Every covalently bonded atom possesses at least one valence electron pair. These valence electrons may be bonding pairs or non-bonding pairs. For example, when their valence shells have been filled by covalent bonding with other atoms hydrogen possesses one valence pair; carbon has four valence pairs; oxygen also has four valence pairs (two bonding and two non-bonding pairs).
The valence bond theory took deep root in chemistry. The molecular orbital theory developed by Muliken has many advantages and simplicities in providing satisfactory explanations on which valence bond theory has failed. Nevertheless, both the theories have been nourished and used as complementary to each other.
What is the VSEPR Theory?Back to Top
VESPR theory is used to predict molecular shape by counting the total number of valence electron pairs surrounding the central atom of the molecule and selecting the corresponding geometric arrangement. It is through the arrangement that the valence pairs present can orient themselves as far apart from one another as possible.
Valence shell electron pair repulsion theory focuses on the bonding and non-bonding electrons in the valence shell of the central atom and the role they play in determining the molecular shape. The three dimensional shapes of molecules and polyatomic ions are the result of the orientation of atoms about the central atom.
How VSEPR Works?Back to Top
The VSEPR method works very well in predicting the molecular geometry. The molecules containing three or more atoms assume a definite shape around the central atom, such as linear, trigonal planar, square planar, trigonal bipyramidal, square pyramidal, octahedral etc. The shapes of the molecule are determined by the following two factors.
- Electron tends to form pairs with the electron of the opposite spin.
- Electron pairs tend to repel each other. The electron pairs therefore arrange themselves as far apart as possible.
Electron pairs are of two types.
- Bond pair (bp) - electrons which actually take part in covalent bond formation.
- Lone pair(lp) - electrons which do not take part in the bond formation.
According to VSEPR theory two electron pairs have stayed as far apart as possible since like charges repel each other. This arrangement of electron pairs (orbitals) in shape minimises the force of repulsion between them.
VSEPR Theory - Lone PairsBack to Top
Presence of lone pair of electron on central atom gives rise to more repulsion in lone pair - bond pair electrons and thereby contraction in bond angles due to repulsion between lone pair of electrons and bond pair os electrons. The repulsion and contraction in bond order follows.
L.P-L.P > L.P-B.P > B.P-B.P
For example consider NH3.
Since N has three unpaired electrons and thus it can form three sigma bonds; Also excitation of electrons in higher d sub shell is not possible since the 2nd shell does not have d orbitals. That is why N does not show +5 covalence like its other group members P, As etc.
Furthermore if all the three p orbitals are involved in sigma bond formation with 1s orbital of hydrogen the bond angles will be at 90o where as NH3 has a bond angle of 106o51'. Thus it has been proposed that N atoms in NH3 is sp3 hybridized to show pyramidal geometry or tetrahedral nature with one position occupied by a lone pair of electron. The presence of the lone pair of electron on the N atom contracts the bond angle from 109o28' to 106o51'.
VSEPR Theory Bond AnglesBack to Top
The bond angles better indicates the three dimensional shape of a species. When lone pairs replace bonding in the VSEPR model, the geometry changes slightly to account for the increased repulsion between the non bonding electron pairs. This becomes apparent when we examine the bond angles of different molecules.
The bond angles of some of the molecules and their expected geometry are tabulated below.
|Number of electron domains
||Arrangement about central atom
||Geometry of domain arrangement
||Expected bond angle
VSEPR Theory Shapes of MoleculesBack to Top
The VSEPR theory is used to predict geometry so that the shapes of molecules can be predicted. By combining the principle that valence electron pairs want to get as far apart as they can with the fact that the lone pairs repel more strongly than do bonding pairs, we can predict the molecular shapes. The total number of valence electron pairs determines the the overall geometry around a central atom. The distribution of these pairs between bonding and nonbonding orbitals adjusts that geometry. So the VSEPR theory can predict several shapes that appear over and over in real life molecules.
The VSEPR Theory Chart explaining the shapes of molecules are tabulated below.
|Number of electron pairs||Electron pair geometry||Bonding pairs||Lone pairs||Molecular shape|
Applying the VSEPR TheoryBack to Top
- VSEPR theory proposes that the structure of a molecule determined by the repulsive interaction of electron pairs in the valence shell of its central atom.
- It is used to predict possible shapes of molecules. It is based on the premise that a covalent molecule takes up the shape in which the repulsion between the bonding and the lone pair is a minimum.
- It predicts the geometry by assuming that electron domains - regions of space that contain bonding electrons, unpaired valence electrons or lone pairs - stay as far apart as possible from each other, while staying as close as possible to the central atom.
- The application of VSEPR theory to triatomic molecules is exemplified by considering water, carbon dioxide, xeon difluoride and a ratio of connected species.
ExamplesBack to Top
According to VSEPR theory the molecule adopts the shape in which the repulsion is less. Some of the examples are shown below.
1. Shape of SF4
In SF4 molecule, there are five electron pairs which should lead to trigonal bipyramidal structure. However, out of the five electron pairs, one is a lone pair. This lone pair could occupy an axial position or an equatorial position. The actual shape of SF4 molecule is
Hence according to VSEPR theory the preferred shape SF4 molecule is the one in which lone pair lies in the equatorial position.
2. Shape of NH3 molecule
This molecule has three bonding electron pairs and one lone pair of electrons. These four electron pairs are directed towards the four corners of a tetrahedron. But because one of three electron pairs is a lone pair, the shape of the NH3 molecule is distorted from tetrahedral geometry and assumes trigonal pyramid shape.