Isotopes are atoms with the same number of protons and electrons, but with different numbers of neutrons. The most important thing to keep in mind is that the number of protons determines the element we’re dealing with. In most areas of chemistry, this is enough to determine the reactivity of the atom, though in nuclear chemistry the number of neutrons is also quite important in determining the nuclear processes an isotope can undergo. (Please refer back to the “Atoms and Molecules” chapter for additional information on isotopes.)