The forces of attraction and repulsion between interacting particles (atoms and molecules) are known as intermolecular forces. This phrase excludes the forces that keep atoms in a molecule together, known as covalent bonds, and the electrostatic forces that exist between the two oppositely charged ions. Van der Waals forces, named for the Dutch physicist Johannes van der Waals (1837-1923), are attractive intermolecular forces that explain why real gases behave differently from ideal gases. This is something that this unit will teach us later on. Van der Waals forces come in three different forms: dipole-induced dipole forces, dispersion forces, also known as London forces, and their magnitudes vary greatly. Hydrogen bonding is a particularly potent kind of dipole-dipole interaction. A select few elements are able to take part.

Dipole-dipole interactions: This type of interaction occur between polar molecules having permanent molecules. Such molecules possess partial charges of opposite sign. The positive end of the one dipole attracts the negative end of the other molecule.

Eg H-Cl

Dipole induced dipole interaction: This type of interaction occurs between a dipole and induced dipole. This interaction occurs between polar molecule and non-polar molecule. The positive end of the dipole attracts the electron clod of non polar molecule and thus polarity is developed.

HFand O2, HCl and N2

Dispersion forces: This types interaction occurs between the nonpolar molecules. The oscillation of electron cloud makes more denser region on one side of molecule makes the molecules instantaneous dipole. This instantaneous dipole distorts the electron distribution of other atoms or molecules and this type of interaction is known as Dispersion forces or London forces.

Eg: N2. Cl2 . O2

Hydrogen bond : The dipole -dipole interactions experienced when hydrogen is bonded to N, O F are unusually strong and this type of interaction is called Hydrogen bond