The identity of an element depends on the number of protons in the nucleus of an atom. Atoms of a given element that differ in the number if neutrons, and consequently in mass, are called isotopes. The number of protons in a nucleus is called the atomic number and is denoted by the symbol Z. The number of neutrons is denoted by the symbol N. The mass number, A, is given by A = N + Z.

Using this atomic notation allows scientists to refer to correct isotopes.