Isobars are atoms (nuclides) of different chemical elements that have the same number of nucleons. Correspondingly, isobars differ in atomic number (or number of protons) but have the same mass number. An example of a series of isobars is S, Cl, Ar, K, and Ca. While the nuclei of these nuclides all contain 40 nucleons, they contain varying numbers of protons and neutrons.