Valence Shell Electron Pair Repulsion Model

In the last section we learned about a simple approach called the Lewis-dot structure that gives a good approximation of how the valence electrons are distributed in a molecule. What Lewis-Dot structures do not tell us is the shape of the molecule. For example, why do XeF₄ and CF₄ have different shapes even though the central atom is coordinated by four Fluorine atoms in both cases?

One way to answer these questions about molecular structure is to use a simple approach that builds on the Lewis-Dot structure approach called the Valence Shell Electron Pair Repulsion Model, or VSEPR model.

The idea behind this approach is that the structure around a given atom is determined principally by minimizing electron repulsions. That is, the bonding and non-bonding electrons around a given atom will be positioned as far apart as possible. For example, BeCl₂ has the Lewis Structure.

There are only two pairs of electrons around Be. The arrangement that puts the bonding electron pairs as far apart as possible is a linear arrangement:

...as far apart as possible - a very simple model.

Using this guiding principle let's look at the possible arrangements that arise when there are different numbers of electron domains (i.e., bonding and non-bonding electrons) surrounding an atom.

Given the arrangements above we use the following rules for predicting the geometry around an atom.

VSEPR Rules for Determining Structure

1. Draw the Lewis Structure.
2. Add together the number of atoms bound to the central atom and the number of lone pair electrons and choose the appropriate arrangement. (i.e., linear, triangular planar, tetrahedral, trigonal bipyramidal, or octahedral).
3. Draw the structure, placing the appropriate number of bonds and lone electron pairs (i.e., electron domains) about the central atom according to the chosen arrangement.

   What's the structure of BF₃?

   The Lewis Dot Structure for BF₃ is:

   

   There are only three atoms around B and no lone pairs so we use a Trigonal Planar arrangement.

   

   The name of structure for BF₃ is also Trigonal Planar.

   

   What's the structure of SO₂?

   The Lewis Dot Structure of SO₂ is:

   

   There are only 2 bonds and one lone pair around S; a total of three "domains". Therefore we use a Trigonal Planar arrangement.

   

   However, we write "bent" for the structure because lone pairs don't count for the name of the shape.

   
4. If more that one structure is possible, then minimize repulsions, keeping in mind that:
   1. 90° repulsions > 120° repulsions > 180° repulsions
   2. Lone Pair-Lone Pair repulsions > Bond-Lone Pair repulsions > Bond-Bond repulsions
   3. Lone Pair-Lone Pair repulsions at 90° > Bond-Lone Pair repulsions at 90° > Bond-Bond repulsions at 90°

What's the structure of XeF₄?

The Lewis Dot Structure is

There are 4 bonds + 2 lone pairs → 6 domains → Octahedral Arrangement. Using this arrangement we find there are two possibilities:

A is better, since B has a 90° lone pair-lone pair repulsions, and A has a 180° L.P.-L.P. repulsions. Therefore, the structure of XeF₄, ignoring the lone pair electrons is "square planar".

Finally, what's the structure of ICl₂^- ?

The Lewis Dot Structure is:

Around Iodine we have 2 bonds + 3 lone pairs → 5 domains so we use a trigonal bipyramidal arrangement. We find there are three possibilities for the structure:

A and B have 90° L.P.-L.P. repulsions. C has none, so C is the most likely structure. Therefore the shape of ICl₂^- is linear.

Using the rules above we name the shape using only the positions of atoms, ignoring the lone pair electrons. Below are the names for different geometries.

Net Electric Dipole Moments for Molecules

The net electric dipole moment for a molecule is the vector sum of the electric dipole moments of all its bonds. For example, from the bent structure of a water molecule we can see how the electric dipole moment vectors for the two O-H bonds will combine into an overall electric dipole moment vector for the water molecule:

Whereas, the electric dipole moment vectors for the two C-O bonds in CO₂ will combine into an overall zero net electric dipole moment vector for the CO₂ molecule:

Using our new understanding of molecular geometries we can now predict whether molecules with polar bonds will have a net electric dipole moment.