Kinetic Molecular Theory of Gases
As indicated by the gas laws, all gases show similar physical characteristics and behavior. A
theoretical model to explain why gases behave the way they do was developed during the second
half of the 19th century. The combined efforts of Boltzmann, Maxwell, and others led to the kinetic
molecular theory of gases, which gives us an understanding of gaseous behavior on a microscopic,
molecular level. Like the gas laws, this theory was developed in reference to ideal gases, although it
can be applied with reasonable accuracy to real gases as well.
The assumptions of the kinetic molecular theory of gases are as follows:
AVERAGE MOLECULAR SPEEDS
According to the kinetic molecular theory of gases, the average kinetic energy of a gas is
proportional to the absolute temperature of the gas; more specifically, the kinetic energy of one
mole of gas is 3/2 RT. Since the kinetic energy is related to the speed (KE = 1/2 mv 2 ), this also means
that the higher the temperature, the faster the gas molecules are moving. However, because the
large number of rapidly and randomly moving gas particles do not all move at the same speed, the
speed of an individual gas molecule is nearly impossible to define and is not a very useful concept.
There will be molecules that move faster and those that move slower than the average value. A
Maxwell-Boltzmann distribution curve shows the distribution of speeds of the gas particles at a
given temperature. The figure below shows a distribution curve of molecular speeds at two
Gases are made up of particles whose volumes are negligible compared to the container
volume.1.
2. Gas atoms or molecules exhibit no intermolecular attractions or repulsions.
Gas particles are in continuous, random motion, undergoing collisions with other particles and
with the container walls.3.
Collisions between any two gas particles are elastic, meaning that no energy is dissipated or,
equivalently, that kinetic energy is conserved.4.
The average kinetic energy of gas particles is proportional to the absolute temperature of the
gas, and is the same for all gases at a given temperature.