The building blocks of atomic and nuclear structure are the electron, proton, and neutron. Atom is electrically neutral if total electron charge equals total proton charge. Electrons are bound to the positively charged nucleus by electrostatic attraction. With the exception of 209 Bi, all nuclei with atomic numbers greater than 82 are unstable. Many nuclei with atomic numbers less than 82 also are unstable.
Unstable nuclei undergo transformations which release energy.
Each transformation of a parent nucleus is called a disintegration. The disintegration rate is proportional to the number of radioactive atoms and the half-life. The half-life is the time required for half of the radioactive atoms to disintegrate. Each radionuclide is unique in terms of the type and energy of the ionizing radiation it emits, and in the duration of its half-life. Below, the classes of decay events that we most commonly encounter are identified.