Answer» - In nature, most elements occur as a mixture of two or more isotopes, each isotope has a certain percentage of natural occurrence.
- 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.
- Isotope of Cl occurs in nature, in two isotopic forms, with masses 35 units and 37 units.
- The isotope with mass 35 is present in 75% in nature. The isotope with mass 37 is present in 25% in nature.
- The average mass of chlorine is \(=[(\,35\,\times\frac{75}{100})\,+\,(\,35\times\frac{25}{100})] =(\frac{105}{4}\,+\,\frac{37}{4}) = \frac{142}{4}=35.5\,units.\)
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