Intermolecular Forces

Intermolecular Forces: an attraction between

molecules caused by opposite charges

Intermolecular forces are: • electrostatic in nature • formed between the positive and

negatively sides of a molecule. • weaker than actual bonds

(covalent, ionic) • temporary they help “stick”

molecules to one another

Three Types of Intermolecular Forces

–London Dispersion –Dipole-Dipole –Hydrogen Bonding

London Dispersion Forces • Weakest intermolecular forces • temporary movement of electrons

causing temporary attraction of “mini-dipoles” (- & + side)

He atom temporary movement to create attraction

• Present among all molecules, but only are significant in nonpolar molecules

Dipole-Dipole

• Still a very weak force, but stronger than London dispersion

• Found between polar molecules- with one positive and one negative side

• The positive side of one molecule attracts the negative of another molecule

Dipole-Dipole - A Short Visual (Not on note sheet)

• One end of a molecule is permanently positive and one end is permanently negative

• Opposites attract

δ+ δ+ δ - δ -

Dipole-Dipole - Another Short Visual (Not on note sheet)

• A quick visual of what this might look like…

• Notice there is not a regular pattern of the molecules - the attraction is not strong enough to hold the molecules in a fixed position.

Hydrogen Bonding

• A weak force of attraction, but strongest of the intermolecular forces

• Occurs when the most electronegative elements – F, O, and N– are directly bonded to H.

• High electronegative elements attract electrons so strongly that the molecule becomes very polar

• Think of it as a dipole –dipole interaction but stronger!

Hydrogen Bonding – A Short Visual (not on note sheet)

• The very polar molecule has a very negative end and a very positive end.

δ-

δ + δ -

δ + • The negative

electron pairs are attracted to the positive hydrogen.

O

H H

O

H H

Hydrogen bonds lead to higher boiling points!