Question
Which one of the following is the smallest in size?
A.
$${N^{3 - }}$$
B.
$${O^{2 - }}$$
C.
$${F^ - }$$
D.
$$N{a^ + }$$
Answer :
$$N{a^ + }$$
Solution :
$${\text{For isoelectronic ions, ionic size }} \propto \frac{1}{{{\text{atomic number }}}}$$
$$\eqalign{
& {\text{Species}}\,\,\,{\text{No}}{\text{.}}\,{\text{of}}\,{e^ - }\,\,\,{\text{At}}{\text{.No}}{\text{.}} \cr
& {{\text{N}}^{ - 3}}\,\,\,\,\,\,\,\,\,\,\,\,\,10\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,7 \cr
& {O^{ - 2}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,10\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,8 \cr
& {F^ - }\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,10\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,9 \cr
& N{a^ + }\,\,\,\,\,\,\,\,\,\,\,\,\,10\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,11 \cr} $$
∴ $$N{a^ + }$$ is largest in size.