherford won the 1908 Nobel Prize in chemistry. Particles named and characterized


3/5

by him include the alpha particle, beta particle and proton.

Nagaoka (1903) - Nagaoka postulated a "Saturnian" model of the atom with flat rings of electrons revolvingaround a positively charged particle.

Abegg (1904) - Abegg discovered that inert gases had a stable electron configuration that lead to their chemicalinactivity

Albert Einstein (1905) - Although Planck said that energy of matter is quantized, he continued to describe energyas travelling in the form of waves. Planck was unwilling to consider that energy might have particle-like

properties. Einstein was prepared to make such a statement. Light, according to Einstein, is quantized. It occursas quanta ofelectromagnetic energy that have particle-like properties. these "particle-like" packets were later called photons.In Einstein's view light and therefore all electromagnetic energy travels in the form of photons of energy. Light isemitted as photons of energy and absorbed as photons of energy. Einstein published the famous equation E= mc2.

Hans Geiger (1906) - Geiger developed an electrical device to "click" when hit with alpha particles.

Robert Millikan (1909) - Millikan conducted his classic oil drop experiment to determine the charge(e=1.602 x 10-19 C) and the mass (m = 9.11 x 10 -28 g) of an electron.

Ernest Rutherford (1909) - In 1909, now at the University of Manchester, Rutherford was bombarding a thin goldfoil with alpha particles when he noticed that although almost all of them went through the gold, one in eightthousand would "bounce" (i.e. scatter) back. The amazed Rutherford commented that it was "as if you fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you." From this simpleobservation, Rutherford concluded that the atom's mass must be concentrated in a small positively-charged nucleus

while the electrons inhabit the farthest reaches of the atom. He thought that the electrons surrounded the nucleusto make the atom neutral, but most of the volume of the atom was empty space. His model was called the NuclearAtom model or Planetary Model because the electrons resemble planets in motion around a central body. Therewere criticisms of Rutherford's model; an atomic nucleus composed entirely of positive charges should fly apart.Furthermore, Rutherford's nuclear atom did not explain the total mass of the atom. An even more significant

problem was that Rutherford's model contradicted the laws of nineteenth century physics. According to the physicsof the time, an electron n motion around a central body must continuously give off radiation. Therefore, ifRutherford's model was correct, a continuous spectrum of light should be observed as the electron gives offradiation.

Niels Bohr (1913) - Bohr proposed a model for the hydrogen atom that accounted for the existence of the lines inthe spectrum of hydrogen gas. He overcame the shortcomings of Rutherford's model by proposing that the energyof the electrons is quantized. He thought that the electrons orbit the nucleus like the planets around the sun, butthat they could travel only in specific allowed orbits. To explain the energy jumps that an electron could take inmoving from one allowed orbit to another Bohr devised an equation relating the energy positions of the single


4/5

electron in hydrogen to the wavelength of its spectral lines. Bohr's realization that the atom's energy is quantizedand that electrons are restricted to specific energy levels was an astounding achievement. The model successfully

predicted lines in the emission spectrum of hydrogen in the visible, ultraviolet and infrared regions. The oneproblem with Bohr's model was that it only explained one-electron systems. It could not explain the emissionpatterns produced by atoms or ions of two or more electrons.

H.G.J. Moseley (1914) - Moseley used x-ray tubes and determined the charges on the nuclei of most atoms. Hewrote, "The atomic number of an element is equal to the number of protons in the nucleus". This work was used to

reorganize the periodic table based upon atomic number instead of atomic mass.

Aston (1919) Ashton discovered the existence of isotopes through the use of a mass spectrograph.

Neils Bohr (1922) - Bohrs model explained the atomic structure that underlies regularities of the periodic table ofelements. His atomic model had atoms built up of successive orbital shells of electrons.

