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 molecular masses by comparing the vapor density of a collection of gases with molecules containing one or more of the chemical element in question. In the 20th century, until the 1960s chemists and physicists used two different atomic-mass scales. The chemists used a "atomic mass unit" (amu) scale such that the natural mixture of oxygen isotopes had an atomic mass 16, while the physicists assigned the same number 16 to only the atomic mass of the most common oxygen isotope (O, containing eight protons and eight neutrons). However, because oxygen-17 and oxygen-18 are also present in natural oxygen this led