Democritus used the word atomos, Greek for unbreakable, to describe the smallest parts of matter. His is an early atomic theory, though he lacked evidence.
History of the Atom
Thomson
The cathode ray experiment helped Thomson realize that the atom could be divided into smaller parts, including a negatively charged part called the electron.
Dalton
Five Statements of Atomic Theory, with evidence
He theorized that the positive part of the atom was like pudding with electrons embedded in it.
All elements are made of atoms that differ from one another. Atoms of the same element are identical. Atoms combine in whole number ratios.
Rutherford
With the gold foil experiment, Rutherford discovered the small, dense center of the atom and he named it the nucleus.
Bohr
By studying the spectral lines of hydrogen, Bohr theorized that electrons move at fixed energies, in paths he called orbits. His model described H perfectly.
Electron Cloud Model
The modern idea of the atom suggests the 90% probability of "locating" an electron in mathematically-determined shapes called orbitals.