Intermolecular forces 12/10/99 There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces We have already looked at intramolecular bonds (ionic, covalent, polar covalent, metallic) Intermolecular forces (IMF) have to do with the attraction between molecules (versus the attraction between atoms in a molecule, metal, or compound) The three IMFs are: 1) H-bonding, 2) dipole - dipole, and 3) London forces (in order of decreasing strength)

Dipole - Dipole attractions 12/10/99 We have seen that molecules can have a separation of charge This happens in both ionic and polar bonds (the greater the ΔEN, the greater the dipoles) H Cl + – Molecules are attracted to each other in a compound by these positive and negative forces + – + – + – + –

Hydrogen - bonding 12/10/99 H-bonding is a special type of dipole - dipole attraction that is very strong It occurs when F, O, or N are bonded to H They are given a special name (H-bonding) because compounds containing these forces are important in biological systems

London forces 12/10/99 Non-polar molecules do not have dipoles like polar molecules. How, then, can non-polar compounds form solids or liquids? London forces (also called van der Waal forces) are due to small dipoles that exist in non-polar molecules Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical The resulting tiny dipoles cause attractions between atoms/molecules (the greater the mass, the greater the London forces)

Summary 12/10/99 The bigger the IMF’s, the more energy it takes to overcome them . . . . . . thus The bigger the IMF’s, the higher the melting and boiling point Bonds are stronger than IMF’s, so the melting and boiling points of covalent network solids (i.e.. diamond), ionic compounds (i.e.. salts), and metals (i.e.. iron) are much higher than molecular substances (i.e.. water)

IMF Intramolecular are stronger. A covalent bond is 100x stronger. 12/10/99 IMF Intramolecular are stronger. A covalent bond is 100x stronger. The molecules gather together as liquids or solids at low temperatures. London forces Are present in all compounds Can occur between atoms or molecules Are transient in nature (dipole-dipole are more permanent). London forces are weaker

12/10/99 a) O2 would be lower because it is smaller in mass. London attractions are smaller and consequently lower melting/boiling points. b) O2 because it has only London forces. NO has a is polar, giving it small dipoles. C8H18 would have the higher melting/boiling point. This is a result of the stronger London forces a large difference in electronegativity and the small sizes of (FON) atoms.