Average Atomic Mass Calculator

The average atomic mass on the periodic table is a weighted mean: each isotope's mass times the fraction of atoms that are that isotope. Enter up to six isotopes with their masses in atomic mass units and their percentage abundances, and this returns the weighted mean along with how much each isotope contributes. The abundances have to total 100 percent, because they are a complete split of every atom of the element, and an entry that misses is rejected rather than quietly scaled. A second action runs it backwards, recovering the two abundances of a two-isotope element from its published average.