What is the formula taking out …

To find the percentage of each isotopes and add there average. For example; Chlorine occurs in nature in two isotopic forms, with masses 35 u and 37 u in the ratio of 3:1. The mass of an atom of any natural element is take as the average mass of all the naturally occuring atoms of that element. If an element has no isotopes, then the mass of it's atom would be the same as the sum of protons and neutrons in it . But if an element occurs in isotopic forms, then we have to know the percentage of each isotopic form and then the average mass is calculated. The average Atomic mass of chlorine atom, on the basis of above data , will be [(35×75÷100+37×25÷100)=(105÷4+37÷4)=142÷4=35.5u]. It means that if you take a certain amount of Chlorine, it will contain both isotopes of Chlorine and the average mass is 35.5 u.