Nuclide: an atom containing a specified number of protons and neutrons in its nucleus—in other words, any particular atom under discussion. Unstable nuclide: one that will spontaneously disintegrate or emit radiation, thus giving off energy and altering to some new form (often another element). The new form may also be unstable; often it will be stable, that is, with no tendency to disintegrate. Unstable nuclides are also referred to as radioactive. Radioactivity: the spontaneous emission of radiation by elements with unstable nuclei. Radionuclide: a radioactive (that is, unstable) nuclide. Radioisotope: another more commonly seen term for radionuclide. Radioactive decay: the process whereby a radionuclide is converted to another form (usually another element) by emitting radiation. Parent nuclide: a nuclide undergoing radioactive decay. Daughter nuclide: the nuclide produced when a parent nuclide decays. Activity: the rate at which a sample of the material decays, usually expressed as the number of disintegrations per unit time. Naturally radioactive elements decay spontaneously by emitting alpha particles, beta particles, and gamma radiation. Other elements can be induced to decay by bombarding them with high-energy particles; this is known as artificial radioactivity. Like chemical reactions, equations representing nuclear reactions must be balanced. However, the method for balancing nuclear equations differs from that used for chemical equations. To balance a nuclear equation, the sum of the atomic numbers or particle charges (subscripts) and the sum of the mass numbers (superscripts) on both sides of the equation must be equal. When a nucleus undergoes alpha decay, it emits a particle that is identical to a helium nucleus, with an atomic number of 2 and a mass number of 4. Since the emission of an alpha particle from the nucleus results in a loss of 2 protons and 2 neutrons, when writing a nuclear reaction involving an alpha decay, subtract 4 from the mass number and 2 from the atomic number.