Standard atomic weight precise value is acceptable, CIAAW has published a single-number conventional atomic weight that can be used for example in trade. For hydrogen, . The twelve elements are: hydrogen, lithium, boron, carbon, nitrogen, oxygen, magnesium, silicon, sulfur, chlorine, bromine and thallium. By using the abridged value, and the conventional value for the twelve interval values, a short IUPAC-defined value (5 digits plus uncertainty) can be given for all stable elements. In many situations, and in periodic tables, this may be sufficiently detailed. Standard atomic weight The standard atomic weight ("A", a relative atomic mass) is the atomic weight ("A") of a
Isotope (counting as stable), and 26 elements have only a single stable isotope (of these, 19 are so-called mononuclidic elements, having a single primordial stable isotope that dominates and fixes the atomic weight of the natural element to high precision; 3 radioactive mononuclidic elements occur as well). In total, there are 253 nuclides that have not been observed to decay. For the 80 elements that have one or more stable isotopes, the average number of stable isotopes is 253/80 = 3.1625 isotopes per element. The proton:neutron ratio is not the only factor affecting nuclear stability. It depends also on evenness or