Most atoms are highly stable, meaning the nucleus will always hold together, barring extreme circumstances. But in some atoms, the energy that binds the nucleus will eventually fail in a process called radioactive decay – the spontaneous disintegration of the nucleus.
The most notorious unstable atoms are elements with a very high number of protons, such as uranium (92 protons). But some lighter elements, eg carbon, can be unstable and radioactive when they have too many or too few neutrons. Neutron-count variations are called isotopes. For example, while garden-variety carbon-12 (six protons and six neutrons) is stable, carbon-14 (six protons and eight neutrons) is radioactive.
Radioactive decay results in the ejection of subatomic particles from the nucleus. In alpha radiation, the atom ejects two protons and two neutrons. In beta…