Question

Ordinary chlorine has two isotopes ${}_{17}{}^{35}Cl$ and ${}_{17}{}^{37}\mathrm{Cl}$ in the ratio $3:1$. Calculate the relative atomic mass (atomic weight) of chlorine.

Answer 1

Isotopes are atoms of the same element having the same atomic number but a different mass number.

Relative atomic mass of chlorine, ${\mathrm{A}}_{\mathrm{r}}$ $=$$({\mathrm{xA}}_1+{\mathrm{yA}}_2)/\mathrm{x}+\mathrm{y}$, where ${\mathrm{A}}_1$ and ${\mathrm{A}}_2$are relative masses of two isotopes and $\mathrm{x}$ and $\mathrm{y}$are their relative abundances.

Therefore, ${\mathrm{A}}_{\mathrm{r}}$ $=$$(3\times 35+1\times 37)/3+1$ $=$$(105+37)/4$

${\mathrm{A}}_{\mathrm{r}}$$=$$142/4$ $=35.5$

Therefore. relative atomic mass of chlorine is $35.5$ .