A diatomic molecule, treated as a rigid rotator has ___________ degree of freedom.

Correct option is B

A diatomic molecule, treated as a rigid rotator, has 5 degrees of freedom.
Translational degrees of freedom: The molecule can move in three directions (x, y, z), which accounts for 3 degrees of freedom.
Rotational degrees of freedom: The molecule can rotate around two axes that are perpendicular to the bond axis. Since the molecule is a rigid rotator (i.e., its bond length remains fixed), there are 2 rotational degrees of freedom. It cannot rotate around the bond axis because it's a linear molecule (i.e., there is no moment of inertia around that axis).

Thus, the total number of degrees of freedom for a rigid diatomic molecule is:

3

(
translational
)
+
2

(
rotational
)
=
5

degrees of freedom
.
3(translational)+2(rotational)=5degrees of freedom.