Atomic Structure Timeline

~ updated  Sept 2017 ~

Welcome to the atomic structure timeline.   This site explores discoveries related to atomic structure including the electron, proton and neutron.   The dates used for events are open to debate since many scientist's spent decades studying  a topic. Check the links for more in depth material. Most of all  enjoy.

Created by Lee Buescher,
Retired from the ScienceDept, Watertown High School
Watertown, Wisconsin 53098 USA

Visit these sites for original papers in chemistry.
1. Selected Classic Papers from the History of Chemistry by Carmen Giunta at Le Moyne College.
2. John Parks Chem Team site on Classic Papers from the History of Chemistry

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 
 + Leucippus
"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").

Lothar Meyer
Classified elements into six families by their valence.

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 ".

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. Hear Thompson talk about the size of an atom. Examine Thompson's 3 experiments to verify atoms had parts.

J.J. Thomson

Studied "canal rays" and found they were associated with the proton H + .
1898 Rutherford
Studied radiations emitted from uranium and named them alpha and  beta.He estimated atom size  and concluded mass was concentrated in the nucleus.

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. Marie was the first person awarded two Nobel prizes. Video of  Marie Curie in her lab in 1923.
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 (Geiger Müller tube) 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.

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