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

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

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

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

• Al³⁺ < Mg²⁺<Na⁺<F^-(increasing ionic size)

B.

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

The electron gain enthalpy is defined as the energy released when an electron is added to a neutral gaseous atom to convert it into a negative ion. Large negative values of electron gain enthalpy indicate that it is easier to add an electron to an atom to convert it into an anion, and hence large amount of energy is released. The electron gain enthalpies in halogens decreases in the order
Cl>F>Br>I

because as we proceed down a group because the size of the atom increases. Therefore the distance of the nucleus from the valence orbital (which accepts the additional electron) increases. Hence the force by which the additional electron is attracted by the nucleus also decreases and the electron gain enthalpies become less negative as we proceed down a group.