Intermolecular forces are responsible for the physical properties of a substance, such as its melting or boiling point, viscosity, solubility, surface tension and evaporation. The types of intermolecular forces are ionic, hydrogen bonding, dipole and induced dipole forces. Except ionic force, all other intermolecular forces occur between neutral molecules, which have a slight polarity.
Ionic forces occur between two ions. The ions may have been formed from neutral atoms or molecules. For example, a neutral sodium atom can lose an electron to a neutral chlorine atom to form sodium chloride, which has a sodium and chlorine ion.
Some covalent bonds are formed by unequal sharing of electrons. One molecule ends up with a partial negative charge and the other with a partial positive charge, thereby making the molecule polar. The molecules are still considered to be neutral since no electrons were exchanged in the process of forming the bond. These molecules are said to be polar molecules or dipoles, and the force between them is called a dipole force.
Nonpolar molecules can distort the electron cloud slightly to become temporary dipoles. They are held together by a weak force called the induced dipole force. The strength of the force depends on the size of the molecule and the ease with which the electron cloud can be moved or distorted.
Hydrogen bonds are a special kind of dipole force involving hydrogen bonding to an electronegative element such as oxygen or nitrogen.