Interatomic distance in hydrogen $\left({\mathrm{H}}_2\right)$ molecule $\left({\mathrm{L}}_{{\mathrm{H}}_2}\right)=74$ pm and interatomic distance in chlorine $\left(\mathrm{C}{\mathrm{l}}_2\right)$ molecule $\left({\mathrm{L}}_{\mathrm{C}{\mathrm{l}}_2}\right)=198\mathrm{ }\mathrm{p}\mathrm{m}$ . Atoms linked by one $\mathrm{\sigma }$ -bond, covalent radius $\left(\mathrm{r}\right)=\frac{\mathrm{I}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{ }\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}}{2}$ . Therefore covalent radius of hydrogen $\left({\mathrm{r}}_{\mathrm{H}}\right)=\frac{{\mathrm{L}}_{{\mathrm{H}}_2}}{2}=\frac{74}{2}=37$ pm and that of chlorine $\left({\mathrm{r}}_{\mathrm{C}\mathrm{l}}\right)=\frac{{\mathrm{L}}_{\mathrm{C}{\mathrm{l}}_2}}{2}=\frac{198}{2}=99$ pm

As ${\text{H}}_2\left(\text{g}\right)+{\text{Cl}}_2\left(\text{g}\right)\to 2\text{HCL}\left(\text{g}\right)$

Therefore bond length of HCl molecule $\left({\mathrm{L}}_{\mathrm{H}\mathrm{C}\mathrm{l}}\right)$

$={\mathrm{r}}_{\mathrm{H}}+{\mathrm{r}}_{\mathrm{C}\mathrm{l}}=37+99=136$ pm.