$r_{n} \propto \frac{n^{2}}{Z}$
$\left( r _{1}\right)_{ H }=\left( r _{ n }\right)_{ Be ^{3-}}$
$\frac{1^{2}}{1}=\frac{ n ^{2}}{4}$
$n =2$
The energy of their orbit is,
$( E )_{B e^{3+}}=-13.6\left(\frac{ Z ^{2}}{ n ^{2}}\right)$
$=-13.6\left(\frac{4^{2}}{2^{2}}\right)$
$=-54.4 \,e.V$
$I. C{H_3}^ + $ $II. N{H_2}^ - $ $III. N{H_4}^ + $ $ IV. N{H_3}$