Atomic number and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the "relative isotopic mass"), is within 1% of the whole number "A". Atoms with the same atomic number "Z" but different neutron numbers "N", and hence different atomic masses, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass)
Isobar (nuclide) Isobar (nuclide) 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 would be S, Cl, Ar, K, and Ca. The nuclei of these nuclides all contain 40 nucleons; however, they contain varying numbers of protons and neutrons. The term "isobars" (originally "isobares") for nuclides was suggested by Alfred Walter Stewart in 1918. It is derived from the Greek word "isos", meaning "equal" and "baros", meaning "weight". The same mass number implies