L7 Qualitative Molecular Orbital Theory Principles

Approximations for electronic energies that ignore electronic repulsion are bad. How do we improve them

The orbital approximation includes electornic interactions within the Hamiltonian

Using matrices for LCAO

Why does He₂ not form

The stabilisation of the in phase combination is smaller than the out of phase destabilisation

What is the overlap S_AB and exchange integral H_AB

Overlap - essentially just how much orbitals overlap. Is very small, is 1 at full overlap

The exchange integral is dependent on the overlap integral unless the internuclear distance is too small.

What does the size of an orbital drawn on an orbital mixing diagram mean

The degree of contribution c_X with larger sizes meaning closer energy and more contribution. The electron density will mostly be on this side (think about how electronegative atoms have lower energy orbitals).

Why do atomic orbitals on the same atom not overlap (mathematically)

They involve multiplying S and AS which integrates to zero

Why are core orbital interactions ignored

Core orbitals have much lower energy than valence orbitals and as such do not overlap enough to be non-negligible.

What symmetry needs to be fulfilled for orbitals to overlap

Their symmetry has to be the same

LiH orbital interactions diagram

The Li orbitals mix thanks to the H orbital. You can see that the combination at the bottom contains p character.