Isotones are atoms that have the same neutron number but different proton number. For example, 3616S, 3717Cl, 3818Ar, 3919K and 4020Ca are all isotones of 20 since they all contain 20 neutrons.
Isodiaphers — Nuclides having different atomic numbers and mass numbers but the same neutron excess, (the difference between numbers of neutrons and protons in the nucleus).
Isobars are elements that have the same number of nucleons (sum of protons and neutrons). The series of elements with 40 Mass number serves as a good example; 4016S, 4017Cl, 4018Ar, 4019K, and 4020Ca.
