Question

Which one of the following orders is correct for the bond dissociation enthalpy of halogen molecules?
(A). ${I_2} > B{r_2} > C{l_2} > {F_2}$
(B) $C{l_2} > B{r_2} > {F_2} > {I_2}$
(C) $B{r_2} > {I_2} > {F_2} > C{l_2}$
(D) ${F_2} > C{l_2} > B{r_2} > {I_2}$

Answer 1

The bond dissociation enthalpy is the measure of the energy which is required in an endothermic process to cause the cleavage of a chemical bond to form two atomic or molecular fragments which are usually the radical species.

Complete step by step answer:
The bond dissociation energies of the halogen family decrease as we go down in the group. This is because the size of the atoms increases on moving down the group and we already know that the bond dissociation energy is inversely proportional to the bond length. And bond length does increase with the increase in the size of the atom. All in all, we can say that, higher is the size of atoms, higher will be the bond length and thus less will be the bond dissociation energy.
Therefore, the order of increasing bond dissociation enthalpy among halogens should be:
${I_2} < B{r_2} < C{l_2} < {F_2}$
But this is not the case, as the bond dissociation enthalpy of fluorine is however lower than those of the chlorine and bromine because of the interelectronic repulsions present in the small atom of fluorine.
Therefore, the correct order of the bond dissociation energy of halogens is: $C{l_2} > B{r_2} > {F_2} > {I_2}$.
Hence, the correct option is (B).

Additional information:
Fluorine has a small atomic size and in fluorine $\left( {{F_2}} \right)$, $2p$ subshell is compact and close to the nucleus. Due to small atomic size, a number of electrons are held in a compact volume and there is strong repulsion among non-bonded electrons. Hence bond becomes weak though bond is short.

Note:
Electronegativity increases as we move from left to right and decreases as we move from top to down in the periodic table. And as the differences in electronegativity of the two bonded atoms decreases the bond dissociation enthalpy also decreases.