Now we can look up the atomic masses of a known element by simply looking at the periodic table. But there's going to be instances where you may not know the identity of the element, or you may not have a periodic table handy. In these cases, we can actually calculate the atomic mass for that element.
Now to the atomic mass of an element can be calculated if you know the isotopic masses and percent abundances of that element. We're going to say here that your isotopic masses are just the masses for all the isotopes of a given element. The percent abundances are just the percentages available for each of the isotopes of a given element. Sometimes these percent abundances are also called natural abundances.
Now, associated with R percent or natural abundances, we have R fractional abundances. It represents the percent abundance of an isotope divided by 100. So remember, when you have a percentage form, such as 25%, to get its fractional form you would divide it by 100. That's what's going on here. We have our percent abundances. We can divide them by 100 and that'll give us our fractional abundances.
With this piece of information we can use the atomic mass formula. Now the atomic mass formula says that your atomic mass equals the isotope mass one. So the mass of your first isotope times its fractional abundance. Plus I just took mass two times its fractional abundances. Now realize that some elements have more than two isotopes, so you just continue on with the formula. So you'd say plus isotope mass, three times its fractional abundance, and so on and so forth.
So just realize that we're going to utilize this atomic mass formula in order to calculate the atomic mass of an element when we don't have a periodic table handy.
