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.

(N/A) Each of the given species (ions) has the same number of electrons ($10$ electrons). Hence,the given species are isoelectronic.
$(b)$ The ionic radii of isoelectronic species increase with a decrease in the magnitude of nuclear charge.
The arrangement of the given species in order of their increasing nuclear charge is as follows:
$N^{3-} < O^{2-} < F^{-} < Na^{+} < Mg^{2+} < Al^{3+}$
Nuclear charge $= +7, +8, +9, +11, +12, +13$
Therefore,the arrangement of the given species in order of their increasing ionic radii is as follows:
$Al^{3+} < Mg^{2+} < Na^{+} < F^{-} < O^{2-} < N^{3-}$