The Evolution of the Atomic Theory

Issac Newton

Oct 19, 1704

Proposed a mechanical universe with small solid masses in motion.

John Dalton

Oct 19, 1803

Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.

Michael Faraday

Oct 19, 1832

Studied the effect of electricity on solutions, coined term "electrolysis" as a splitting of molecules with electricity, developed laws of electrolysis. Faraday himself was not a proponent of atomism.

J. Plucker

Oct 19, 1859

Built one of the first gas discharge tubes ("cathode ray tube").

Dmitri Mendeleev

Oct 19, 1869

Arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law.

James Clerk Maxwell

Oct 19, 1873

Proposed electric and magnetic fields filled the void.

Sir William Clerkes

Oct 19, 1879

Discovered cathode rays had the following properties: travel in straight lines from the cathode; cause glass to fluoresce; impart a negative charge to objects they strike; are deflected by electric fields and magnets to suggest a negative charge; cause pinwheels in their path to spin indicating they have mass.

E. Goldstein

Oct 19, 1886

Used a CRT to study "canal rays" which had electrical and magnetic properties opposite of an electron.

G.J. Stoney

Oct 19, 1894

Proposed that electricity was made of discrete negative particles he called "electrons ".

Wilhelm Roentgen

Oct 19, 1895

Using a CRT he observed that nearby chemicals glowed. Further experiments found very penetrating rays coming from the CRT that were not deflected by a magnetic field. He named them "X-rays".

Henri Becquerel

Oct 19, 1896

While studying the effect of x-rays on photographic film, he discovered some chemicals spontaneously decompose and give off very pentrating rays.

J.J. Thomson

Oct 19, 1897

Used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 10 8 coulombs/gram. Studied "canal rays" and found they were associated with the proton H + .

Rutherford

Oct 19, 1898

Studied radiations emitted from uranium and thorium and named them alpha and beta.

Marie Sklodowska Curie

Oct 19, 1898

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

Oct 19, 1900

Observed spontaneous disintegration of radioactive elements into variants he called "isotopes" or totally new elements, discovered "half-life", made initial calculations on energy released during decay.

Max Planck

Oct 19, 1900

Used the idea of quanta (discrete units of energy) to explain hot glowing matter.

Nagaoka

Oct 19, 1903

Postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.

Abegg

Oct 20, 1904

Discovered that inert gases had a stable electron configuration which lead to their chemical inactivity.

Albert Einstein

Oct 20, 1905

Published the famous equation E=mc 2

Hans Geiger

Oct 20, 1906

Developed an electrical device to "click" when hit with alpha particles.

R.A. Millikan

Oct 20, 1909

Oil drop experiment determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 gram) of an electron.

Ernest Rutherford

Oct 20, 1911

Using alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil . He established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus.

H.G.J. Moseley

Oct 20, 1914

Using x-ray tubes, determined the charges on the nuclei of most atoms. He wrote"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

Oct 20, 1919

Discovered the existence of isotopes through the use of a mass spectrograph.

Niels Bohr

Oct 20, 1922

Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of sucessive orbital shells of electrons.

de Broglie

Oct 20, 1923

Discovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. Supported Einstein.

Heisenberg

Oct 20, 1927

Described atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Indeterminancy - you can not know both the position and velocity of a particle.

Cockcroft/Walton

Oct 20, 1929

Built an early linear accelerator and bombarded lithium with protons to produce alpha particles

Shrodinger

Oct 20, 1930

Viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom.

Paul Dirac

Oct 20, 1930

Proposed anti-particles . Anderson discovered the anti-electron (positron) in 1932 and Segre/Chamberlain detected the anti-proton in 1955..

James Chadwick

Oct 20, 1930

Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron.

Lise Meitner,Hahn , Strassman

Oct 20, 1938

Conducted experiments verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction.

Glen Seaborg

Oct 20, 1941

Synthesized 6 transuranium elements and suggested a change in the layout of the periodic table.

Enrico Fermi

Oct 20, 1942

Conducted the first controlled chain reaction releasing energy from the atoms nucleus.