Standard atomic weight real life substances—for example, in pharmaceuticals and scientific research. Atomic weights are specific to single sources and samples of an element, such as the atomic weight of carbon in a particular bone from a particular archeological site. Standard atomic weight generalizes such values to the "range of atomic weights" which a chemist might expect to derive from many random samples from Earth. This range is the cause of the "interval notation" in some standard atomic weight values. Out of the 118 known chemical elements, 84 are stable and have this Earth-environment based value. Typically, such a value is, for example
Relative atomic mass requires the sources be terrestrial, natural, and stable with regard to radioactivity. Also, there are requirements for the research process. For 84 stable elements, CIAAW has determined this standard atomic weight. These values are widely published and referred to loosely as 'the' atomic weight of elements for real-life substances like pharmaceuticals and commercial trade. Also, CIAAW has published abridged (rounded) values and simplified values (for when the Earthly sources vary systematically). Atomic mass ("m") is the mass of a single atom, with unit "Da" or "u" (the unified atomic mass unit). It defines the mass of a specific isotope, which