Democritus was an Ancient Greek philosopher remembered today for his formulation of an atomic theory of the universe. He was perhaps the first person to come up with the idea of the atom.
Drew upon the Ancient Greek idea of an 'atom'. His theory stated that atoms were indivisible, those of a given element were identical, and compounds were combinations of other elements.
Discovered electrons. Won a Nobel Prize. Was the maker of the 'plum pudding' model, which shows an atom composed of electrons scattered throughout a spherical cloud of positive charge.
Disproved the plum pudding model. He fired alpha particles at gold foil and showed that most of an atom is empty space, the nucleus has a positive charge and is 'massive'- it contains most of the atom's mass.
He suggested that electrons orbit the nucleus.
Stated that electrons did not move in a fixed orbit around the nucleus, but that they moved in waves. he said it was impossible to know the exact positions of the electrons.