An atom is unstable (radioactive) if the forces among the particles that make up the nucleus are unbalanced, that is to say, if the nucleus has an excess of internal energy. See also: stable nucleus
U.S. Environmental Protection Agency, Radiation Glossary, Office of Radiation and Indoor Air Radiation Protection Division, Washington, DC, USA 2013. http://www.epa.gov/radiation/glossary/index.html#a , (Dzstr/ Rad Emr - Amer) {823}