Chemistry - GENERAL CHEMISTRY Question with Solution | TestHub

Noble gas configuration$= {\mathrm{ns}}^2{\mathrm{np}}^6$

$\left[{}_{38}{\mathrm{Sr}}^{2+}\right]={[}_{36}\mathrm{Kr}]$

$\left[{\mathrm{Cs}}^{+}\right]={[}_{54}\mathrm{Xe}]$

$\left[{{\mathrm{Yb}}_{70}}^{2+}\right]={[}_{54}\mathrm{Xe}]4{\mathrm{f}}^{14}$

$\left[{}_{57}{\mathrm{La}}^{2+}\right]={[}_{54}\mathrm{Xe}]5 {\mathrm{d}}^1$

$\left[{}_{82}{\mathrm{Pb}}^{2+}\right]={[}_{54}\mathrm{Xe}]4{\mathrm{f}}^{14}5 {\mathrm{d}}^{10}6 {\mathrm{s}}^2$

$\left[{}_{26}{\mathrm{Fe}}^{2+}\right]={[}_{18}\mathrm{Ar}]3{\mathrm{d}}^6$

${\mathrm{Cs}}^{+}$ and  ${\mathrm{Sr}}^{2+}$ have noble gas configuration, which are $\mathrm{Xe}$ and $\mathrm{Kr}$, respectively.

Hence, the answer is $2$.