He believed all matter is made of atoms that are indivisible and indestructible.  He also believed that atoms of different elements have different masses and properties.

He suggested the electrons orbit the nucleus in energy levels that have a set size and energy.  He also suggested that energy is absorbed or emitted when an electron moves from one energy level to another.

His model is called the "plum pudding" model and he believed that atoms contained randomly distributed negative particles called electrons which are embedded randomly in a positive sphere.

This scientist thought most of the atom was empty space with a positive center that contained most of the mass of the atom.  He also discovered protons and the nucleus.