to predict the molecular shape for molecules using Valence Shell Electron Pair Repulsion (VSEPR)...

• to predict the molecular shape for molecules using Valence Shell Electron Pair Repulsion (VSEPR) theory.

• shapes to know: – linear– bent– trigonal planar– tetrahedral

Review / Warmup• Draw the lewis dot structure for:

1) CO2 - Carbon Dioxide 2) H2O - Water

3) BF3 - Boron Triflouride 4) CH4 - methane

• Lewis dot structures are good representations of how molecules are put together, but they fail to show the correct three-dimensional shape of molecules.

• The VSEPR theory states that electron pairs repel each other, so molecules acquire shapes that make sure these pairs are as far apart as possible.

What does VSEPR stand for?

• VSEPR = Valence Shell Electron Pair Repulsion

Three step process for determining the shape of a molecule

1. First draw the Lewis dot structure

2. Count the number bonded atoms and lone pairs on the central atom.

3. Select the shape that matches the count for bonded atoms and lone pairs on the central atom.

• CO2 has two bonded oxygens to the central atom.• Each oxygen bonded to the central carbon is

positioned as far apart from each other as possible, giving the carbon dioxide molecule a linear shape.

• The bond angle between each oxygen atom is now 180o.

http://chemconnections.org/VSEPR-jmol/Carbondioxide.html

180o

carbon dioxide Lewis Dot Structure

• Water has two bonded hydrogens and two unshared electron pairs on the central atom.

• The two hydrogen atoms are also repelled by the two lone pairs on the central atom, which give the molecule a bent shape.

• The bond angle between each hydrogen atom is now 104o.

e-

e- e-

e-H2O Lewis Dot Structure

104oH

H

O

http://chemconnections.org/VSEPR-jmol/Sulfurdioxide.html http://chemconnections.org/VSEPR-jmol/Water.html

• Each fluorine bonded to the central boron is positioned as far apart from each other as possible, giving the boron trifluoride molecule a trigonal planar shape.

• The bond angle between each fluorine atom is now 120o.

Lewis Dot structure for boron trifluoride:

B

F

F

F

http://chemconnections.org/VSEPR-jmol/Borontrifluoride.html

• Each hydrogen bonded to the central carbon is positioned as far apart from each other as possible, giving the methane molecule a tetrahedral shape.

• The bond angle between each hydrogen atom is now 109.5o.

Lewis Dot structure for CH4

http://chemconnections.org/VSEPR-jmol/Methane.html

The molecular geometry of a molecule is determined by the number of bonded atoms and lone pairs of electrons around the central atom of that molecule.

The molecular shape of a molecule can be predicted if the number of bonded atoms and lone pairs of electrons around the central atom is known.

# of bonded atoms

# of lone pairs

(central atom)

Molecular Geometry Molecule Bond

Angles

2 0 Linear 180o

# of bonded atoms

# of lone pairs (cent. atom)

Molecular Geometry Molecule Bond

Angles

2 1 Bent 109o

# of bonded atoms

# of lone pairs (cent. atom)

Molecular Geometry Molecule Bond

Angles

2 2 Bent<109o

typically104o

# of bonded atoms

# of lone pairs (cent. atom)

Molecular Geometry Molecule Bond

Angles

3 0 Trigonal Planar 120o

# of bonded atoms

# of lone pairs (cent. atom)

Molecular Geometry Molecule Bond

Angles

4 0 Tetrahedral 109.5o

• to predict the molecular shape for molecules using Valence Shell Electron Pair Repulsion (VSEPR) theory.

• shapes to know: – linear– bent– trigonal planar– tetrahedral

http://chemconnections.org/VSEPR-jmol/index.html