Question

The ions $O ^{2-}, F ^{-}, Na ^{+}, Mg ^{2+}$ and $Al ^{3+}$ are isoelecronic. Their ionic radii show :

• A. An increase from $O ^{2-}$ to $F ^{-}$ and then decrease from $Na ^{+}$ to $Al ^{3+}$
• B. An decrease from $O ^{2-}$ to $F ^{-}$ and then increase from $N a ^{+}$ to $Al ^{3+}$
• C. A significant increase from $O ^{2-}$ to $Al ^{3+}$
• D. A significant decrease from $O ^{2-}$ to $Al ^{3+}$

Answer 1

Answer: (D)

A significant decrease from $O ^{2-}$ to $Al ^{3+}$

Answer 2

On increasing atomic number of isoelectronic species ionic radii decreases due to increasing effective nuclear charge
(Z$_{eff})$
\hspace20mm Radius ? \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.
$\, \, \, \, \, \, \, \, O^{2-} > F^- > Na^+ > Mg^{2+} > Al^{3+}$