Why is the atomic mass of chlorine taken as $35.5\, u$ and not a whole number like $35\, u$ or $36\, u$? Explain.

(N/A) Chlorine exists in nature as two isotopes with atomic masses $35\, u$ and $37\, u$ in the ratio of $3:1$.
Therefore,the average atomic mass of chlorine is calculated as:
$\text{Average Atomic Mass} = (35 \times \frac{3}{4}) + (37 \times \frac{1}{4}) = 26.25 + 9.25 = 35.5\, u$.
This does not imply that any individual chlorine atom has a fractional mass of $35.5\, u$. It indicates that in any natural sample of chlorine,both isotopes are present,and the weighted average mass of these atoms is $35.5\, u$.