Louis de Broglie (1924) - By the early 1920's it was accepted that energy had matter-like properties. de Broglie

suggested that matter had wave-like properties. Experimental evidence to support this was put forth in 1927 when

researchers suggested that electrons had wave-like properties based on the diffraction patterns observed from

streams of moving electrons. This was the beginning of the wave-particle duality of matter theory. These ideas

supported Einstein.

Erwin Schroedinger (1926) - Schroedinger used mathematics and statistics to combine de Broglie's idea of matter

waves and Einstein's idea of quantized energy particles (photons). Schroedinger's mathematical equations andtheir interpretations, together with Heisenberg's uncertainty principle, resulted in the birth of the field of quantummechanics. Schroedinger used concepts from quantum mechanics to propose a new atomic model called thequantum mechanical model of the atom. This model describes atoms as having certain allowed quantities ofenergy because of wave-like properties of their electrons. He stated that an electron that is attracted to the nucleuscan have only certain three dimensional wave patterns. He devised a wave equation to describe possible wave

patterns of electrons. This wave equation contains variables (integers) called quantum numbers to describe thevarious electron orbitals. He described an orbital as a region of space where there is a high probability that anelectron will be found.

Werner Heisenberg (1927) - The current model of the atom describes the volume of space surrounding the nucleus

of the atom as being indistinct or cloud-like because of a scientific principle called the uncertainty principle. Using

mathematics, Heisenberg showed that it is impossible to know both the position and the momentum of an electron

at the same time. In 1927 he was puzzling over the basic quantum properties of electrons. He realized that the act

of measuring an electron's properties by hitting it with gamma rays would alter the electron's behavior. Indeed, you


5/5

could measure the position of an electron (or other particle) OR you could measure its momentum. But the more

precisely you measure one property, the more you throw the other off. He tied this up in an equation using Planck's

constant, and called it the uncertainty principle. While many resisted this idea, it eventually became accepted as a

fundamental law of nature. According to this principle, if you can know an electrons precise position and path

around the nucleus, as you would by defining its orbit, you cannot know with certainty its velocity. Similarly, if

you know its precise velocity, you cannot know with certainty its position. Based on the uncertainty principle,

Bohr's atomic model is flawed because you cannot assign fixed paths (orbits) to the motion of the electrons.

Cockcroft / Walton (1929) Cockcroft and Walton built an early linear accelerator and bombarded lithium withprotons to produce alpha particles.

Paul Dirac (1930) - Dirac proposed anti-particles. Anderson discovered the anti-electron (positron) in 1932 andSegre/Chamberlain detected the anti-proton in 1955.James Chadwick (1932) - Chadwick used alpha particles discovered a neutral atomic particle with a mass close to a

proton. Thus was discovered the neutron.

Lise Meitner, Hahn, Strassman (1938) - They conducted experiments verifying that heavy elements captureneutrons and form unstable products that undergo fission. This process ejects more neutrons continuing the fission

chain reaction.

Glenn Seaborg (1941-51) - Seaborg synthesized 6 transuranium elements and suggested a change in the layout ofthe periodic table. He suggested the lanthanide and actinide series should be removed form the main body of the

periodic table

Enrico Fermi (1942) - Fermi conducted the first controlled chain reaction releasing energy from the atoms nucleus.

Murray Gell-Mann (1950s) - During the 1950s, discoveries of new subatomic particles were proliferating soquickly such that scientists spoke of a "particle zoo." Gell-Mann turned his attention to some particles that behaved

particularly strangely. He proposed a new quantum property of particles he called the "strangeness number." Whilestudying particles, he found even more general characteristics that allowed him to sort them into eight "families."He called this grouping the eightfold way, referring to Buddhist philosophy's eight attributes of right living. Then

he found that the eightfold way could really best be explained by a particle, undiscovered as yet, that had three

parts (hadrons), each holding a fraction of a charge. He called them "quarks" with a nod to James Joyce, whosenovel Finnegan's Wake contains the passage: "Three quarks for Muster Mark!" Fractional charge seemed anoutrageous suggestion at first, but proof came for his theoretical quarks in 1974.

historical development of aioms

Documents