Matter exists in solid, liquid and gaseous states. Solids have fixed shape and volume, liquids have fixed volume but no fixed shape, gases have neither fixed shape nor fixed volume. We will interact about the arrangement of constituting particles of different states of matter, their characteristic properties and discuss the laws governing their behavior.

Intermolecular Forces. The force of attraction between molecules which keep molecules together are called intermolecular forces.

Molecular Interactional Energy. The energy due to force of attraction between molecules is called molecular interactional energy. Molecular interactions lead to intermolecular attractive and repulsive forces.

Thermal Energy. The energy which moves the molecules apart is called thermal energy. It is defined as energy possessed by matter by virtue of its temperature and is also measure of thermal motion or movement of molecules.

Change in State. It involves competition between ordering molecular interaction energy and disor­dering thermal energy.

Gaseous State. In gases, molecules have almost no molecular interactions and thermal motion is random translatory movement of molecules that is why they do not have fixed shape and volume because they have large intermolecular space, therefore weak force of attraction. For gases like hydrogen, helium intermolecular forces can be neglected at high temperature and low pressure.