Ncert Exercises & Solutions

Consider the isoelectronic species, ${Na}^{+}, {Mg}^{2 +}, F^{-}, and O^{2 -}$ . The correct order of increasing length of their radii is :

1. $F^{-} < O^{2 -} < {Mg}^{2 +} < {Na}^{+}$

2. ${Mg}^{2 +} < {Na}^{+} < F^{-} < O^{2 -}$

3. $O^{2 -} < F^{-} < {Na}^{+} < {Mg}^{2 +}$

4. $O^{2 -} < F^{-} < {Mg}^{2 +} < {Na}^{+}$

Hint: In isoelectronic species anion is larger than cation

In case of isoelectronic species, ionic radii

\propto \frac{1}{positive charge on cation} \propto negative charge on anion

The ionic radii increases as the positive charge decreases or the negative charge increases.

Ion radii order {Mg}^{2 +} < {Na}^{+} < F^{-} < O^{2 -}