Valence Shell Electron Pair Repulsion Theory (VSEPR) is a qualitative theory that allows us to predict the geometry of molecules based upon their Lewis structures.
This model looks at the repulsion of different electron regions (either bonding or non-bonding) and chooses a geometry that minimizes their repulsions.
For more practice with the VSEPR theory and VB Hybridization see the worksheet on the worksheet page. This page also has a very helpful sheet on electronic and molecular geometries.
In VSEPR theory, we are interested in the geometry around a particular atom typically referred to as the central atom. The goal is to determine the geometry of the atoms that are bonded to this central atom. To determine this, we must first determine the electronic geometry. The electronic geometry is an idea of where the regions of electron density are surrounding the central atom. A region of electron density can either be bonding or non-bonding. Electrons in a covalent bond between two atoms are counted as one region regardless of whether it is a single, double, or triple bond (or anything in between). Non-bonding regions are counted by counting the number of lone pairs on the central atom.
The electronic geometry is determined by the geometry that minimizes the repulsions between these regions (moves them as far apart from each other as possible). This geometry depends on the number of regions. If there are two regions, the farthest apart they can get form each other is to be on opposite sides of the central atom. As the two regions and the central atom would now all be in a line, we call this electronic geometry linear. Similarly, the farthest apart we can get three regions is if each is 120° apart from each other forming a triangular shape in the same plane. We call this geometry trigonal planar. Four regions leads to a tetrahedral geometry, five a trigonal bipyramid, and six an octahedral geometry.
We can then use the electron geometry to determine the molecular geometry (where the nuclei are).
The molecular geometry is what we actually want to know about a molecule. Where are the atoms! The electronic geometry is an idea. The molecular geometry is something we can probe in the lab.
To get the molecular geometry, we simply have to take the electronic geometry and look at how many electron regions are bonding and how many are non-bonding.
If there are no lone pairs (non-bonding regions), then the molecular geometry and the electronic geometry are the same. If there are lone pairs, then we need to give the molecular geometry a new name based on where the atoms are located.
For example, for a tetrahedral geometry if there is one lone pair (like ammonia), then we call this geometry trigonal pyramidal because the atoms sit at the corners of a pyramid that has triangular sides. If the electronic geometry is tetrahedral and there are two lone pairs (like water), then the atoms are arranged in an angular or bent shape that we creatively call "angular" or "bent."
When the central atom has a Lewis structure that is an expanded octet, there can be 5 or 6 regions of electron density. This leads to a lot of possible geometries depending on the number of lone pairs. The following videos look at the expanded octet geometries.
When a molecule has a dipole we call it a polar molecule. In order for a molecule to have a dipole there are two key criteria. First, the molecule must have some polar bonds. Second, the dipoles created by these bonds must not cancel out as a result of the symmetry of the molecule. This is one of our key interests in the shape of the molecule. We would like to know if certain regions within the molecule have a higher electron density than other regions. This will strongly affect both how the molecule interacts with other molecules as well as the molecule's chemistry.
Let's look at a couple of key examples to demonstrate this idea. One of the most important chemical substances in the world is water. The properties of water derive in many ways from the means in which the electrons are distributed in the molecule. Water consists of a central oxygen atom covalently bonded to two hydrogen atoms. Using the VSEPR model we would deduce that water has a bent structure. To examine if water is polar, we need to examine the bonds within the molecule. We would classify the O-H bonds in water as polar covalent bonds since oxygen is significantly more electronegative than hydrogen. This means that each of these bonds has a dipole that arises from the partial positive charge on the hydrogen and the partial negative charge on the oxygen. To see if the overall molecule has a dipole, we need to look at the geometrical arrangement of these dipoles. Because water is bent, when we add up the dipoles of the bonds, we find that there is a net dipole. That is, essentially water has a positive side (where the hydrogens are) and a negative side (the oxygen end). The dipole is symmetric in the "middle" of the molecule since each "side" with a hydrogen is symmetric. This is most easily seen in the diagram of the dipole for water. The net (or molecular dipole) is shown in green.
In contrast to this example, carbon dioxide does not have a molecular dipole despite the fact that it has polar covalent bonds. This is a direct result of the geometry of the molecule. The oxygens in the molecules have a greater electron density and thus have a partial negative charge. This makes the carbon oxygen double bonds polar. However, since the molecule is linear the two dipoles from the two bonds in the molecule are pointing in exactly opposite directions. As a result, the dipoles cancel and the overall molecule has no net dipole.
Having gone through the expanded octet you have now seen All the VSEPR geometries.
These ideas are summarized on this very helpful chart from Dr. Paul McCord......
A copy of the chart can be downloaded below.
Electronic-and-Molecular-Geometries-Help-Sheet
Looking for more? Check out the site under "links" on the top menu. For example this is a nice site from the University of Sheffield on VSEPR .