3.10. The VSEPR Theory

How can you determine a molecule's structure?

"VSEPR" = Valence Shell Electron Pair Repulsion

• Each electron pair in a molecule is repelled by the other pairs.
• Maximal distance between the areas with high electron density ⇒ structure!

Method

1. Count the total number of valence electrons + extra charges.
2. Divide by two (to get the number of pairs).
3. Connect the atoms with single bonds.
4. Place electron pairs so that all ligands have noble gas structure.
5. Move bonds to the central atom, so that all atoms have noble gas structure.
6. Electron pairs repel each other ⇒ conclusion about the molecule’s structure.

Example 1: Methane

1. Number of valence electrons

• C: 4
• H: 4 × 1 = 4
• In total: 8

2. Divide by two

• Number of electron pairs: 8/2 = 4 pairs

3. Connect the atoms with single bonds

Below, the Lewis dot structure for methane is shown.

4–5. Nobel gas structure + move bonds

The four hydrogen atoms are connected to the carbon atom with covalent bonds.

• All the atoms already have noble gas structure.

6. Conclusion about the molecule's structure

Four areas with high electron density ⇒ The hydrogen atoms end up in the corners of a tetrahedron.

The structure of a methane molecule is tetrahedrical.

Example 2: Carbon dioxide, CO₂

1. Number of valence electrons

• C: 4
• O: 6 × 2 = 12
• Totalt: 16

2. Divide by two

• Number of electron pairs: 16/2 = 8 pairs

3. Connect the atoms with single bonds

The carbon atom is placed centrally, the oxygen atoms are ligands:

4. Place the rest of the valence electrons

The electrons are placed so the ligand atoms get noble gas structure:

5. Move electron pairs

The carbon atom also gets noble gas structure:

6. Conclusion about the molecule's structure

Two areas with high electron density (the double bonds) ⇒ 180° between the bonds, the molecule is linear.

The carbon dioxide molecule is linear.

Example 3: Beryllium chloride, BeCl₂

1. Number of valence electrons

• Be: 2
• Cl: 7 × 2 st = 14 st
• Totalt: 16

2. Divide by two

• Number of electron pairs: 16/2 = 8 pairs

3. Connect the atoms with single bonds

The beryllium atom is placed centrally, the chlorine atoms are ligands:

4. Place the rest of the valence electrons

The electrons are placed so the ligand atoms get noble gas structure:

5. Move electron pairs

The carbon atom also gets noble gas structure:

6. Conclusion about the molecule's structure

Two areas with high electron density (the double bonds) ⇒ 180° between the bonds.

• Just like the CO₂-molecule, BeCl₂ is linear.

Example 4: Sulfur dioxide, SO₂

1. Number of valence electrons

• S: 6
• Cl: 6 × 2 st = 12
• Totalt: 18

2. Divide by two

• Number of electron pairs: 18/2 = 9 pairs

3. Connect the atoms with single bonds

The sulfur atom is placed centrally, the oxygen atoms are ligands:

4. Place the rest of the valence electrons

The electrons are placed so the ligand atoms get noble gas structure.

• Six of the remaining electron pairs are placed on the oxygen atoms.
• The last electron pair is placed on the sulfur atom:

5. Move electron pairs

The sulfur atom also gets noble gas structure:

6. Conclusion about the molecule's structure

Three areas with high electron density (including the non-binding electron pair on the sulfur atom) ⇒ 120° (approx.) between the bonds.

• The SO₂-molecule is bent.

The sulfur dioxide molecule is bent.

Exercises

Use the VSEPR theory to determine the structures of each of the following molecules: