Question

Consider the following species :

$N^{3-}$,$O^{2-}$,$F^{-}$,${Na}^{+}$,${Mg}^{2+}$ and ${Al}^{3+}$.

(a) What is common in them?

(b) Arrange them in the order of increasing ionic radii.

Answer 1

Negative sign means more of electrons and positive sign means lack of electrons.

Here $N^{3-}$ with atomic no. 7 has 7+3= 10 electrons

$O^{2-}$ with atomic no. 8 has 8+2 = 10 electrons

$F^{-}$ with atomic no. 9 has 9+1= 10 electrons

$N{a}^{+}$ with atomic no. 11 has 9-1= 10 electrons

$M{g}^{2+}$ with atomic no. 12 has 12-2= 10 electrons

$A{l}^{3+}$ with atomic no. 13 has 13-3= 10 electrons

Since all these ions have 10 electrons in their shell, therefore, these are iso-electronic species

The more + the charge, the smaller the ionic radius. Remember that - means adding electrons.

These electrons go in the outermost shells.

Also, when an atom loses electrons, it clings ever more tightly to the ones it has left, further reducing the ionic radius. therefore the order of ionic radii will be :

$A{l}^{3+}<M{g}^{2+}<N{a}^{+}<F^{-}<O^{2-}<N^{3-}$ (increasing order)