London Dispersion Forces

• A nonpolar molecule is a molecule that has a symmetrical shape.

• A nonpolar molecule is a molecule in which the electrons are usually able to distribute themselves in a balanced manner.

• A nonpolar molecule is a molecule with NO permanent positive or permanent negative end.

London Dispersion Forces

C

H

H

HHH H

OO C

London Dispersion Forces

• As the electrons move about the molecule, they are usually spread out in a balanced manner.

• This balanced arrangement of the electrons causes the molecule to have no permanent positive or negative end.

London Dispersion Forces

• How do molecules without permanent positive and negative ends attract each other?

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London Dispersion Forces

• In their random movements, the electrons sometimes are NOT distributed in a balanced manner around the molecule!

London Dispersion Forces

• When this happens, a temporary polar molecule is formed!

δ+ δ-

• London dispersion forces are weak intermittent forces between nonpolar molecules.

• A temporary polar molecule will cause neighbor-ing nonpolar molecules to become temporary polar molecules.

δ+ δ- δ+ δ-

London Dispersion Forces

• The forces that hold the individual atoms in CO2 molecules together are covalent bonds.

• The forces that hold CO2 molecules to each other in a sample of dry ice are London dispersion forces.

OO C= =

OO C= =

OO C= =

OO C= =

OO C= =

OO C= =

OO C= =

OO C= =

OO C= =

London Dispersion Forces

• Generally, the more massive the nonpolar molecule is, the stronger the London dispersion forces will be and the stronger the London dispersion forces is, the higher the boiling point will be!

H2---H2 O2---O2 CO2---CO2 MM = 2 g/mol MM = 32 g/mol MM = 44 g/mol

bp = 20 K bp = 90 K bp = 195 K

Cl2---Cl2 Br2---Br2 MM = 71 g/mol MM = 160 g/mol

bp = 238 K bp = 332 K

London Dispersion Forces