The atomic weight of an element is the average weight of all the isotopic masses of the element, calculated on the basis of their relative abundance in nature. The atomic weights are set on a "carbon-12" scale. This is the standard weight scale that is used worldwide to express atomic weights. Exploring this further, we can say that 12 atomic mass units (amu) make up the mass of one atom of isotope. In other words, one amu is equal to 1/12 the mass of one carbon-12 atom. We can also say that the atomic weight of carbon-12 is 12 amu. Even though it is popular to use the term atomic weight, atomic mass is a more appropriate term since we are really talking about the mass rather than the weight.