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
Atomic mass unit of carbon-12 in 0.012 kg of carbon-12. This can be expressed mathematically as Molecular masses of proteins are often expressed in kilodaltons (kDa or kD). For example, a molecule of a protein with molar mass has a mass of 64 kDa. In research and commerce, the degree of polymerization of synthetic polymers is conventionally expressed in daltons. The US Supreme Court based a major precedent of appellate law on a disputed case of counting daltons for a molecular distribution. Atomic mass unit The unified atomic mass unit or dalton (symbol: u, or Da) is a standard unit of mass that