Atomic weight is the average weight of an element's atoms found on Earth. Since elements occur as variuos different isotopes in different concentrations, a weighted average is used. The unit used for this value is the so called atomic mass unit, or u. It is approximately equal to 1.66×10^-27, which is roughly the mass of a proton or neutron.

Note that some elements don't occur in nature in a stable state, and they therefore don't have an atomic weight value.

IUPAC is used as our data source.