The problem of determining an accurate molecular geometry is rather complex.
Given an arrangement of atoms with coordinates
the total energy, , of the assembly is a function of these coordinate vectors
and is determined by the electronic structure:
Fortunately, there exists a simple theory for determining molecular geometries at a qualitative level, which is useful for understanding the basic shapes of molecules. This theory is known as the VSEPR theory, which is an acronym for the valence shell electron pair repulsion theory. It is useful for estimating the shapes of molecules for which there is a central atom bonded to several other atoms surrounding it.
The basic principle of the VSEPR theory is that molecular geometry can be predicted based on the notion that electron pairs in molecules tend to repel each other and achieve a maximum separation from each other. This applies both to bonding electrons as well as lone pairs.
In order to use the VSEPR principle, one needs to compute a number
known as the steric number. This is given by
The second number can be determined directly from the Lewis structure.
Once the steric number is known, the basic shape of the molecule is given by the following assignment of shapes:
The basic shapes are shown in the left column of the figure below:
The reason for these assignments will become clear through a study of several examples:
The Lewis diagram for CO is
The Lewis diagram for this cation is was shown earlier.
The steric number of the central chlorine is
Again, this geometry can be seen to achieve a maximum separation between the three pairs of shared electrons around the chlorine.
The Lewis structure for this anion is a resonant form:
Because of the resonant structure, the bond lengths are the same.