VSEPR
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Last edited: 2011-11-03 09:44 AM

Readings for this section.

Petrucci: Section 10-7

Valence Shell Electron Pair Repulsion Theory

Molecular Geometries

NOTE All the coloured pictures have java-enabled rotatable models available.  Click on the image to open the page containing the java applet.

Experimental evidence clearly shows us that the Lewis Model of molecular bonding, while having it's merits is far from complete. Take for example the molecule Chlorofluoromethane (CH₂FCl) If we draw the Lewis Dot Structure for this molecule, we get one of two possibilities:

These structures seem to show that there are two different versions of this molecule, one in which the chlorine is adjacent to the fluorine and one where it is across from it. Experimental evidence shows us that there is only one molecule with the formula CH₂FCl. At the same time evidence points to the fact that the molecule CH₂FCl has two distinct forms with different physical properties (optical properties). There must be further theories that can explain these observations.

It turns out that the flat representations produced in the Basic Lewis structures are the problem. Molecules are not generally flat but exist in three dimensions.

or

Can you superimpose these?

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VSEPR

The Valence Shell Electron Pair Repulsion Theory (VSEPR), as it is traditionally called helps us to understand the 3d structure of molecules. Although we will speak often of electron pairs in this discussion, the same logic will hold true for single electrons in orbitals, and for double bonds, where one could think of the bond as consisting of two pairs of electrons. In general, the region in space occupied by the pair of electrons can be termed the domain of the electron pair. The domain is related to the orbitals we have discussed earlier (and will elaborate on later) but the two do not necessarily refer to the same thiing.

The general concept is that the pairs of electrons repel each other and wish to locate themselves as far as possible from each other about a given nucleus. Hence, for two pairs of electrons on a nucleus, the two pairs would locate themselves exactly opposite each other, forming a bond angle of exactly 180º. If three pairs exist, they will locate themselves in a plain about the nucleus at angles of 120º from each other. higher numbers of electrons form 3d arrangements as follows.

Table: Geometry and Electron Pair Arrangements   The angles given are the ideal angles for such an arrangements.

Electron Pairs	e– pair (domain) Geometry	e– pair diagram
2	Linear
3	Trigonal planar
4	Tetrahedral
5	Trigonal bipyramidal
6	Octahedral

Examples:

We'll now go through a set of example molecules and/or ions and discuss their geometries. It is important to note that the shape of the molecules as we discuss them here is not always the same as the electron domain geometries described above.

We will consider the molecular shapes, starting with the simplest and working up to the more complicated examples. In addition, I'll mention a classification system which may be helpful in counting electron domains used herein.

The classification system follows:

A represents a central atom (any atom being considered. It need not actually be the centre of the molecule)
X represents an atom bonded to A. It could be single, double or triple bonded. It makes no difference to the scheme.
E represents a non-bonded electron domain (lone pair).

For example, methane CH₄ is an AX₄ molecule while ammonia NH₃ is an AX₃E molecule. Both of these molecules have four electron domains and hence would have a tetrahedral domain geometry as listed above. However, the shape of the molecules are not the same as we will see below.

2 pairs of electrons

BeCl2

LINEAR

This molecule is linear. The Be does not fill its octet shell in this situation. To do so would put a large negative charge on it and a positive charge on the Chlorine atoms. This would simply not happen since Cl is so much more electronegative than Be.

3 pairs of electrons

BF3

Trigonal Planar shape

4 electron pairs

CH4		AX4	tetrahedral
NH3		AX3E	trigonal pyramidal
H2O		AX2E2	bent or angular
HF		AX1E3 or just AXE3	linear

Note that in all these cases, the electron-domain geometry is tetrahedral. However, the molecular shape is not always so. In the case of CH₄, the molecule is actually tetrahedral in shape with a perfect Tetrahedral angle of 109.5º. The next two examples have lone pairs which occupy a larger domain volume (push more on the bonding pairs) and reduce the bond angle to less than 109.5º. The last case, HF, is simply a liner diatomic molecule. There is no bond angle.

5 electron pairs

PCl5		AX5	trigonal bipyramidal
SF4		AX4E	See Saw or disphenoidal
ClF3		AX3E2	T-shaped
XeF2		AX2E3	Linear

In these cases, the electron-domain geometry is always trigonal bipyramidal. However, only the first molecule is that shape with the ideal angles of 90 and 120 degrees for the axial and equatorial bonds, respectively.

In the case of SF₄, there is one lone pair and four bonding pairs. The lone pair will preferentially locate itself in an equatorial position since that position has only two other pairs of electrons within 90 degrees while an axial position would have three. Thus, the molecular would be see-saw shaped or the more technically correct name, disphenoidal. The bond angles would be less than the ideal angles of 90 and 120 degrees.

ClF3 has two lone pairs and they both locate themselves in equatorial positions for the same reasons as described in the previous case. This molecule is T-shaped with bond angles of less than 90 degrees.

6 electron pairs

SF6		AX6	Octahedral
ClF5		AX5E	Square Pyramidal
XeF4		AX4E2	Square Planar

In all these cases, the electron-domain geometry is octahedral and in the case of SF₆, so is the shape. The molecule ClF₅ has one lone pair and five bonding pairs but since all positions in the octahedral geometry are equivalent, it doesn't matter which position the lone pair takes. I drew it on the bottom position here for visual effect. In the case of XeF₄, the two lone pairs will locate themselves on opposite sides of the square planar molecule. In the case of the XeF₄ molecule, the lone pairs will orient themselves in a square plane and the molecule will be linear in shape.

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Prof. Michael J. Mombourquette.
Copyright © 1997
Revised: November 03, 2011.

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