Motion of Molecules in an Ideal Gas !

Motion of Molecules in an Ideal Gas

An animation illustrating the motion of molecules in an ideal gas.

The kinetic theory describe a gas as a large number of submicroscopic particles (atoms or molecules), all of which are in constant rapid motion that has randomness arising from their many collisions with each other and with the walls of the container.

Kinetic theory explains macroscopic properties of gases, such as pressure, temperature, viscosity, thermal conductivity, and volume, by considering their molecular composition and motion. The theory posits that gas pressure is due to the impacts, on the walls of a container, of molecules or atoms moving at different velocities.

Kinetic theory defines temperature in its own way, not identical with the thermodynamic definition.

Under a microscope, the molecules making up a liquid are too small to be visible, but the jittery motion of pollen grains or dust particles can be seen. Known as Brownian motion, it results directly from collisions between the grains or particles and liquid molecules. As analyzed by Albert Einstein in 1905, this experimental evidence for kinetic theory is generally seen as having confirmed the concrete material existence of atoms and molecules.


The theory for ideal gases makes the following assumptions:

  • The gas consists of very small particles known as molecules. This smallness of their size is such that the total volume of the individual gas molecules added up is negligible compared to the volume of the smallest open ball containing all the molecules. This is equivalent to stating that the average distance separating the gas particles is large compared to their size.
  • These particles have the same mass.
  • The number of molecules is so large that statistical treatment can be applied.
  • These molecules are in constant, random, and rapid motion.
  • The rapidly moving particles constantly collide among themselves and with the walls of the container. All these collisions are perfectly elastic. This means, the molecules are considered to be perfectly spherical in shape, and elastic in nature.
  • Except during collisions, the interactions among molecules are negligible. (That is, they exert no forces on one another.)
  • The volume occupied by the individual particles of a gas is negligible compared to the volume of the gas itself.
  • The particles of an ideal gas exert no attractive forces on each other or on their surroundings.
  • Gas particles are in a constant state of random motion and move in straight lines until they collide with another body.
  • The collisions exhibited by gas particles are completely elastic; when two molecules collide, total kinetic energy is conserved.
  • The average kinetic energy of gas molecules is directly proportional to absolute temperature only; this implies that all molecular motion ceases if the temperature is reduced to absolute zero.



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