Intermolecular forces are the forces of attraction and repulsion between interacting particles (atoms and molecules) and are always much weaker than the chemical bonds that hold atoms of a molecule together.
VAN DER WAALS FORCES
Intermolecular attractions hold two or more molecules together. These are weakest chemical forces and can be of the following types.
These are electrostatic attractions between the oppositely charged ends of permanent dipoles. It exists between polar molecules and due to this force, gas can be liquefied.
Dipole-induced dipole attraction
It exists between polar and non-polar molecules.
Dispersion force or London forces
It is an Instantaneous dipole-Instantaneous induced dipole attraction.
It exists among the non-polar molecules like $H_2,O_2,Cl_2$H2,O2,Cl2 etc. in solid or liquid states. Even in atoms or molecules which have no permanent dipole, instantaneous dipoles will arise as a result of momentary unbalances in electron distribution. London forces are extremely short range in action and the weakest of all attractive forces. The London forces increase rapidly with molecular weight, or more properly with the molecular volume and the number of polarizable electrons.
Nitrogen, oxygen and ﬂuorine are the highly electronegative elements. When they are tied to a hydrogen atom to form a covalent bond, the electrons of the covalent bond are shifted towards the more electronegative atom. This partially, positively charges the hydrogen atom forming a bond with the other more electronegative atom. This bond is called a hydrogen bond and is weaker than a covalent bond. For example, in HF molecule, the hydrogen bond exists between the hydrogen atom of one molecule and ﬂuorine atom of another molecule as given below :
Thus, hydrogen bond can be defined as the attractive force which binds the hydrogen atom of one molecule with the electronegative atom (F, O or N) of another molecule. Hydrogen bonding is said to be formed when slightly acidic hydrogen attached to a strongly electronegative atom such as F, N and O is held with weak electrostatic forces by the lone pair of electrons of the electronegative atom i.e. the strongly positive H and the strongly electronegative lone pair tend to line up and hold the molecules together. Other atoms with high electronegativities, such as Cl, can also form hydrogen bonds in strongly polar molecules such as chloroform, $CHCL_3$CHCL3 .
Conditions required for H-bond :
(i) Molecule should have a more electronegative atom (F, O, N) linked to H-atom.
(ii) Size of the electronegative atom should be smaller.
(iii) A lone pair should be present on an electronegative atom.