Periodic Classification and Periodic Properties - Result Question 51
####48. Arrange the following in order of their
(i) decreasing ionic size $Mg^{2+}, O^{2-}, Na^{+}, F^{-}$
(ii) increasing first ionisation energy $Mg, Al, Si, Na$
(iii) increasing bond length $F _2, N _2, Cl _2, O _2$
$(1985,3 M)$
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Solution:
- (i) $Mg^{2+}, O^{2-}, Na^{+}$and $F^{-}$are all isoelectronic, has 10 electrons each. Among isoelectronic species, the order of size is cation $<$ neutral $<$ anion.
Also, between cations, higher the charge, smaller the size and between anions, greater the negative charge, larger the size. Therefore, the decreasing order of ionic radii : $O^{2-}>F^{-}>Na^{+}>Mg^{2+}$
(ii) First ionisation energy increases from left to right in a period. However, exception occur between group 2 and 13 and group 15 and 16 where trend is reversed on the grounds of stability of completely filled and completely half-filled orbitals. Therefore, Ionisation energy (1st) : $Na<Al<Mg<Si$
(iii) If the atoms are from same period, bond length is inversely proportional to bond order. In a group, bond length is related directly to atomic radius. Therefore,
bond length $N _2<O _2<F _2<Cl _2$
