[Shadow]

How is the H-F bond stronger than H-H bond? In H-H two orbitals of the same symmetry,energy and size interact while in H-F it's not the case.

[Irlanur]

what do you mean by stronger bond? what's you're argument that it shouldn't be the case?

[Shadow]

More energy is needed to break it. The overlap in H₂ should be perfect as two same orbitals interact, while the F p orbital is different than H s orbital, so the overlap should be worse.

[Irlanur]

I guess you never looked at MO-theory in any quantitative way?

The overlap in H2 should be perfect as two same orbitals interact

what should "perfect" mean? they are obviously not the same, they are in different positions.

After all, I can guess what you mean. but why do you directly conclude that the bond should be stronger? after all, fluor has a different nuclear charge.

[Corribus]

Overlap isn't the only thing that determines stabilization energy in MO theory. In fact it's only one of three integrals that matters, the others being Coulomb interaction and electron exchange function.

[AdiDex]

I think..

Other than relative size of orbitals ( which are overlapping ) there is one more factor..(And in this case the size of orbitals is almost same)
The bond energy also depends upon the the fact that how much electrons are closer to the atom .
And the atomic size of hydrogen is 53 pm and fluorine's size is 42 pm ,
In case of HF the electrons are more closer to the nucleus as the fluorine is smaller and main thing is that fluorine is most ELECTRONEGATIVE atom..

[AdiDex]

Hey Shadow ,

Do you agree with me .? ?

[Shadow]

I don't like it. I would like to learn more about electron exchange function.

[AdiDex]

Shadow ,
I have a question ,

Is there any mistake in my concept .
As it look logical the more electrons approaches towards the nuclei , the sytem will release more energy and it get stablized .
If there is any mistake so please correct me..!!

[Irlanur]

I don't like the electronegativity argument because it has no obvious connection with MO-theory. it's a phenomenological and intuitive explanation when MO theory should be able to explain this issue without such things (it does, we will get there).

shadow: what you probably think is the following: in H2, the interaction between the orbitals is very strong, or i another way, one orbital is raised in energy really high and one is lowered. in HF on the other hand, the orbitals of H and F are not influenced very much because they are very different in energy and "volume" (a really bad term in MO-theory). But that's only half of the story. the "strength" of the bond is the difference in energy between the bonded and the non-bonded state. this is obviously much more in HF.

[Shadow]

Ok.

[AdiDex]

Why the difference between bonding and nonbonding Molecular orbitals is greater in case of HF .

[Irlanur]

because the fluorine orbitals are much lower in energy from the beginning.

[AdiDex]

That means hydrogen's orbital are higher in the energy than that of fluorine's orbitals...??

[Shadow]

Yes, they are.