Now, how do we calculate atomic mass when no fractional abundances are given? Well, remember, adding up the fractional abundances off all isotopes for an element gives a total of one. Because remember, all of them together constitute 100% for that particular element. And dividing it by ah 100 gives us it's fractional abundance in that equals one. Now, this is important for atomic mass questions where there are two isotopes with unknown fractional abundances. So basically, if you're gonna have a question where you have to calculate the fraction abundances of all isotopes, it will only be for two isotopes, not three or more.