He first proposed the theory of the "atom." He thought atoms were small, hard particles; are uncuttable; and that atoms constantly move.
He improved Democritus' first theory. He discovered that atoms of different elements are different from others, and that atoms of the same element are the same size. He also discovered that atoms combine in specific proportions and that all substances are made of atoms. To find this out, he worked with gases.
He conducted an experiment with a cathode-ray.Through this, Thomson discovered that there are small, negatively charged particles inside an atom. With all this, he proposed the Plum-Pudding Model.
He did an expirement that involved shooting a beam of particles at a thin sheet of gold. Through this he discovered that there is a small, dense, positively charged nucleus; most of an atom's mass in the atom's nucleus; atoms contain mostly empty space; electrons move in empty space in the atom.
He brought up the idea of Electron shells, which was later proved wrong. He thought that electrons jumped from level to level, and they travel in definite paths.
They determined that electrons paths cannot actually be predicted, and electrons are found in electron clouds, not in definite paths.