Question
The ions $${O^{2 - }},{F^ - },N{a^ + },M{g^{2 + }}$$ and $$A{l^{3 + }}$$ are isoelectronic. Their ionic radii show
A.
an increase from $${O^{2 - }}$$ to $${F^ - }$$ and then decrease from $$N{a^ + }$$ to $$A{l^{3 + }}$$
B.
a decrease from $${O^{2 - }}$$ to $${F^ - }$$ and then increase from $$N{a^ + }$$ to $$A{l^{3 + }}$$
C.
a significant increase from $${O^{2 - }}$$ to $$A{l^{3 + }}$$
D.
a significant decrease from $${O^{2 - }}$$ to $$A{l^{3 + }}$$
Answer :
a significant decrease from $${O^{2 - }}$$ to $$A{l^{3 + }}$$
Solution :
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 $${O^{2 - }} > {F^ - } > N{a^ + } > M{g^{2 + }} > \,\,\,A{l^{3 + }}$$