Please briefly explain why is bond energy defined only for gases? Thanks for your help again.

Answer 1

Bond energy is the energy required to make or break a bond between two atoms. That is the energy required when the molecule is a gas. As a liquid, you must add additional energy to vaporize it before breaking the bond; as a solid you must add additional energy to melt it then to vaporize it before breaking the bond. So "bond" energies for solids or liquids would include other kinds of energy than just for breaking the bonds.