Atomic weight is the average mass of atoms of an element relative to some standard; the present standard is the carbon- 12 isotope, which is assigned an atomic weight of exactly 12 atomic mass units (amu). Mass numbers (the sum of an atom's protons and neutrons) are always whole numbers, but the atomic weight of an element is the average of the weights of its isotopes, taking the frequency of their natural occurrence into account, and is not usually a whole number. A gram atomic weight is a quantity of an element in grams that has the same numerical value as the element's atomic weight; the gram atomic weight of carbon is 12 grams. Such a quantity always contains 6.022 times 10(23) atoms (one mole). For the atomic weights of the elements, see
Last modified on: Monday, October 20, 1997.