# 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 XeF_{4} and CF_{4} have different shapes even though the central atom is coordinated by four Fluorine atoms in both cases?

**Demo:**

- Ball and Stick Models of XeF
_{4}and CF_{4}Structures

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, BeCl_{2} 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.

**Demo:**

- Show that 4 balloons tied together adopt tetrahedral arrangement.
- Show that 3 balloons tied together adopt trigonal planar arrangement.
- Show that 2 balloons tied together adopt linear arrangement.

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

- Draw the Lewis Structure.
- 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). - 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 BF**_{3}?The Lewis Dot Structure for BF

_{3}is:There are only three atoms around B and no lone pairs so we use a Trigonal Planar arrangement.

The name of structure for BF

_{3}is also Trigonal Planar.**What's the structure of SO**_{2}?The Lewis Dot Structure of SO

_{2}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.

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

**What's the structure of XeF _{4}?**

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 XeF_{4}, ignoring the lone pair electrons is "square planar".

**Finally, what's the structure of ICl _{2}^{-} ?**

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 ICl_{2}^{-} 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 CO_{2} will combine into an overall zero net electric dipole moment vector for the CO_{2} molecule:

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

#### Homework from Chemisty, The Central Science, 10th Ed.

9.3, 9.6, 9.9, 9.11, 9.13, 9.15, 9.17, 9.19, 9.21, 9.23, 9.25, 9.27, 9.29

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