Directed valence

The localized 2c MO  picture rely on hybrid AOs that point in the direction of other atoms and provide directed valence. Combining the s with one p orbital in a valence shell provides two sp hybrids directed at 180° apart. With s the two p orbitals make sp² hybrids directed at 120° in a plane.  These can be employed to explain a trigonal planar molecule  like BF₃. Combining s with all three p orbitals gives sp³ hybrids directed in the direction of the corners of a tetrahedron. These are geometrical arrangements assumed by VSEPR for two, three and four electron pairs, correspondingly. In the 2c MO  explanation of the methane CH₄, each of the sp³ hybrids on carbon is combined  to make a bonding  MO with one hydrogen 1s orbital. The four equal bonding MOs  are occupied by two electrons each.

Nonbonding  electron pairs can also be assumed to occupy hybrids on the central atom. So in ammonia  NH₃, three hybrids on nitrogen are directed in the direction of hydrogen atoms and form bonding combinations.  The fourth one does not overlap with a hydrogen atom and remains nonbonding. There are two bonding MOs and two nonbonding in water H₂O.  The bond angles in these molecules (107° in NH₃, 104.5° in H₂O compared with the perfect tetrahedral angle of 109.5° found in CH₄)  suggest that the hybrids employed for bonding  and nonbonding  MOs  are not quite equal. A smaller bond angle corresponds to more p character and less s in the hybrid. (The angle among pure p orbitals is 90°; the Valence s orbitals are more tightly bound to an individual atom than are p orbitals and so do not contribute as much to bonding  MOs . Alternatively, hybrid AOs with some s character are more strongly directed than are pure p orbitals and so you can overlap more strongly with neighboring atoms. So the degree of hybridization  depends on a balance of factors. H₂O and NH₃ have angles quite close to  the ideal sp³ prediction,  even though  the  bonding  orbitals have little more  p  character and  the  nonbonding  MOs  will  have respectively more s. In PH₃  and H₂S the angles are closer to 90°, displaying that the balance has changed and that bonding MOs are constructed mainly with valence p orbitals with s remaining largely nonbonding.  This fashion can be attributed to the weaker bond strengths (compared with s-p energy separations) for elements lower in a group. The description  of bond angles provided  by VSEPR  is very dissimilar.