mass atomic weight The weighted average of the atomic masses of the naturally occurring stable isotopes and long-lived radioactive isotopes of an element. Atomic weight is typically expressed in atomic mass units. For example, the atomic weight of carbon, 12.011 amu, is calculated based on the natural abundance of carbon-12 (approx. 98.9%) and carbon-13 (approx. 1.1%), with carbon-14 contributing a negligible amount.