WAEC Chemistry 2014 Paper

Concept:The relative atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes.Explanation:Chlorine exists as a mixture of isotopes, mainly $\mathrm{Cl}$-35 and $\mathrm{Cl}$-37.These isotopes have different masses: $35$ and $37$.They also occur in different natural abundances: about $75\%$ $\mathrm{Cl}$-35 and $25\%$ $\mathrm{Cl}$-37.So the relative atomic mass is calculated as the weighted average:$$A_r = \frac{(75 \times 35) + (25 \times 37)}{100} = 35.5$$Since the result depends on the fractional abundance of each isotope, it is not a whole number.This is why chlorine has a relative atomic mass of $35.5$ instead of an integer value.Answer:It is not a whole number because it is the weighted average mass of all its various isotopes.Therefore, the correct option is A.