Atomic weight

Atomic weight (atomic mass) is a value that expresses the relative mass of the atoms of an element. It is normally defined by setting the atomic weight of a carbon atom to 12. Because this comparison allows the mass of atoms of other elements to be expressed in relative terms, it is an important measure for understanding chemical reactions and the composition of substances. Atomic weight is calculated by taking into account the natural abundance of each element's isotopes, so each element has its own distinct value.