JEE Main · 2010 · 11 Apr 2010 · Offline · Classification of Elements and Periodicity

The correct sequence which shows decreasing order of the ionic radii of the elements is

(A) $Al^{3 +} > Mg^{2 +} > Na^{+} > F^{-} > O^{2 -}$
(B) $Na^{+} > Mg^{2 +} > Al^{3 +} > O^{2 -} > F^{-}$
(C) $Na^{+} > F^{-} > Mg^{2 +} > O^{2 -} > Al^{3 +}$
(D) $O^{2 -} > F^{-} > Na^{+} > Mg^{2 +} > Al^{3 +}$ ✓ Correct

Answer: (D)

Solution

All five ions carry ten electrons, so they are isoelectronic and we compare them by nuclear charge. With the same electron count a higher nuclear charge pulls the cloud in more tightly and makes the ion smaller. The charges rise through

O^{2 -} (8) < F^{-} (9) < N a^{+} (11) < M g^{2 +} (12) < A l^{3 +} (13)

, so the sizes fall in the same order:

O^{2 -} > F^{-} > N a^{+} > M g^{2 +} > A l^{3 +}

JEE Main · 2010 · 11 Apr 2010 · Offline · Single correct option

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JEE Main · 2010 · 11 Apr 2010 · Offline · Classification of Elements and Periodicity

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