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2.2.4 the valence shell electron pair repulsion (VSEPR) model

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chem modelsofbondingandstructure

Structure 2.2.4 - the valence shell electron pair repulsion model enables the shapes of molecules to be predicted from the repulsion of electron domains around a central atom

an electron domain is the area in which electrons are most likely to be found, bonding and non-bonding

electron pairs are an over-simplification, since molecules often have multiple pairs of shared electrons. electron domain should be used, non-bonding, single, double, or triple pairs. the total number of electron domains around the central atom is what’s important

  • the total number of electron domains around the central atom determines the geometrical arrangement of the electron domains
  • the shape of the molecule is determined by the angles between the bonded atoms
  • non-bonding pairs and multiple bonds cause more repulsion than a bonding pair for these reasons:
    • lone pairs have a higher concentration of charge as they are not shared between two atoms
    • multiple bonds have a higher concentration as they contain two or three pairs of electrons

as a result, molecules with lone pairs or multiple bonds on the central atom have some distortions in the structure that reduce the angle

two domains
  • linear molecule
C#C
C#N
three domains
  • triangular planar electron domain geometry
  • if all 3 domains are bonding, then the molecular geometry will also be triangular planar
[H]B([H])([H])
Cl[Al](Cl)(Cl)
[H][C]([H])=O

there is some distortion in HCOH due to the increased repulsion of the double bond, so it should look more like

the Lewis formula of ozone, has three electron domains around the central oxygen atom. therefore, the electron domain geometry of the molecule is still triangular planar. however, only two of the three electron domains contain bonding electrons. thus, the molecular geometry will be bent or v-shaped instead of triangular planar

[O]=[O]-[O]
four domains
  • tetrahedral electron domain geometry
  • if all domains are bonding, the molecular geometry will be tetrahedral
  • consider bonding pairs otherwise


notice the greater reduction in bond angle in compared to . this is due to the presence of two lone pairs that cause more repulsion

see 2.2.13 molecules with an expanded octet (HL)

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