Class 12 > Unit # 15:Molecular Theory of Gases > Postulates of Kinetic Theory of Gases

Q.4 State the basic postulates of the Kinetic Theory of Gases.

Postulates of Kinetic Theory of Gases

1. Molecular Composition: A gas consists of extremely small particles called molecules. Depending upon the chemical nature of the gas, each molecule may consist of a single atom or a group of atoms. All molecules of a specific gas in a stable state are considered completely identical in mass and size.
2. Large Number Assumption: Any finite volume of a gas contains a very large number of these constituent molecules. This assumption is verified by experiments; at standard temperature and pressure (STP), there are approximately $3 \times 10^{25}$ molecules contained in a single cubic meter.
3. Intermolecular Separation: The gas molecules are separated by distances that are vastly larger than their own molecular dimensions. The average finite diameter of a gas molecule, modeled as a rigid sphere, is roughly $3 \times 10^{-10}\text{ m}$. Consequently, the actual physical volume occupied by the gas molecules themselves is negligible compared to the total volume of the container.
4. Random Motion & Elastic Collisions: The molecules are in a continuous state of random, chaotic motion, traveling in all possible directions with a wide distribution of speeds. During this motion, they undergo perfectly elastic collisions with one another and with the inner boundary walls of the container. The walls of the container can be treated as perfectly smooth.
5. Negligible Intermolecular Forces: Molecules exert no attractive or repulsive forces on one another except during the immediate instant of a collision. Therefore, in the intervals between consecutive collisions, the molecules move freely along perfectly straight-line paths at constant velocity.
6. Applicability of Classical Mechanics: Standard Newtonian mechanics is fully applicable to the translation, collision, and individual motion of these gas molecules.