Important: Degrees of Freedom of a Single Gas Particle
In the kinetic-molecular model of an ideal gas, we consider only those degrees of freedom of its particles which result in some form of mechanical energy.
- Monoatomic gas molecules have degrees of freedom, since each particle is just a point whose velocity has three independent components. We consider only the translational kinetic energy, since single points cannot rotate and the kinetic-molecular model of an ideal gas is simple and does not include potential energies.
- Diatomic gas molecules have degrees of freedom, since each particle is represented by a molecule consisting of two atoms represented as points. There are degrees of freedom again due to the molecule’s velocity which has three independent components, but there are an additional degrees of freedom which come from the fact that the molecule has two possible axes of rotation. The axes are only two and not three, since rotation abound the line joining the two atoms is not possible - a point cannot rotate around itself.