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Question:

In which of the following options the order of arrangement does not go with the variation of property indicated against it?

Li<Na<K<Rb(increasing metallic radius)

Al3+<Mg2+<Na+<F⁻(increasing ionic size)

B<C<N<O(increasing first ionisation enthalpy)

I<Br<Cl<F(increasing electron gain enthalpy)

Solution:

The electron gain enthalpy of chlorine is more than fluorine because in it the added electron goes to the smaller energy level (n = 2) and hence suffers significant repulsion from the electrons already present in this shell. The first ionisation enthalpy of nitrogen is more than oxygen due to a half-filled p orbital which makes the nitrogen more stable.