the mass in grams of 1 mol (6.02 × 1023 atoms) of an atomic species; the mass of an atom of a chemical element in relation to the mass of an atom of carbon-12 (12C), which is set equal to 12.000, thus a ratio and therefore dimensionless (although the actual mass, numerically the same, is sometimes expressed in daltons); not necessarily the weight of any individual atom of an element, since most elements are made up of several isotopes of different masses; e.g., the atomic weight of chlorine is 35.4527, because it is composed of 35Cl and 37Cl in proportions that give an average of 35.4527.molecular weight.
