An atom is stable because of a balanced nucleus that does not contain excess energy. If the forces between the protons and the neutrons in the nucleus are unbalanced, then the atom is unstable. Stable atoms retain their form indefinitely, while unstable atoms undergo radioactive decay. Most naturally occurring atoms are stable and do not decay.
The nucleus of an atom is made up of protons and neutrons. The forces inside the nucleus of a stable atom are balanced because the nucleus contains the proper number of protons and neutrons. These atoms are theoretically resistant to all forms of decay except proton decay, which is a hypothetical form of decay that has never been observed in a laboratory or in nature.
Unstable atoms are radioactive and decay after a certain amount of time. Different unstable atoms decay through different processes, such as the ejection of a proton or a neutron; the conversion of a proton to a neutron or a neutron to a proton; and the emission of the excess energy in the form of photons. In many cases, the resulting atom is still unstable. This atom then decays again to a new atom. The process continues in a decay chain until a stable form is reached.