Question

The bond dissociation energy is highest for

• A. Cl₂
• B. I₂
• C. F₂
• D. Br₂

Hint:

When comparing bond dissociation energies:
• Consider bond length: shorter bonds generally have higher bond energy.
• Check for lone pair-lone pair repulsions, which can weaken bonds, as seen in F₂.

Answer 1

The Correct Option is A

 To determine which diatomic halogen molecule has the highest bond dissociation energy, we should consider the bond strength and atomic size of the halogens: fluorine (F₂), chlorine (Cl₂), bromine (Br₂), and iodine (I₂).

1. The trend of bond dissociation energy in halogens is generally influenced by atomic size and bond strength. As the atomic size increases, the bond distance increases, which usually results in a weaker bond. Consequently, the bond dissociation energy decreases with increasing atomic size in a group.
2. Among the given options, the sizes of the atoms increase in the order: F < Cl < Br < I.
3. In F₂, despite being the smallest atom, the bond is unusually weak compared to Cl₂ due to significant electron-electron repulsion caused by the small size of fluorine, which adversely affects bond strength.
4. Cl₂ has a significantly stronger bond than F₂ because the optimal bond length minimizes repulsion, making the bond dissociation energy of Cl₂ the highest among the halogens listed.
5. Br₂ and I₂ have larger atomic sizes resulting in weaker bonds and thus lower bond dissociation energies compared to Cl₂.

Therefore, among the choices given, the bond dissociation energy is highest for chlorine, Cl₂.

Molecule	Bond Dissociation Energy (kJ/mol)
F2	158
Cl2	242
Br2	193
I2	151

Based on the above reasoning and data, the correct answer is Cl₂.