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. An isotope is a version of an atom that has the same number of protons but different numbers of neutrons. Isotopes are very important in nuclear chemistry. A proton is a is a particle in the nucleus of an atom that has a positive charge. A neutron is also a particle in the nucleus of an atom, but it does not have an electric charge.