Intermolecular Forces
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
London Dispersion Forces • Weakest intermolecular forces • temporary movement of electrons
causing temporary attraction of “mini-dipoles” (- & + side)
He atom temporary movement to create attraction
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