Democritus a Greek philosopher, developed and systematized classical atomism, a theory credited to his teacher Leucippus. The theory postulated a world made up of hard, indivisible (hence atomic, from Greek atoms, "uncuttable") particles of matter moving through empty space.

Experiments with gases that first became possible at the turn of the nineteenth century led John Dalton in 1803 to propose a modern theory of the atom based on the following assumptions. 1. The matter is made up of atoms that are indivisible and indestructible. 2.

In 1897, J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged. ... Thomson realized that the accepted model of an atom did not account for negatively or positively charged particles.

In 1911, he was the first to discover that atoms have a small charged nucleus surrounded by largely empty space, and are circled by tiny electrons, which became known as the Rutherford model (or planetary model) of the atom.

Niels Bohr proposed the Bohr Model of the Atom in 1915. ... The Bohr Model is a planetary model in which the negatively charged electrons orbit a small, positively charged nucleus similar to the planets orbiting the sun (except that the orbits are not planar)

Until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. In 1932, James Chadwick bombarded beryllium atoms with alpha particles. Unknown radiation was produced.