Intra-molecular bonds or inter-atomic bonds are true chemical bonds and occur within molecules. i.e. covalent, polar covalent, dative covalent, ionic, and metallic bonds. On the other hand, intermolecular bonds or forces occur between molecules and keep the liquid and solid particles together in these states. Intermolecular forces are much weaker and act over greater distances than intra-molecular bonds. Intramolecular and intermolecular bonds 1

The forces keeping atoms together in molecules, ions in ionic bonds and atoms in metallic bonds have already been described previously. Called inter-atomic forces or bonds. The question that now needs to be answered is: What keeps the molecules or atoms together in a liquid or a solid and prevents them from changing into gases? Intermolecular forces 2

The attracting forces between molecular units are called intermolecular forces. Two groups:1. forces between ions and molecules. 2. forces between molecules. The forces between molecules are called van der Waals forces, of which there are 3 types: 3

Ion-dipoleIon-induced dipole Dipole-dipole Dipole-induced dipole Momentary dipoles The last 3 are called van der Waals forces London or momentary dipoles 4

Types and examples of intermolecular forces: 1. Ion – dipole (NaCl in H 2 O). 2. Ion - induced dipole (NaCl in CCl 4 ). 3. Dipole - dipole or Keesom forces (H 2 S in H 2 O). 4. Dipole - induced dipole or Debye forces (H 2 S in CCl 4 ). 5. Momentary dipoles, or London, or dispersion forces (I 2 ). Stronger forces involving ions Weaker, van der Waals forces 5

1. Ion – dipole force Atom or neutral molecule 2. Ion - induced dipole force NaCl in H 2 O NaCl in CCl 4 Ion dipole forces 6

3. Dipole - dipole 5. Momentary dipoles 4. Dipole – induced dipole H 2 S – H 2 O I 2 – I 2 H 2 S - CCl 4 Visualizing van der Waals forces 7

Properties: - weak forces of attraction between molecules/particles - they act through distances that are greater than intra-molecular distances (bond length). - they are exerted in all directions. 8

Van der Waals Weakest Strongest Intermolecul ar forces 9

Special case of dipole-dipole bonding: Hydrogen bonded covalently to small atoms (e.g. F, O, N) of high electronegativity, forms polar bonds with relatively large positive and negative charges. Why would these molecules form particularly strong dipole-dipole bonds? These are stronger dipoles than the van der Waals dipoles. 10

There are three good examples of hydrogen bonding: H 2 O, NH 3 & HF. The dotted line represents the hydrogen bond. Hydrogen bonding in water 11

Hydrogen bond HF hydrogen bonds 12

Positioning of water molecules - each surrounded by 5 – 7 molecules Positioning of ice molecules – each bonded to 4 molecules 13

Water vapour molecules much further apart – easy to compress. Intermolecular forces summary 14

The different intermolecular forces & bond lengths determine the different physical properties of solids & liquids. If molecules have van der Waals forces between the particles, the larger the molecule, the greater the intermolecular force. This means higher Melting and Boiling points as well as various other physical properties. Intermolecular forces & properties of solids, liquids & gases 15

Depends upon type, strength and length of the intermolecular forces: PropertyBrief description Vapour pressurePressure of evaporated particles Surface tensionEnergy reqd. to increase surface area SolubilityAbility of solvent to dissolve solute M.P. & B.P.Temp. solids melt & V.P. = atm. Press. CapillarityAbility to flow against gravity ViscosityAbility to pour or flow DensityMass per unit volume Thermal conduction Ability to conduct heat Thermal conductivity Viscosity Capillary action 16

Forces of cohesion - forces between molecules of the same kind in a liquid. Forces of adhesion - intermolecular forces between the liquid molecules and the other substance with which it is in contact – usually the container. Water has greater adhesive forces, while Hg has greater cohesive forces Cohesive & adhesive forces 17

Intermolecular forces: Non-polar liquids (e.g. CCl 4 ) - London forces. Polar liquids (e.g. ethanol) - dipole-dipole forces. Hydrogen bonds (H bonded to O, F or N). Molten ion compounds - ion-ion forces. Molten metals - metallic bonds. Salt & sugar dissolving in water Like dissolves like 18

Comparing intermolecular forces of several liquids: H 2 O has hydrogen bonds compared with van der Waals forces in others Intermolecular forces and boiling points 19

Evaporation & boiling: Evaporation occurs at the surface of a liquid. Escaping H 2 O molecules exert pressure on surface. This pressure is called vapour pressure. As temp increases, evaporation rate increases. When vapour pressure = atmospheric pressure, evaporation can occur below surface – liquid is boiling. Vapour pressure and boiling points 20

Molecular solids - weak van der Waals forces (e.g. iodine and dry ice) or hydrogen bonds (e.g. ice). Low melting points, soft, often sublime easily. Network- or macromolecular solids - atoms which are covalently bonded (e.g. diamond). Ionic solids - ions which are bonded in a crystal lattice by strong coulomb forces (NaCl). Metals - positive atomic kernels surrounded by negatively charged “sea” of electrons (Zn). Intermolecular forces and solids 21

Solids have the following characteristics: Crystals: Have regular, flat sides & fixed angles. Allotropes: Different crystalline forms of same element. Polymorphs: Different crystal forms of same compound. Crystal lattice: Orderly arrangement of particles. Unit cell: Smallest unit of crystal form. Polymorphs Allotropes of carbon Growing crystals 22

Graphite & diamonds Metallic bonding 23

I 2 Diamond NaCl Copper Different structures in crystals 24