In physics, a charge particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its properties are quite different from solids, liquids and gases (plasma is the most common state of matter in the universe). Particles either have a positive, negative or no charge (being neutral).