Why is the bond energy of Cl2 243 kJ/mol greater than F2 is 159 kJ/mol, the bond energy & Br2 193 kJ/mol?

Answer 1

In general, the bigger the atom, the weaker the bond. This explains why Br-Br is weaker than Cl-Cl. It is reckoned that the lone pairs if F atoms are too repulsive for F-F to be the strongest, which it ought to be! This keeps the F atoms further away from each other than predicted, and the bond weaker than predicted.