How do we determine the atomic mass of an element with its isotopes?

Explanation

1) In nature, most elements occur as a mixture of wo or more isotopes, each isotope has a certain percentage of natural occurrence.
2) The atomic mass of an element is taken as an average mass of all the naturally occuring atoms of the sample element.
Ex : Let us calculate the atomic mass of chlorine.
3) Isotope of `Cl` occurs in nature, in two isotopic forms, with masses 35 units and 37 units.
4) The isotope with mass 35 is present in `75%` in nature.
5) The isotope with mass 37 is present in `25%` in nature.
6) The average mass of chlorine is
`=[(35xx(75)/(100))+(37xx(25)/(100))]`
`=((105)/(4)+(37)/(4))`
`=(142)/(4)="35.5 units"`