Timeline of Atomic Theory

(Democritus' Atomic Theory) Democritus theorized that all matter is composed of individual elements (Atomos)

(John Dalton's Atomic Theory) John Dalton developed a theory that matter is composed of atoms of different weights and that they are shaped like spheres

(J.J Thomson's Atomic Theory) His research in cathode rays led to the discovery of the electron, and he pursued further innovations in atomic structure exploration.

(Ernest Rutherford's Atomic Theory) Ernest used gold foil and alpha particles emitted from a radioactive element to discover his theory that the atom has a tiny and heavy nucleus.

(Niels Bohr's Atomic Theory) He introduced the idea that an electron could drop from a higher energy orbit to a lower one. He also used the planetary orbit theory that electrons orbit around the nucleus

(James Chadwick's Atomic Theory) James Chadwick discovered the neutron. Neutrons are located in the nucleus. They have neither a positive or negative charge but they contribute to the atomic weight.