Elements have a set number of protons that determines their atomic number (Z). For example, all atoms with eight protons are oxygen; however, the number of neutrons can vary for atoms of the same element. The sum of the number of protons and the number of neutrons is the mass number (A). Atoms with the same atomic number but different mass numbers are called isotopes. Elements can have multiple isotopes, for example, carbon-12, carbon-13, and carbon-14.

An element's atomic mass, or weight, is a weighted average of the masses of the element's isotopes. The weighted average reflects the relative abundance of the different isotopes in the sample. In other words, the masses of the most common isotopes contribute most strongly to the average.

Radioactive decay can alter the number of protons in an element, changing its identity. Many elements have stable isotopes, but most have at least one radioactive isotope, known as a radioisotope. Elements with atomic numbers of 84 or higher are all unstable and decay into elements with lower atomic numbers.

Radiation can help determine a material's age and can be used to diagnose and track medical conditions and treat cancer.