Vsepr Theory

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ar geometry is determined by how many bonding and non-bonding electron groups surround the central atom, the first thing one needs to do is count how many of each there are. There is a notation that simplifies this bookkeeping:
Valence Shell Electron Pair Repulsion theory,VSEPR theory, starts with the simple axiom that electrons repel each other. Since a bond is a concentration of electron density between two nuclei, they will repel each other also. Bonds and unshared pairs all involve these regions of electron density called domains. If you have a central atom and a set of bonds or unshared electron pairs around it, you would expect them to get as far as possible away from each other to minimize the repulsion. That means the angle between say any two bonds needs to be as large as practical to minimize the energy of the molecule.
The steps to applying VSEPR theory to a molecule are:

Draw a Lewis structure for the molecule.
Determine the number of domains around the central atom.
Determine the geometry and hybridization around the central atom.
Determine the bond angles and shape of the molecule.


Consider the molecule BeCl2. Figure 1 is the Lewis structure. Note that there are no unshared pairs on Be and only two single bonds. Therefore there are two domains around Be. The best geometry for two domains is shown in figure 2. In order for the two domains to be as far as possible from each other, they must be on opposite sides in a line. This is called the linear geometry. The bond angle is 180o.

How is this linear geometry achieved? The beryllium atom has two valence electrons in a 2s orbital. To form two bonds we need two orbitals with one electron each. We can promote one of the 2s electrons to a 2p orbital to get that. However if we form sigma bonds to chlorine...