What Does the Number Next to Isotopes Signify?

The number next to isotopes signifies the sum of the number of protons plus the number of neutrons in the nucleus of an atom. For a particular element, the number of protons never changes, but the number of neutrons can vary.

All atoms of the element chlorine, have 17 protons in their nucleus. The isotope chlorine-28 has 11 neutrons in its nucleus, while chlorine-37 has 20 neutrons. The number of protons and neutrons largely determines the atomic mass of an atom, so different isotopes of an element differ in their mass. Uranium-238, which has 92 protons and 146 neutrons, has a higher atomic mass than does uranium-235, which has only 143 neutrons.