The prediction of the detailed molecular structure (including bond angles) is not as simple as shown in Table 1. In molecules with either lone pair electrons or multiple (double or triple) bonds the angles about the central atom are distorted due to the increased electron repulsion (Figure 3). The differences in repulsion caused by a lone pair or a bonding pair may be rationalized in a simple manner by a lone pair taking up more space than a bonding pair (Figure 4).

Figure 3: The order of decreasing repulsion between non-bonding (unshared) and bonding electron pairs.

Figure 4: Representation of the relative space taken up by (a) a lone pair of electrons and (b) a bonding pair of electrons.

Water is one of the classic cases in considering the issue of non-bonding (unshared) electron pairs.

From this an idealized tetrahedral geometry would give the H-O-H angle as 109.5°, however, from Figure 3 we know that the lone pair^...lone pair repulsion is greater than the lone pair^...bonding pair repulsion which is greater than the bonding pair^...bonding pair repulsion, and thus, the H-O-H angle should be decreased from the ideal tetrahedral. The experimentally determined H-O-H angle in water is in fact 104.5°.

Ethylene is a good case in considering the issue of multiple bonds. Ethylene contains both σ-bond and π-bond between the carbon atoms. This combination can be thought of as a super bond, and as such its effect is similar to a lone pair.

From this an idealized tetrahedral geometry would give the H-C-H angle as 120°, however, the π-bond repulsion is greater than the σ-bond repulsion, and thus, the H-C-H angle should be decreased from the ideal tetrahedral. The experimentally determined H-O-H angle in water is in fact 118.3°.

If a molecule or ion has two or more resonance forms it is necessary to consider each form before angles are predicted. For example, the carbonate anion, CO₃^-, can be drawn as a single structure from which it would be predicted that two groups of O-C-O angles would result (Figure 5).

Figure 5: VSEPR predicted structure of a single resonance form of CO₃^2-.

However, CO₃^- should actually be drawn in each of its resonance forms as in Figure 6.

Figure 6: Resonance forms of CO₃^2-.

From Figure 6 it is clear that the real structure will be an average of the three resonance forms, and hence there will be a single O-C-O angle = 120° (Figure 7).

Figure 7: Structure of CO₃^2-.

In an A-X bond where the atom electronegativities are very different the bonding pair is assumed to occupy less space than in bond between two atoms of similar electronegativities. As the bonding pair occupies less space it will repel neighboring electron pairs less.

For example, based on the above, a comparison of the tetrahedral compounds H₂O and F₂O would suggest that the F-O-F angle be smaller than the H-O-H angle since fluorine has a higher electronegativity than hydrogen (4.0 and 2.1, respectively). This is indeed observed (Figure 8).

Figure 8: Structures of H₂O and F₂O.

In many cases, inter-ligand steric interactions can also be used to explain the difference in angle. For example, while fluorine is more electronegative than chlorine (4.0 versus 3.0, respectively) it is also significantly smaller (Table 2). Thus, a larger Cl-X-Cl angle than a F-X-F angle in the homologous compound, can be attributed to a greater steric interactions, rather than difference in electronegativities.

There is the potential for more than one isomer for molecules that adopt structures in which there is a symmetry difference between at least two of the ligand positions. For example a trigional bipyramidal compound of the formula PXY₄ has two possible structures. One where the X occupies an axial position (Figure 9a) and the other where it occupies an equatorial position (Figure 9b).

Figure 9: The two possible structures of PXY₄.

Through the consideration of structures Henry Bent suggested a rule: More electronegative substituents prefer hybrid orbitals with less s character, and conversely, more electropositive substituents prefer hybrid orbitals with greater s character.

For example, in PFCl₅ the fluorine is the most electronegative substituent and will therefore occupy the axial (p-character only) position.