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NEET Chemistry QUESTION #7387
Question 1
Among the isoelectronic species \(\text{O}^{2-}\), \(\text{F}^-\), \(\text{Na}^+\), \(\text{Mg}^{2+}\) and \(\text{Al}^{3+}\), their ionic radii show which trend?
  • An increase from \(\text{O}^{2-}\) to \(\text{F}^-\) then decrease from \(\text{Na}^+\) to \(\text{Al}^{3+}\)
  • A decrease from \(\text{O}^{2-}\) to \(\text{F}^-\) then increase from \(\text{Na}^+\) to \(\text{Al}^{3+}\)
  • A significant overall increase from \(\text{O}^{2-}\) to \(\text{Al}^{3+}\)
  • A significant overall decrease from \(\text{O}^{2-}\) to \(\text{Al}^{3+}\)✔️
Correct Answer Explanation
Isoelectronic species all have 10 electrons. Their size depends on nuclear charge:

\(\text{O}^{2-}(Z=8) > \text{F}^-(Z=9) > \text{Na}^+(Z=11) > \text{Mg}^{2+}(Z=12) > \text{Al}^{3+}(Z=13)\)

More protons = stronger pull on same electron cloud = smaller radius. There is a steady significant decrease across the series. No increase occurs — all species have the same number of electrons but increasing nuclear charge.