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Q.
What is common between given cations and anions, $O^{2-},\, F^{-},\, Na^{+},\, Mg^{2+},\, Al^{3+}$ ?
Classification of Elements and Periodicity in Properties
Solution:
$O$ and $F$ are in period 2 while $Na , Mg$ and $Al$ are in period 3 .
Number of electrons in $O ^{2-}=8+2=10, F ^{-}=9+1=10,\, Na ^{+}=11-1=$
$10,\, Mg ^{2+}=12-2=10,\, Al ^{3+}=13-3=10$
Given ions have same number of electrons thus are isoelectronic species.
Thus the order of atomic radii is: $O ^{2-}> F ^{-}> Na ^{+}> Mg ^{2+}> Al ^{3+}$
All are isoelectronic species having 10 electrons.