Walsh diagrams

Walsh diagrams show the change of molecular orbital energies with a variable. Below is a diagram of how the orbital energies of a XH₂ molecular change with bond angle:

Non-valance orbitals are typically unaffected by bond angle and therefore excluded.

Walsh’s rule for predicting shapes of molecules states that a molecule will generally adopt a structure that provides the most stability for its HOMO (highest order molecular orbital). If a particular structural change does not perturb the HOMO, the closest occupied molecular orbital governs the shape.

For example, gas phase BH₂ has four valance electrons, which would go into the 1σ_g and 1σ_u molecular orbitals. Using the above diagram we can that the HOMO is most stable at 180°.

Water has eight valance electrons, you should be able to tell from the above diagram that it would be bent towards 90°.