Atomic Theory

Democritus

400 BCE

His mentor, Leucippus, originally came up with the atomic theory, but it was then adopted by Democritus. The atomic theory stated that “The universe is composed of two elements: the atoms and the void in which they exist and move.” According to Democritus atoms were miniscule quantities of matter.

Isaac Newton

Jan 25, 1704

Isaac Newton theorized a mechanical universe with small, solid masses in motion.

John Dalton

Jan 25, 1803

Dalton's atomic theory remains valid in modern chemical thought and his ideas were that all matter is made of atoms, atoms are indivisible and indestructible, and compounds are formed by a combination of two or more different kinds of atoms.

Dmitri Mendeleev

Jan 25, 1869

Dmitri Mendeleev published his periodic table of elements and it arranged the known elements according to their chemical properties and in order of their relative atomic mass.

J.J. Thomson

Jan 25, 1897

J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube.

Marie Curie

Jan 25, 1898

Marie Curie was a pioneer in researching radioactivity, winning the Nobel Prize in Physics and in Chemistry.

Hans Geiger

Jan 25, 1906

Hans Geiger was known for the Marsden experiment which discovered the atomic nucleus.

Robert Millikan

Jan 25, 1909

Robert Millikan and his oil drop experiment helped to quantify the charge of an electron, which contributed greatly to our understanding of the structure of the atom and atomic theory.

Ernest Rutherford

Jan 25, 1911

Ernest Rutherford 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. Moseley

Jan 25, 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".

Niels Bohr

Jan 25, 1922

Developed an explanation of atomic structure that underlies regularities of the periodic table of elements.

Werner Heisenberg

Jan 25, 1927

Heisenberg contributed to the atomic theory by including quantum mechanics, the branch of mechanics, based on quantum theory, used for interpreting the behavior of elementary particles and atoms.

James Chadwick

Jan 25, 1932

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

Lise Meitner

Jan 25, 1938

Lise Meitner 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.