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Question: Answered & Verified by Expert
The ions $\mathrm{O}^{2-}, \mathrm{F}^{-}, \mathrm{Na}^{+}, \mathrm{Mg}^{2+}$ and $\mathrm{Al}^{3+}$ are

isoelectronic. Their ionic radii show:
ChemistryClassification of Elements and Periodicity in PropertiesBITSATBITSAT 2013
Options:
  • A A decrease from $\mathrm{O}^{2-}$ to $\mathrm{F}^{-}$ and then increase from $\mathrm{Na}^{+}$ to $\mathrm{Al}^{3+}$
  • B A significant increase from $\mathrm{O}^{2-}$ to $\mathrm{Al}^{3+}$
  • C A significant decrease from $\mathrm{O}^{2-}$ to $\mathrm{Al}^{3+}$
  • D An increase from $\mathrm{O}^{2-}$ to $\mathrm{F}^{-}$ and then decrease from $\mathrm{Na}^{+}$ to $\mathrm{Al}^{3+}$
Solution:
2114 Upvotes Verified Answer
The correct answer is: A significant decrease from $\mathrm{O}^{2-}$ to $\mathrm{Al}^{3+}$
Amongst isoelectronic species, ionic radii of anion is more than that of cations. Further size of anion increase with increase in -ve charge and size of cation decrease with increase in $+$ ve charge. Hence ionic radii decreases from $\mathrm{O}^{-}$ to $\mathrm{Al}^{3+}$

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