Valence Shell Electron Pair Repulsion Theory
Last Update: March 8, 2005
Valence Shell Electron Pair Repulsion (VSEPR) theory is a convenient way to turn a Lewis dot structure into a three-dimensional representation of a polyatomic chemical species in the gaseous state. Because Lewis dot structures do not give a highly accurate model for bonding in most molecules, VSEPR structures are limited in the same ways that Lewis dot structures are limited. In addition, VSEPR only gives an accurate prediction of structure in the gaseous state because there are no interactions between particles in a gas, which can have a significant influence on structure in liquids and solids.
1. Start from the Lewis dot structure and count areas of electron density around the central atom.
In VSEPR a lone pair, a single unpaired electron and any bond (single, double or triple) each count as one area of electron density. For species with more than one “central atom”, treat each “central atom” separately.
2. From the number of areas of electron density around the central atom determine the electron pair geometry.
In VSEPR there are only five electron pair geometries that maximize the distance between areas of electron density. These are given in Table 1.
|Areas of Electron Density||Electron Pair Geometry|
Table 1. VSEPR's electron pair geometries.
3. Place the bonds (and their associated atoms), lone pairs and unpaired electrons around the central atom in the correct electron pair geometry.
There are two special cases:
- •in the trigonal bipyramidal geometry, the lone pairs are always placed in one of the three equatorial positions, and
- •in the octahedral geometry when there is a single lone pair, it is always shown in the bottom “axial” position by convention, and when there are two lone pairs they are placed across from each other in the “axial” positions.
4. Convert the electron pair geometry to the molecular structure (more commonly referred to simply as the structure) by omitting the lone pairs and unpaired electrons.
NOTE! Omit does not mean that we redraw the structure (this is the most common error in converting the electron pair geometry to an actual structure). The placement of all bonds and atoms does not change! We, in essence, go from the electron pair geometry to the structure by covering up the lone pairs and draw everything that remains in exactly the same position as in the electron pair geometry.
There are several new structures that derive from the presence of lone pairs in electron pair geometries. They are shown in Table 2.
Where does It come from?
|Bent||from a trigonal planar electron pair geometry and one lone pair, or a tetrahedral electron pair geometry and two lone pairs|
|Trigonal pyramidal||from a tetrahedral electron pair geometry and one lone pair|
|Bisphenoid/See-saw||from a trigonal bipyramidal electron pair geometry and one lone pair|
|T-shaped||from a trigonal bipyramidal electron pair geometry and two lone pairs|
|Square pyramidal||from an octahedral electron pair geometry and one lone pair|
|Square planar||from an octahedral electron pair geometry and two lone pairs|
Table 2. Structures arising from electron pair geometries due to the presence of lone pairs.
A linear structure can be obtained from some electron pair geometries because of the presence of lone pairs. These cases are: a tetrahedral electron pair geometry with three lone pairs, and a trigonal bipyramidal electron pair geometry with three lone pairs.
5. Predict or Explain Deviations from Ideal Structures.
VSEPR can also be used to predict how a polyatomic species will distort relative to the idealized structure. However, it is best used as a tool to explain why a given structure is distorted rather than as a predictive tool. In explaining why the distortions occur we need to balance three competing influences:
- •lone pairs take up more space than bonds. Repulsion between electrons in a lone pair and the other electrons (either in bonds or other lone pairs) may cause distortions in the structure.
- •triple bonds are fatter than double bonds, which are fatter than single bonds. The amount of electron-electron repulsion experienced between a bond and lone pairs or other bonding pairs decreases in the order: triple > double> single. Therefore, we expect that a triple bond may cause more distortions in the structure than either a double or single bond and that a double bond will cause more distortion when compared to a single bond.
- •bonds which involve a significant difference in electronegativity between the atoms in the bond will have the electrons in the bond distorted toward the more electronegative atom. This will decrease electron density near the central atom and lessen the repulsion between this bonding pair and other electron pairs in the molecule.