Question

For $Na^{+}, Mg^{2+}, F^{-}$ and $O^{2-}$; the correct order of increasing ionic radii is :

• A. $O^{2-} < F^{-} < Na^{+} < Mg^{2+}$
• B. $Na^{+} < Mg^{2+} < F^{-} < O^{2-}$
• C. $Mg^{2+} < Na^{+} < F^{-} < O^{2-}$
• D. $Mg^{2+} < O^{2-} < Na^{+} < F^{-}$

Answer 1

Answer: (C)

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

Answer 2

In representative ( $s$ - and $p$ -block) elements, there is a general decrease of atomic radius in a period because the effective nuclear charge $\left(Z_{ eff }\right)$ increases across a row. When a negative ion is formed, one or more electrons are added to an atom and the effective nuclear charge is reduced. The electron cloud expands and the size of negative ion becomes more. Hence, the correct order of increasing ionic radii is $Mg ^{2+}< Na ^{+}< F ^{-}< O ^{2-}$.