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 a decrease from $N{a}^{+}$ to $A{l}^{3+}$
• B. A decrease from $O^{2-}$ to $F^{-}$ and then an 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 1

The correct option is D A significant decrease from $O^{2-}$ to $A{l}^{3+}$

All the five ions are isoelectronic but the ionic radii differs because each ion possesses different nuclear charges.
As the effective nuclear charge increases, the attractive force or the inward pull experienced by the valence electrons also increases and hence atomic radius or ionic radius decreases with increase in effective nuclear charge.

The cation with a greater positive charge will have a smaller radius owing to the increase in the effective nuclear charge.

The anion with a greater negative charge will have a larger radius owing to the repulsion between the electrons.

Thus, the order of decreasing ionic radii is: $O^{2-}>F^{-}>N{a}^{+}>M{g}^{2+}>A{l}^{3+}$