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JEE Main levelSingle correct optionDifficulty 2 of 3pt077

The correct increasing order of ionic radii for the isoelectronic ions $O X^{2 -}$ , $F X^{-}$ , $Na X^{+}$ and $Mg X^{2 +}$ is:

A $Mg X^{2 +} < Na X^{+} < O X^{2 -} < F X^{-}$
B $O X^{2 -} < F X^{-} < Na X^{+} < Mg X^{2 +}$
C $Mg X^{2 +} < Na X^{+} < F X^{-} < O X^{2 -}$
D $Na X^{+} < Mg X^{2 +} < F X^{-} < O X^{2 -}$

Answer: C

For isoelectronic ions the radius falls as the nuclear charge rises:

Z = 12, 11, 9, 8

for

Mg X^{2 +}, Na X^{+}, F X^{-}, O X^{2 -}

. So the radii increase in the order

Mg X^{2 +} < Na X^{+} < F X^{-} < O X^{2 -}

.

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