Which of the following order of radii is correct?

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$H^{+} < {Li}^{+} < H^{-}$

$Li < Be < Mg$

${Na}^{+} > F^{-} > O^{2 -}$

$O < F < Ne$

answer is B.

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Detailed Solution

As we move across the period, ionic radii decrease as $e^{-}$ are filled in the same shell thereby increasing effective nucleus charge and resulting in larger attraction. Hence, the size (radii) decreases.

As we move down the group, ionic radii increase as they are $e^{-}$ added to the new shell due to which the effective nuclear charge decreases and thus the size increases.

Also, cations are smaller in size than the anions as in cations' number of $p + >$ no. of $e^{-}$ due to which $e^{-}$ are more strongly pulled towards the nucleus thereby decreasing the size.

Anions are bigger in size as no. of $p + <$ no. of $e^{-}$ , so the $e^{-}$ pull by the nucleus is less hence bigger size.

∴ $H^{+} < {Li}^{+} < H^{-}$ is the correct order.

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