Democritius established his atom theory.

John added onto Democritus' theory

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

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

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.

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.

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.

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