Atomic weight (in atomic mass) - a number that shows how many times the mass of an atom of a given chemical element more international units of atomic masses, that is 1/12 of the mass of the atom isotope of carbon With^{12}.

Atomic weight atomic weight,mean value of the mass of an atom of a chemical element, which is expressed in international units of atomic masses. G 1960 as an international unit of atomic masses adopted the so-called carbon unit equal to 1/12 of the mass of an atom of carbon isotope With^{12}, atomic weight, which is considered equal to 12,000. Atomic weight is a fundamental characteristic of a chemical element and is the basis for the calculation of molecular weights of simple and complex substances and for all kinds of calculations, based on the chemical formulas and equations. In practice, the mass of a chemical element is often measured in grams-atoms. Gram-atom - is the mass of an element in grams, numerically equal to its atomic weight. For example, gram-atom of sulfur is 32,06 g (atomic weight of sulfur - 32,06). Gram-atom of each element contains the same number of atoms of 6.02·10^{23} (Avogadro's number). The numerical values of the atomic weights of chemical elements, see the Periodic system of chemical elements.