Early history

Our understanding of the atom has evolved through the centuries thanks to contributions made by many different people. Here are some of the most important contributors:

Democritus proposed that matter could be divided until an indivisible particle was reached: the atom.
John Dalton
Dalton devised with an atomic theory with six postulates: matter is composed of indestructible, indivisible atoms that are the same for one element but different from other elements.
J.J. Thompson
Thompson discovered subatomic particles—the atom is a positively charged sphere with negatively charged particles embedded within it. This is called the raisin-bun model.
Sir Ernest Rutherford
Rutherford concluded that the atom is a nucleus consisting of positively charged protons, surrounded mostly by empty space, some of which is occupied by negatively charged electrons (solar-system model).
Sir James Chadwick
Chadwick suggested the existence of neutrons within the nucleus.
Niels Bohr
Bohr experimented with hydrogen gas and its emission and absorption spectra. He concluded that the atom has specific allowable energy levels, called stationery states. Electrons only emit energy when they transfer from one energy level to another. He used Max Planck’s quantum theory to support his own.