Consider the following species: $N^{3-}, O^{2-}, F^{-}, Na^{+}, Mg^{2+}, Al^{3+}$
$(a)$ What is common in them?
$(b)$ Arrange them in the order of increasing ionic radii.

(N/A) All these ions have the same number $(10)$ of electrons. Therefore,these are called isoelectronic species.
$(b)$ Since the number of electrons is the same,the ionic size decreases with an increase in nuclear charge. Therefore,the ions can be arranged in increasing order of ionic radii as $Al^{3+} < Mg^{2+} < Na^{+} < F^{-} < O^{2-} < N^{3-}$.