An electron is an elementary particle carrying a unit charge of negative electricity. An atom consists of a small, dense, positively charged nucleus surrounded by electrons that whirl about it in orbits, forming a cloud of charge.

Ordinarily there are just enough negative electrons to balance the positive charge of the nucleus, and the atom is neutral. If electrons are added or removed, a net charge results, and the atom is said to be ionized atomic electrons are responsible for the chemical properties of matter (Valence).

The electron was discovered in 1897 by Joseph John Thomson, who showed that cathode rays are composed of electrons. The electron is the lightest known particle having a non-zero rest mass. The positron, the electron's antiparticle was discovered in 1932.