He believed that the world is an empty space in which atoms moved around. He also believed that changes in matter are only possible because the atoms are being rearranged.
Aristotole did not support the Atomic Theory, that all matter is made up of atoms, but rather proposed that everything was made up of the four elements, Earht, Fire, Water, and Air.
He discovered that air has weight and can cause pressure. He found this out when he created the "Barometer" .
He proposed the Law of Conservation of Matter/Mass, meaning that mass cannot be destroyed, only changed.
Confirmed Democratis' theory that all matter consists of tiny particles. He also theorized that atoms are indestructible and unchangeable, which is untrue. He said that atoms of the same elements are identical, this was later corrected to "Elements are characterized by their atomic number.". He also discovered chemical change in atoms.
He experimented with the "Cathode Ray Tube" through which he discovered the electron, and couldn't find any positive in the atoms (for a while). He described the atom like "Plum Pudding"; a bunch of positive stuff with electrons able to be removed.
He used uranium to produce alpha particles. He theorized that the atom was mostly empty space with a positive center (Nucleus).
He discovered the "Electron Cloud", and the stages of an atom.
He discovered that all matter has wave properties.