Question

The number of rotational degree of freedom for a diatomic gas is

• A. $2$
• B. $3$
• C. $5$
• D. $7$

Answer 1

The correct option is A $2$

Diatomic gas will have linear structure as shown.

Since the atoms are modelled as points, we need to discard the rotation around the molecule's axis. It has virtually no moment of inertia for that axis. Therefore, no rotation about $x-$ axis
But linear structure can rotate about $y$ and $z$ axis.


$\therefore$ Diatomic gas has $2$ rotational degree of freedom.