Atomic Weights

... are based on the mass of the carbon–12 atom.

The atomic weight of a given sample of an element (more correctly known as its relative atomic mass) is defined as the ratio of the average mass of its atoms, according to their relative abundance, to one twelfth of the mass of a carbon–12 atom (known as the atomic mass constant).

This is why atomic weights are not whole numbers.

© Haydn Thompson 2020