Editing VSEPR theory (section)

===Degree of repulsion===
The overall geometry is further refined by distinguishing between ''bonding'' and ''nonbonding'' electron pairs. The bonding electron pair shared in a [[sigma bond]] with an adjacent atom lies further from the central atom than a nonbonding (lone) pair of that atom, which is held close to its positively charged nucleus. VSEPR theory therefore views repulsion by the lone pair to be greater than the repulsion by a bonding pair. As such, when a molecule has 2 interactions with different degrees of repulsion, VSEPR theory predicts the structure where lone pairs occupy positions that allow them to experience less repulsion. Lone pair–lone pair (lp–lp) repulsions are considered stronger than lone pair–bonding pair (lp–bp) repulsions, which in turn are considered stronger than bonding pair–bonding pair (bp–bp) repulsions, distinctions that then guide decisions about overall geometry when 2 or more non-equivalent positions are possible.<ref name=Petrucci/>{{rp|410–417}} For instance, when 5 valence electron pairs surround a central atom, they adopt a ''trigonal bipyramidal'' molecular geometry with two collinear ''axial'' positions and three ''equatorial'' positions. An electron pair in an axial position has three close equatorial neighbors only 90° away and a fourth much farther at 180°, while an equatorial electron pair has only two adjacent pairs at 90° and two at 120°. The repulsion from the close neighbors at 90° is more important, so that the axial positions experience more repulsion than the equatorial positions; hence, when there are lone pairs, they tend to occupy equatorial positions as shown in the diagrams of the next section for steric number five.<ref name=Miessler/>

The difference between lone pairs and bonding pairs may also be used to rationalize deviations from idealized geometries. For example, the H<sub>2</sub>O molecule has four electron pairs in its valence shell: two lone pairs and two bond pairs.  The four electron pairs are spread so as to point roughly towards the apices of a tetrahedron.  However, the bond angle between the two O–H bonds is only 104.5°, rather than the 109.5° of a regular tetrahedron, because the two lone pairs (whose density or probability envelopes lie closer to the oxygen nucleus) exert a greater mutual repulsion than the two bond pairs.<ref name=Petrucci/>{{rp|410–417}}<ref name=Miessler/>

A bond of higher [[bond order]] also exerts greater repulsion since the [[pi bond]] electrons contribute.<ref name=Miessler/> For example, in [[isobutylene]], (H<sub>3</sub>C)<sub>2</sub>C=CH<sub>2</sub>, the H<sub>3</sub>C−C=C angle (124°) is larger than the H<sub>3</sub>C−C−CH<sub>3</sub> angle (111.5°). However, in the [[carbonate]] ion, {{chem|CO|3|2−}}, all three C−O bonds are equivalent with angles of 120° due to [[resonance (chemistry)|resonance]].