Valence Shell Electron Pair Repulsion Theory (VSEPR)

In Chapter 2, we learned about the detailed internal structure of an atom’s electron cloud, including how electrons inhabit regions of space called atomic orbitals.

VSEPR theory ignores orbitals, and focuses on the big picture. Basically, it says two things:

• Covalent bonds and lone pairs both represent “charge clouds” where negative charge is concentrated.

• To determine the 3-D structure of a molecule, assume that all the charge clouds will array themselves so they are as far apart as possible.

H HO

Valence Shell Electron Pair Repulsion Theory (VSEPR)

To build a 3-D model based on VSEPR theory:

1. Draw the Lewis dot structure for the molecule.

2. Identify the number of charge clouds present on all “central” atoms.

• Bonds (regardless of order) count as 1 cloud.

• Lone pairs also count as 1 cloud.

O S O

Sulfur is the only central atom in SO2

1 lone pair2 double bonds+

TOTAL: 3 charge clouds

3. Pick the 3-D shape that separates clouds as much as possible—this is your electronic geometry.

4. Now, just consider the shape of the atoms alone—this is your molecular shape.

O S O SOO

SOO

Electronic Geometry:Trigonal Planar

Molecular Shape:Bent

Lone pairs and bond angles

The angle between two adjacent bonds about a central atom is called a bond angle.

SOO

O C O

180°

Lone pairs take up more space around the central atoms than bonded groups. As a result, they “squash” the bonded groups together, whenever there is an asymmetric distribution of lone pairs.

BF

F

F

120°120°

120°

> 240°

< 120°

VSEPR: Determining a 3-D shape based on charge clouds

Charge Clouds

Electronic Geometry Bond Angles # Bonding

Groups# Lone Pairs Molecular Shape Example

2 180° 2 0 Linear

3

120° 3 0 Trigonal Planar

< 120° 2 1 Bent

BF3

CO2

SO2

Charge Clouds

Electronic Geometry Bond Angles # Bonding

Groups# Lone Pairs Molecular Shape Example

4

109.5° 4 0 Tetrahedral

< 109.5° 3 1 Trigonal Pyramidal

< 109.5° 2 2 Bent

VSEPR: Determining a 3-D shape based on charge clouds

CH4

NH3

H2O

Charge Clouds

Electronic Geometry Bond Angles # Bonding

Groups# Lone Pairs Molecular Shape Example

5

120°and 90° 5 0 Trigonal

Bipyramidal

< 120°and < 90° 4 1 See-Saw

< 90° 3 2 T-Shaped

180° 2 3 Linear

VSEPR: Determining a 3-D shape based on charge clouds

SF4

PF5

ClF3

I3–

VSEPR: Determining a 3-D shape based on charge clouds

Charge Clouds

Electronic Geometry Bond Angles # Bonding

Groups# Lone Pairs Molecular Shape Example

6

90° 6 0 Octahedral

90°and < 90° 5 1 Square

Pyramidal

90° 4 2 Square Planar

BrF5

SF6

XeF4

Practice with VSEPR

Determine the electronic geometry and molecular shape for the following compounds. If there is more than one central atom, the molecular shape is determined for each central atom individually.

BH3 PCl3 SeO2 IF5

HCN ICl3 CH2Cl2 XeF4

CHALLENGE ROUND:

NH2COOH C2H4O (there are two possible isomers!)

Polarity of molecules

If a molecule contains polar bonds, it may have a dipole moment.

electron-richregion

electron-poorregion

d+ d-• Existence/Lack of a dipole moment is very important to molecular properties

• VSEPR can predict if a molecule is polar or not:

Dipoles don’t cancel:Molecule is polar.

Dipoles cancel:Molecule is nonpolar.

Polarity of molecules

Molecules containing net dipole moments are polar molecules: Polar molecules are unsymmetric

Molecules containing no overall dipole moments and are nonpolar molecules: Nonpolar molecules are symmetric

For all molecules (octet and exceptions), polar molecules contain:

§ A central atom bonded to more than one type of element (ex: CH2Cl2)

AND/OR

§ A molecule with an unsymmetric geometry

Polarity of molecules (and shapes): Practice

Predict if these molecules (drawn earlier in these lectures) are polar or nonpolar. (Also, practice finding molecular shapes again!)

BH3 PCl3 SeO2 IF5

HCN ICl3 CH2Cl2 XeF4

CHALLENGE ROUND:

NH2COOH C2H4O (there are two possible isomers!)