Question

Which of the following orders of ionic radii is correctly represented?

• A. $H^{-}>H^{+}>H$
• B. $N{a}^{+}>F^{-}>O^{2-}$
• C. $O^{2-}>F^{-}>N{a}^{+}$
• D. $A{l}^{3+}>M{g}^{2+}>N^{3-}$

Answer 1

The correct option is C

$O^{2-}>F^{-}>N{a}^{+}$

(a) $H^{-}>H^{+}>H$
It is known that radius of a cation is always smaller than that of a neutral atom due to decrease in the number of orbits. Whereas, the radius of anion is always greater than a cation due to decrease in effective nuclear charge. Hence, the correct order is
$H^{-}>H>H^{+}$

(b) $N{a}^{+}>F^{-}>O^{2-}$
The given species are isoelectronic as they contain same number of electrons. For isoelectronic species,
$Ionicradii\propto \frac{1}{atomicnumber}$
Ion: $N{a}^{+}{F}^{-}{O}^{2-}$
Atomic number: 11, 9, 8
Hence, the correct order of ionic radii is
$O^{2-}>F^{-}>N{a}^{+}$

(c) Similarly, the correct option is
$O^{2-}>F^{-}>N{a}^{+}$

(d) Ions: $A{l}^{3+}M{g}^{2+}{N}^{3-}$
Atomic number : 13, 12, 7
Hence, the correct order is, $N^{3-}>M{g}^{2+}>A{l}^{3+}$