The behavior and properties of gases can be explained by the Kinetic Molecular Theory (KMT). A gas that behaves according to the Kinetic Molecular Theory is known as an ideal gas.

The principles of this theory are:

All gases consist of atoms or molecules. Nearly all gases are composed of molecules. For example, hydrogen gas, H2, is composed of hydrogen molecules. The noble gases, however, are composed of atoms rather than molecules. For example, helium gas is composed of helium atoms (He).

Gas molecules are always in motion, and collisions between them are perfectly elastic. Gas molecules move in straight lines until they collide elastically. This means that there is no net loss of kinetic energy when the gas molecules collide with each other and with the walls of the container.

The distance between gas molecules is very large compared with the size of the molecules themselves. Consequently, nearly all of the volume of a gas is empty space.

The temperature of a gas represents the average kinetic energy of its molecules. The molecules move faster when the temperature rises, and slower when the temperature decreases.

The molecules of an ideal gas do not exert any attractive or repulsive force on each other.