CBSE Class 9 Answered

The naturally occurring elements such as chlorine often consist of a mixture of isotopes. These results in relative atomic mass of an element which represents the average mass of one atom of that element, taking into account the different isotopes and their relative proportions.

For example, chlorine consists of two isotopes ³⁵Cl and ³⁷Cl. When pure ³⁵Cl is present then the relative atomic mass is 35 or when pure ³⁷Cl is present, then relative atomic mass is 37. But it rarely occurs that 100% pure compound is present.

If there is 75% ³⁵Cl and 25% ³⁷Cl is present, then relative atomic mass of chlorine is

=  (35 x 75/100) + (37 x 25/100)

=  26.25 + 9.25

= 35.50

 Hence, relative atomic masses are not rarely whole numbers.