The premise of VSEPR is that the valence electron pairs surrounding an atom mutually repel each other and will therefore adopt an arrangement that minimizes this repulsion thus determining the molecular geometry The number of electron pairs surrounding an atom both bonding and nonbonding is called its

VSEPR theory is usually compared and contrasted with valence bond theory which addresses molecular shape through orbitals that are energetically accessible for bonding Valence bond theory concerns itself with the formation of sigma and pi bonds Molecular orbital theory is another model for understanding how atoms and electrons are assembled into molecules and polyatomic ions

VSEPR theory has long been criticized for not being quantitative and therefore limited to the generation of "crude" even though structurally accurate molecular geometries of covalent molecules However molecular mechanics force fields based on VSEPR have also been developedVGS Box Journal of Molecular Modeling 1997 3, 124-141

The number of electron pairs in the valence shell of a central atom is determined by drawing the Lewis structure of the molecule expanded to show all lone pairs of electrons alongside protruding and projecting bonds Where two or more resonance structures can depict a molecule the VSEPR model is applicable to any such structure For the purposes of VSEPR theory the multiple electron pairs in a multiple bond are treated as though they were a single "pair"

These electron pairs are assumed to lie on the surface of a sphere centered on the central atom and since they are negatively charged tend to occupy positions that minimizes their mutual electrostatic repulsions by maximising the distance between them The number of electron pairs therefore determine the overall geometry that they will adopt

For example when there are two electron pairs surrounding the central atom their mutual repulsion is minimal when they lie at opposite poles of the sphere Therefore the central atom is predicted to adopt a

This overall geometry is further refined by distinguishing between

This distinction becomes important when the overall geometry has two or more non-equivalent positions For example when there are 5 electron pairs surrounding the central atom the optimal arrangement is a trigonal bipyramid In this geometry two positions lie at 180° angles to each other and 90° angles to the other 3 adjacent positions whereas the other 3 positions lie at 120° to each other and at 90° to the first two positions The first two positions therefore experience more repulsion than the last three positions Hence when there are one or more lone pairs the lone pairs will tend to occupy the last three positions first

Based on the steric number and distribution of

Steric No | Basic Geometry 0 lone pair | 1 lone pair | 2 lone pairs | 3 lone pairs |
---|---|---|---|---|

2 | linear | |||

3 | trigonal planar | bent | ||

4 | tetrahedral | trigonal pyramid | bent | |

5 | trigonal bipyramid | seesaw | T-shaped | linear |

6 | octahedral | square pyramid | square planar | |

7 | pentagonal bipyramid | pentagonal pyramid |

Molecule Type | Shape | Electron arrangement^{†} | Geometry^{‡} | Examples |
---|---|---|---|---|

AX_{1}E_{n} | Diatomic | HF O_{2} | ||

AX_{2}E_{0} | Linear | BeCl_{2} HgCl_{2} CO_{2} | ||

AX_{2}E_{1} | Bent | NO_{2}^{−} SO_{2} O_{3} | ||

AX_{2}E_{2} | Bent | H_{2}O OF_{2} | ||

AX_{2}E_{3} | Linear | XeF_{2} I_{3}^{−} | ||

AX_{3}E_{0} | Trigonal planar | BF_{3} CO_{3}^{2−} NO_{3}^{−} SO_{3} | ||

AX_{3}E_{1} | Trigonal pyramidal | NH_{3} PCl_{3} | ||

AX_{3}E_{2} | T-shaped | ClF_{3} BrF_{3} | ||

AX_{4}E_{0} | Tetrahedral | CH_{4} PO_{4}^{3−} SO_{4}^{2−} ClO_{4}^{−} | ||

AX_{4}E_{1} | Seesaw | SF_{4} | ||

AX_{4}E_{2} | Square Planar | XeF_{4} | ||

AX_{5}E_{0} | Trigonal Bipyramidal | PCl_{5} | ||

AX_{5}E_{1} | Square Pyramidal | ClF_{5} BrF_{5} | ||

AX_{6}E_{0} | Octahedral | SF_{6} | ||

AX_{6}E_{1} | Pentagonal pyramidal | XeOF|5}}^{−} ^{2−} | ||

AX_{7}E_{0} | Pentagonal bipyramidal | IF_{7} |

When the substituent (X) atoms are not all the same the geometry is still approximately valid but the bond angles may be slightly different from the ones where all the outside atoms are the same For example the double-bond carbons in alkenes like C

The ammonia molecule (NH

A steric number of seven is possible but it occurs in uncommon compounds such as iodine heptafluoride The base geometry for this is pentagonal bipyramidal

The most common geometry for a steric number of eight is a square antiprismatic geometry Examples of this include the octafluoroxenate ion (XeF) in nitrosonium octafluoroxenate octacyanomolybdate (Mo(CN)) and octafluorozirconate (ZrF)

- Molecular geometry
- Linear combination of atomic orbitals
- Molecular modelling
- List of software for mechanics modeling|Software for molecular modeling]
- Valency interaction formula

- 3D Chem - Chemistry Structures and 3D Molecules
- IUMSC - Indiana University Molecular Structure Center