Atomgewichten

Atomgewichten, in English atomic weights, are dimensionless numbers that express the average mass of atoms of an element relative to the mass of carbon-12, which is defined as exactly 12. The atomic weight of an element reflects its isotopic composition as it occurs in nature and is used in chemical calculations to relate mass to amount.

In more detail, the atomic weight Ar is calculated from the masses of the element’s naturally occurring isotopes and their fractional abundances: Ar = sum(fi × mi), where mi is the mass of isotope i (in atomic mass units) and fi is its natural abundance. Because isotopic abundances vary among elements, Ar is often not an integer. For elements with a single stable isotope, the atomic weight is essentially the mass number of that isotope. For elements with multiple isotopes, the atomic weight can differ slightly between samples, and IUPAC publishes this value as the standard atomic weight, sometimes with a stated range or uncertainty to reflect natural variation.

The standard atomic weight is used to express the molar mass of elements in g/mol, providing a convenient link between mass and the amount of substance in stoichiometric calculations. Atomic weights are determined through measurements of isotopic abundances, primarily by mass spectrometry, and are compiled by standardizing bodies such as IUPAC. In practice, atom weights are essential in chemistry, geology, and related fields for converting masses to moles and for comparing compositions across samples. The term Atomgewichte is the German equivalent of atomic weights.