ATOMIC THEORY TIMELINE

Democritus

480 BCE

He made the formula of the atomic theory of the universe. He also said all matter is made up small indestructible units he called atoms.

John Dalton

Aug 6, 1766

John Dalton developed the concept of the mole and proposed a system of symbols to represent atoms of different elements. He also developed a relative atomic weight scale from percent composition data and assumed atomic ratios.

J.J. Thompson

Dec 18, 1856

J.J. Thompson came up with the theory that the electron was a component of all matter and calculated the charge to mass ratio for the electron.

Ernest Rutherford

Aug 30, 1871

Rutherford stated that all of the positive charge and all of the mass of the atom occupied a small volume at the center of the atom and that most of the volume of the atom was empty space occupied by the electrons.

Niels Bohr

Oct 7, 1885

In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities.

Erwin Schrödinger.

Aug 12, 1887

Erwin Schrödinger showed that the quantization of the hydrogen atom's energy levels that appeared in Niels Bohr's atomic model could be calculated from the Schrödinger equation, which describes how the wave function of a quantum mechanical system (in this case, a hydrogen atom's electron) evolves.

James Chadwick

Oct 20, 1891

James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron.

Werner Heisenberg

Dec 5, 1901

Werner Heisenberg contributed to atomic theory through formulating quantum mechanics in terms of matrices and in discovering the uncertainty principle, which states that a particle's position and momentum cannot both be known exactly.