[Ng Jun Wei]

I recently came across this question in my chemical bonding tutorial.
The bond energies for the following bonds are as below
H-H : 436 kJ/mol
P-P  :208 kJ/mol
P-H :322 kJ/mol
Cl-Cl :244 kJ/mol
H-Cl :431 kJ/mol
The bond energy of P-H is the mean (average) of H-H and P-P values. Explain why the H-Cl bond energy is not the mean of H-H and Cl-Cl values?
My opinion:
Has it got to do with the electronegativity difference between the 2 elements as P and H are relatively similar whereas H and Cl are quite drastically different?

[Babcock_Hall]

This is outside of my area of expertise, but I believe that you are correct.  Pauling's electronegativity scale correlated electronegativity differences with bond strengths IIUC.

[Enthalpy]

The mean value is an exception. If it were always the case, then reactions wouldn't release nor absorb heat.

So I don't see why it needs a justification. P-H is just one lucky value.

Or are there deeper reasons? Cl-Cl is weak because the molecule has too many electrons.  The best bond is N≡N; O=O has to accommodate two electrons more on antibonding orbitals, Cl-Cl four, which makes it reactive. But the same reasoning is difficult for P-P which is only a part of a more complex molecule.