Term: magic number: the atomic number of an extra stable strongly bound atomic nucleus: 2, 8, 20, 28, 50, 82 or 126
Broader: atomic number: the order of an element in Mendeleyev's table of the elements; equal to the number of protons in the nucleus or electrons in the neutral state of an atom of an element | Unrelated: bad debt: a debt that is unlikely to be repaid