Now the atomic mass of an element can be calculated if you know the isotopic masses and percent abundances. Isotopic masses are the masses for all the isotopes of a given element. And percent abundance, sometimes referred to as natural abundances, are the percentages available for each of the isotopes of a given element. Sometimes they're referred to as percent natural abundances. I know it's a little bit redundant, but just remember, you might see percent abundances, natural abundances, or percent natural abundances.

Isotopic abundance, also called fractional abundance, is the percent abundance of an isotope divided by 100. Remember, when you divide a percentage by 100, you're changing it from its percentage form to its fractional or decimal form. All of this together gives us our atomic mass formula.

The atomic mass formula, we are going to say, equals the isotopic mass of isotope 1 times its isotopic abundance plus the isotopic mass of isotope 2 times its isotopic abundance. Of course, if you have more than 2 isotopes for a given element, this would just continue. You would keep adding, let's say, plus isotope mass 3 times its isotopic abundance plus isotope mass 4 times its isotopic abundance. In this example, we're just showing that this particular element we're talking about has 2 isotopes involved with it, and they both have their own masses. But again, this formula can be expanded to even more isotopes depending on the element. Elements such as manganese have various isotopes, so their formulas would be a lot larger.

In MathML format, the atomic mass formula can be presented as follows:

Where mi is the isotopic mass and fi is the fractional abundance of isotope i.