Which of the following represents the correct order of ionic radii?

Which of the following represents the correct order of ionic radii?
  1. $\mathrm{Al}^{3+}>\mathrm{Mg}^{2+}>\mathrm{Na}^{+}>\mathrm{O}^{2-}>\mathrm{F}^{-}$
  2. $\mathrm{O}^{2-}>\mathrm{F}^{-}>\mathrm{Na}^{+}>\mathrm{Mg}^{2+}>\mathrm{Al}^{3+}$
  3. $\mathrm{Mg}^{2+}>\mathrm{Al}^{3+}>\mathrm{O}^{2-}>\mathrm{F}^{-}>\mathrm{Na}^{+}$
  4. $\mathrm{O}^{2-}>\mathrm{F}^{-}>\mathrm{Al}^{3+}>\mathrm{Mg}^{2+}>\mathrm{Na}^{+}$

Solution

$\mathrm{InO}^{2-}>\mathrm{F}^{-}>\mathrm{Na}^{+}>\mathrm{Mg}^{2+}>\mathrm{Al}^{3+}$ All these ions have neon configuration $1 s^2, 2 s^2, 2 p^6$ but different nuclear charges, that is why, they are isoelectronic species (different species having the same number and configuration of electrons). To understand the decrease in radius from $\mathrm{O}^{2-}$ to $\mathrm{Al}^{3+}$. The ionic radii of isoelectronic species increases, with a decrease in magnitudes of nuclear charge. Nuclear charge $=+8,+9,+11,+12,+13$

Asked in: AP EAMCET 2018 (22 Apr Shift 2)

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