In gases, atomic particles dart about rapidly, colliding millions of times each second and changing their directions of motion just as frequently. Each gas particle has a kinetic energy proportional to the product of its mass and the square of its velocity. Collectively, however, the gas particles will have some average kinetic energy, that changes only when energy is added to the gas or removed from it. Another way of saying this is that the average kinetic energy changes when the gas is heated or cooled. Thus what we call heat or thermal energy is no more than the collective kinetic energies of atomic constituents.
Temperature is a measure of the average kinetic energy of gas particles.
The motion of the particles composing a body, such as ice or water or water vapor, is called random thermal motion and is illustrated in Figure 2. It increases as the temperature goes up, and it decreases as the temperature goes down. Absolute zero is reached when the average kinetic energy is zero. When we heat the air in a vessel, we are increasing the kinetic energy of each particle, which means a higher average kinetic energy. As a result, the gas particles move about faster, and they collide more frequently and more violently with their surroundings.