The chlorine isotope with 18 neutrons has an abundance of 0.7577 and a mass number of 35 amu. To calculate the average atomic mass, multiply the fraction by the mass number for each isotope, then add them together.
Aug 14, 2019 · To get fractional abundance, you usually divide the percentage abundance of each isotope by 100. And when you add all the fractional abundance values of all the isotopes, you will notice they all add up to 1. To calculate the percent abundance of each isotope in a sample of an element, chemists usually divide the number of atoms of a particular isotope by the total number of atoms of all isotopes of that element and then multiply the result by 100.