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The ideal-gas equation, which is empirical, describes the behavior of gases by establishing relationships between their macroscopic properties. For example, Charles’ law states that volume and temperature are directly related. Gases, therefore, expand when heated at constant pressure. Although gas laws explain how the macroscopic properties change relative to one another, it does not explain the rationale behind it.

The kinetic molecular theory is a microscopic model that helps understand what happens to gas particles at the molecular or atomic level when conditions such as pressure or temperature change. In 1857 Rudolf Clausius published a complete and satisfactory form of the theory, which effectively explains the different gas laws through the postulates that were developed based on hundreds of experimental observations of the behavior of gases.

The salient features of this theory are:

1. Gases are composed of particles (atoms or molecules) that are in continuous motion, traveling in straight lines and changing direction only when they collide with other molecules or with the walls of a container.
Examine a sample of argon gas at standard temperature and pressure. It shows that only 0.01% of the volume is taken up by atoms with an average distance of 3.3 nm (atomic radius of argon is 0.097 nm) between two argon atoms. The distance is far greater than its own dimension.
2. The molecules composing the gas are negligibly small compared to the distances between them. Therefore, the combined volume of all gas particles is negligible relative to the total volume of the container. The particles are considered to be “points” that have mass but negligible volume.
3. The pressure exerted by a gas in a container results from collisions between the gas molecules and the container walls.
4. Gas molecules exert no attractive or repulsive forces on each other or the container walls; therefore, their collisions are elastic (do not involve a loss of energy).
During elastic collisions, energy is transferred between the colliding particles. The average kinetic energy of the particles, therefore, stays constant and does not change with time.
5. The average kinetic energy of the gas molecules is proportional to the kelvin temperature of the gas.
All gases, regardless of their molecular mass, have the same average kinetic energy at the same temperature.

This text is adapted from Openstax, Chemistry 2e, Chapter 9.5 The Kinetic-Molecular Theory.

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