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.
The electron is the lightest known particle having a non-zero rest mass. The positron, the electron's antiparticle , was discovered in 1932. The Concise Columbia Encyclopedia is licensed from Columbia University Press. Copyright 1995 by Columbia University Press. All rights reserved.