Which of the following orders is correct for the size? (1) $\mathrm{Mg}^{2+}…
Which of the following orders is correct for the size?
(1) $\mathrm{Mg}^{2+} < \mathrm{Na}^{+} < \mathrm{F}^{-} < \mathrm{Al}$
(2) $\mathrm{Te}^{2-}>\mathrm{I}^->\mathrm{Cs}^{+}>\mathrm{Ba}^{2+}$
(3) $\mathrm{Fe}^{3+} < \mathrm{Fe}^{2+} < \mathrm{Fe}^{4+}$
(4) $\quad \mathrm{Mg}>\mathrm{Al}>\mathrm{Si}>\mathrm{P}$
$(1),(2) \&(4)$
$(2),(3) \&(4)$
(1), (3) \& (4)
$(1),(2),(3) \&(4)$
Solution
(1) $\mathrm{Mg}^{2+}, \mathrm{Na}^{+}$ and $\mathrm{F}^{-}$ are isoelectronic and thus follows the order $_{12} \mathrm{Mg}^{2+} < _{11} \mathrm{Na}^{+} < _{\mathrm{q}} \mathrm{F}$
Al belongs to third period and has no charge so it is largest. $\mathrm{Na}^{+}=102 \mathrm{pm}, \mathrm{Mg}^{2+}=72 \mathrm{pm}$, Al $=143 \mathrm{pm}, \mathrm{F}^{-}=133 \mathrm{pm}$
(2) Isoelectronic series of ions, all have the Xenon electron configuration
Ionic radius $\propto \frac{1}{\text { muclear charge }}$
Atomic number $\mathrm{Te}=52, \mathrm{I}=53, \mathrm{Cs}=55, \mathrm{Ba}=56$
(4) Across the period the muclear charge increases and thus the size of atoms decreases.
$\mathrm{Mg}=160 \mathrm{pm}, \mathrm{Al}=143 \mathrm{pm}, \mathrm{Si}=118, \mathbf{P}=110 \mathrm{pm}$
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