Every element has atoms which consist of protons, electrons, and neutrons. When an element has more than one form having same number of protons but different number of neutrons in the nucleus, they are called isotopes. They either occur naturally or are artificially produced. The element chlorine (Cl) has more than 50 isotopes, of which Cl-35 and Cl-37 are two stable isotopes. They have same atomic number, which is 17; however, their mass number is 35 and 37, respectively. For any element, abundance is expressed as the percentage of an isotope to the total amount of all isotopes of that element. Note that the sum of the abundance percentages of all the isotopes for any element must equal 100. Definition of percentage abundance for X in Y can be best explained in two possible ways as follows:

1. how many percent of Y is X
2. how much of X is in Y

The formula to find the percent abundance of an element with two isotopes is as follows: In order to find the abundance of an element, it's necessary to compute the average of atomic masses of its isotopes in the first place. Proceed by substituting either one of the isotopes in terms of the other. Add the similar terms in the equation to get the final answer. Consider that an element has two isotopes. If the atomic masses of these isotopes are given, calculating the percentage abundance of each of the isotopes is possible. Given below is a problem, with its detailed solution explaining how to find the same. The value of atomic mass of copper is 63.546 ± 0.003 u. ⁶³Cu and ⁶⁵Cu are two isotopes of copper. The atomic masses of ⁶³Cu and ⁶⁵ are 62.9296 and 64.9278 amu, respectively. Given their atomic masses, calculate the percentage abundance of ⁶³Cu and ⁶⁵Cu. Calculation of percent abundance of isotopes is usually done in chemistry worksheets.