A cation has always the lesser ionic size than a metal atom due to loss of electrons and an anion has always the greater size than metal atom due to gain of electrons. The given species are isoelectronic species as they contain same number of electrons.
For isoelectronic species ionic radii $$ \propto \frac{1}{{{\text{atomic number}}}}.$$
$$\eqalign{
& {\text{Ion}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,:C{a^{2 + }}\,\,\,\,{K^ + }\,\,\,\,Ar\,\,\,\,\,\,{S^2}\,\,\,Cl \cr
& {\text{Atomic}}\,\,{\text{number}}\,\,\,:\,20\,\,\,\,\,\,\,\,\,19\,\,\,\,\,\,\,\,18\,\,\,\,\,16\,\,\,\,\,\,\,\,17 \cr
& {\text{So, the correct order of size is as}} \cr
& C{a^{2 + }} < {K^ + } < Ar < C{l^ - } < {S^{2 - }} \cr} $$
183.
Among the following, the one which is most basic is
$$ZnO$$ and $$A{l_2}{O_3}$$ are amphoteric oxide, $${N_2}{O_5}$$ is oxide of non-metal, so it is acidic and hence, $$MgO$$ is most basic among $$A{l_2}{O_3},ZnO,{N_2}{O_5}$$ and $$MgO.$$
184.
Which is the correct order of second ionization potential of $$C, N, O$$ and $$F$$ in the following ?
On increasing atomic number of isoelectronic species ionic radii decreases due to increasing effective nuclear charge $$\left( {{Z_{eff}}} \right).$$
$${\text{Radius}} \propto \frac{1}{{{\text{Atomic}}\,{\text{number}}}} \propto \frac{1}{{{Z_{eff}}}}$$
So, as the negative charge increases ionic radii increases while on increasing positive charge ionic radii decreases. Anions having higher ionic radii than the cation.
Hence, order of radii $${O^{2 - }} > {F^ - } > N{a^ + } > M{g^{2 + }} > \,\,\,A{l^{3 + }}$$
186.
An element having electronic configuration $$1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}$$ forms