Atom Theory



465 BC

Democritius established his atom theory.

John Dalton


John added onto Democritus' theory

J.J. Thompson


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

Niels Bohr


Proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities.

Robert Millikan


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.

Erwin Schrödinger


Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom.

Sir 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.

Ernest Rutherford


Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure.