Chlorine has two isotopes, ${}_{17}^{35}\text{Cl}$ and ${}_{17}^{37}\text{Cl}$. Calculate the average atomic mass of chlorine atom (in a.m.u.) based on the given information.

$$\begin{array}{ccc}\text{S.No}& \text{Isotopes}& \text{Percentage abundance}\text{(\%)}\\ 1& {}_{17}^{35}\text{Cl}& 75\\ 2& {}_{17}^{37}\text{Cl}& 25\end{array}$$

1. 35.5

The correct option is A 35.5
Average atomic mass =(Atomic mass of isotope 1 x percentage abundance of isotope 1 + Atomic mass of isotope 2 x percentage abundance of isotope 2 )

Average atomic mass =$35\times \frac{75}{100}+37\times \frac{25}{100}$
= 35.5 a.m.u.