Chemical element isotopes: specifically the elements without any stable isotopes are technetium (atomic number 43), promethium (atomic number 61), and all observed elements with atomic numbers greater than 82. Of the 80 elements with at least one stable isotope, 26 have only one single stable isotope. The mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element. The largest number of stable isotopes that occur for a single element is 10 (for tin, element 50). The mass number of an element, "A", is the number of nucleons (protons and neutrons) in the atomic nucleus. Different isotopes
Symbol (chemistry) Symbol (chemistry) In relation to the chemical elements, a symbol is a code for a chemical element. Many functional groups have their own chemical symbol, e.g. Ph for the phenyl group, and Me for the methyl group. Chemical symbols for elements normally consist of one or two letters from the Latin alphabet, but can contain three when the element has a systematic temporary name (as of March 2017, no discovered elements have such a name), and are written with the first letter capitalized. Earlier chemical element symbols stem from classical Latin and Greek vocabulary. For some elements, this is because