# What is the difference between a rigid diatomic molecule and non-rigid diatomic molecule?

I started a mock paper for one of my entrance exams with this fairly easy question. I rearranged the equation and got 5/2, knowing that a diatomic molecule has a Cv of 5/2, i concluded that the answer must be either A or C, but I do not know the difference between rigid diatomic and non rigid diatomic, can someone explain the difference?

Rigid diatomic molecule has $$C_v = 5/2$$ and non rigid has $$C_v = 7/2$$. From this we can fairly distinguish between both. Examples of rigid molecules include $$\mathrm{Cl_2}, \mathrm{Br_2}, \mathrm{N_2}, \mathrm{O_2}$$ and so on. An example of a non-rigid molecule is $$\mathrm{CO}$$.