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.
