Atomic Structure Timeline

Atomic Structure Timeline

Early theories of the structure of matter were not based upon experiments. 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.

Year Scientist(s)  Discovery

Greek
era

Democritus

“by convention bitter, by convention sweet, but in reality atoms and void”

1704

Isaac Newton

Proposed a mechanical universe with small solid masses in motion.

1803

John Dalton

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

1832

Michael Faraday

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.

1859

J. Plucker

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

1869

Dmitri Mendeleev

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.

1873

James Clerk Maxwell

Proposed electric and magnetic fields filled the void.

1879

Sir William Crookes

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.

1886

E. Goldstein

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

1894

G.J. Stoney

Proposed that electricity was made of discrete negative particles he called electrons “. (Link to info on electrons)

1895

Wilhelm Roentgen

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”.

1896

Henri Becquerel

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

1897

J.J. Thomson

Used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 10 8 coulombs/gram.

1897

J.J. Thomson

Studied “canal rays” and found they were associated with the proton H + .

1898

Rutherford

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

1898

Marie Sklodowska 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.

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 during decay.

1900

Max Planck

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

1903

Nagaoka

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

1904

Abegg

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

1905

Albert Einstein

Published the famous equation E=mc 2

1906

Hans Geiger

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

1909

R.A. Millikan

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.

1911

Ernest Rutherford

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.

1914

H.G.J. Moseley

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.

1919

Aston

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

1922

Niels Bohr

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.

1923

de Broglie

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

1927

Heisenberg

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.

1929

Cockcroft / Walton

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

1930

Schrodinger

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

1930

Paul Dirac

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

1932

James Chadwick

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

1938

Lise Meitner, 
Hahn , Strassman

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.

1941 – 51

Glenn Seaborg

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

1942

Enrico Fermi

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

1950’s –

New findings/particles

Follow this link to current theories about atomic stucture.

References
Asimov,I. 1965,A short history of chemistry,  Anchor, NY
CHEMS , 1966, Chemistry-An experimental science, Freeman, San Fran
Dampier,W. 1971, A History of Science, Cambridge,England
Jaffe, B. 1976, Crucibles: The story of chemistry, Dover, NY
Pais,A. 1991, Niels Bohr’s Times, Oxford

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