Instructions1Gather the necessary information. You need to know the average atomic mass of the element in question. You will also need to know the atomic mass of each individual isotope for which you want to calculate the abundance percentage.2Enter the information into the formula (b)x + (1-x)(c)=a, where "x" represents the unknown abundance, "a" represents the average atomic        mass of the element, "b" represents the atomic mass of the first isotope, and "c" represents the atomic mass of the second isotope. For example, let's say that we are dealing with an element that has an average atomic mass of 5. Let's also assume that this element has two potential isotopes with atomic masses of 6 and 4 respectively. After plugging these numbers into the formula we would have (6)x + (1-x)(4)=5.Isotope is a chemistry term used to describe the elements of the periodic table.