Question

Explain the trend in the bond dissociation enthalpies of halogen molecules. Why melting and boiling points of halogens increase on moving down the group?

Answer 1

1. The bond dissociation enthalpies of halogen molecules decrease down the group with increase in atomic size.
2. However, the bond dissociation enthalpy of fluorine is lower than that of chlorine and bromine because the F – F bond is weak.
3. The bond dissociation enthalpies of the halogen molecules decreases in the order,
   Cl - Cl > Br - Br > F - F > 1 - 1.
4. Van der Waals forces between molecules get stronger as one moves down the halogen group due to the increasing atomic size of these molecules. These forces are weaker in the lighter elements, F and Cl, which is why they are gases at room temperature and have low melting and boiling temperatures.