Question

Chlorine atoms have $2$ isotopes: $Cl-35$ and $Cl-37$.
Which isotopes is most abundant?

• A. $Cl-35$
• B. $Cl-37$
• C. Both are equally abundant
• D. There is not enough information to answer this questions.

Answer 1

The correct option is A $Cl-35$

> The average atomic mass of chlorine is 35.5 amu.

> Chlorine consists of two isotopes : $C{l}^{35}$ and $C{l}^{37}$.

> It's average mass depends on the relative abundance of the two isotopes.

Let x be the relative abundance of $C{l}^{35}$ then (1-x) is the relative abundance of $C{l}^{37}$.

Average atomic mass $=\frac{x\left(35\right)+(1-x)37}{x+(1-x)}$

$35.5=35x+37-37x$

$1.5=2x$

$x=0.75$

This implies that out of every four atoms of chlorine, there are 3 atoms of Cl-35 and 1 atom of Cl-37. Hence, Cl-35 is more abundant than Cl-37.