Percent abundance describes the prevalence of each of an element's isotopes in nature. The percent abundance of each isotope is used in the calculation of an element's average atomic mass.
Many elements exist as two or more isotopes, which are versions of the same element that possess the same number of protons per atom but different numbers of neutrons, resulting in different atomic masses. Isotopes exist in varying proportions in nature, so their atomic masses cannot be factored in evenly when calculating an element's average atomic mass, which is the mass number that appears on the periodic table. Rather, a weighted average of the atomic masses is calculated giving proportionally more weight to those isotopes that occur with greater abundance.
For example, the element rubidium has two isotopes, 85Rb and 87Rb, with 85 AMU and 87 AMU being their respective atomic masses. 85Rb has a percent abundance of 72.2 percent, meaning it accounts for 72.2 percent of all rubidium atoms found in nature. 87Rb has a percent abundance of 27.8 percent. Thus, the formula (85 x 0.722) + (87 x 0.278) is used to calculate rubidium's average atomic weight of 85.56 AMU. The amount is closer to 85 than it is to 87 because 85Rb occurs with greater frequency than 87Rb.