Assertion : The molecules of a monoatomic gas have three degrees of freedom.Reason : The molecules of diatomic gas have five degrees of freedom.

A monoatomic gas molecules like He consists of single atom. It can have translational motion in any direction inspace.Thus, it has three translational degrees of freedom f = 3. It can also rotate but due to its small moment of inertia,rotational kinetic energy is neglected.The molecules of a diatomic gas (like O2, CO2 , H2) cannot only move but also rotate about anyone of the threecoordinates.Hence, it can have two rotational degree of freedom.Thus, a diatomic molecule has 5 degree of freedom, i.e. 3 translational and 2 rotational.