particle theory

Concept that all matter is made up of particles. Elements are made up of particles called atoms. Atoms are built up from three types of particle: protons, neutrons, and electrons. Other types of particles in matter are molecules and ions. A molecule is a small group of atoms that are held together by covalent bonds. An ion is a charged particle. It is formed by an atom either gaining or losing electrons. The properties of solids, liquids, and gases can be explained by the kinetic theory of matter, the theory that all matter is made up of very small particles which are in constant motion.