Thermal Energy

Thermal energy is the energy of a body arising from motion of its atoms or molecules. It is directly proportional to the temperature of the substance. It is the measure of average kinetic energy of the particles of the matter and is thus responsible for movement of particles. This movement of particles is called thermal motion.

Intermolecular Forces vs Thermal Energy:

Intermolecular forces tend to keep the molecules together but thermal energy of the molecules tends to keep them apart.

The three states of matter are the result of balance between the intermolecular forces and the thermal energy of the molecules. As the temperature is increased the molecules become larger, the number of electrons increases and the intermolecular forces become stronger and they start to push away from each other. Thus on applying thermal energy to a solid it changes to liquid which in turn changes to a gas. Consequently, a gas on compression changes to a liquid which on further compression changes to a solid.