1Induced dipole-dipole forces

2Permanent dipole-dipole forces

3Hydrogen bonds

Induced dipole-dipole forces are also called london (dispersion) forces, or van der Waals' forces.

Weak intermolecular forces that exist between all molecules.

1. Movement of electrons produces a temporary dipole in a molecule.
2. The temporary dipole induces a dipole on a neighbouring molecule.
3. The induced dipole induces further dipoles on neighbouring molecules, which then attract one another.

1. Molecular size - Larger molecules have more electrons, which can be more easily polarised to create stronger temporary dipoles.
2. Molecular surface area - Molecules with larger surface areas have more of their electron cloud exposed, allowing for greater interactions with neighbouring molecules.

Physical properties, such as melting and boiling points.

Electrostatic forces of attraction that occur between the partial positive end of one polar molecule and the partial negative end of another.

Only the weak intermolecular forces (and not the strong covalent bonds).

A special type of permanent dipole-dipole force found between molecules containing an electronegative atom with a lone pair of electrons and a hydrogen atom attached to an electronegative atom.

Highly electronegative atoms such as oxygen, nitrogen, or fluorine, and hydrogen atoms directly attached to them.

Hydrogen bonds

The open lattice structure of ice, formed by hydrogen bonds, holds water molecules slightly apart and decreases the density of ice compared to liquid water.

Each water molecule can form 2 hydrogen bonds due to the 2 lone pairs on the oxygen atom.

A hydrogen bond is represented by a straight dashed line from the lone pair on one molecule to the H atom on another. 

For example, a hydrogen bond between H₂O molecules is represented below.

Water (H₂O), unlike other group 6 hydrides, can form hydrogen bonds; these strong intermolecular forces between water molecules require more energy to overcome than the weaker induced dipole-dipole forces in other group 6 hydrides, resulting in higher melting and boiling points.

Repulsion between electron pairs increases in the order:

Bonded pair - bonded pair   <  Bonded pair - lone pair  <  Lone pair - lone pair

Lone pairs repel more strongly than bonding pairs becuse they are slightly closer to the central atom and occupy more space.

Although the strong covalent bonds between the atoms are very strong, the intermolecular forces between the molecules are weak, so only require a small amount of energy to overcome.

Covalent bonds are much stronger.

Molecules cannot conduct electricity because they have no overall electrical charge.