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
Avogadro constant value for "d"(Si) is , a relative standard uncertainty of , corresponding to a unit cell volume of . The isotope proportional composition of the sample used must be measured and taken into account. Silicon occurs in three stable isotopes (Si, Si, Si), and the natural variation in their proportions is greater than other uncertainties in the measurements. The atomic weight "A" for the sample crystal can be calculated, as the standard atomic weights of the three nuclides are known with great accuracy. This, together with the measured density "ρ" of the sample, allows the molar volume "V" to be