Question

Which one of the following ions has the highest value of ionic radius ?

• A. $O^{ 2 - }$
• B. $B^{ 3 + }$
• C. $Li^+$
• D. $F^-$

Answer 1

Answer: (A)

$O^{ 2 - }$

Answer 2

$O^{2-}$ has the highest value of ionic radius and this can be explained on the basis of Zle ( nuclei charge/no. of electrons) ratio.
When Z/e ratio increase the : size decreases and where Zle ratio decreases, the size increase
$L i^{+}(Z=3): 1 s^{2}$
$F^{-}(Z=9): 1 s^{2} 2 s^{2} 2 p^{6}$
$B^{3+}(Z=5): 1 s^{2}$
$O^{2-}(Z=8): 1 s^{2} 2 s^{2} 2 p^{6}$
The correct order of ionic radii is
$B^{3+} < Li^{+} < F^{-} < O^{2-}$
For isoelectronic species, more the nculear charge, smaller the size.
Further $Li^{+}$ and $B^{3+}$ have only one shell and as such smaller than $F^{-}$and $O^{2-}$ when have two shells.