Explain the degrees of freedom for
(i) An atom
(ii) A diatomic molecule.

Explanation

Gas molecules of all types have p and z-components of velocity that are entirely independent of one another. Thus, they have three degrees of translational freedom
An atom (or a monatomie molecule), treated as a point mass, has no rotational energy Hence, it has only three degrees of translational freedom.
A diatomic molecule, in addition to translation, can rotate about axes perpendicular to the line connecting the atoms, as shown in the following figure, but not about that line itself. Therefore, it has only two degrees of rotational freedom.

Further, there is the possibility of vibrational motion in which the two atoms oscillate alternately toward and away from one another along the line joining them, as if attached to opposite ends of a spring. Because a harmonic oscillator can have potential energy and kinetic energy, a diatomic molecule is regarded to have two degrees of vibrational freedom. Thus, at high enough temperatures, a diatomic molecule has seven degrees of freedom : three of translation, two each of rotation and vibration.