Of the two type of bonds (pi and sigma), which is capable of rotation around that bond and which is not? why?

The sigma bonds are capable of rotating, the pi bonds not.

Explanation:

Sigma bonds are the strongest type of bonds, there are related to the overlapping in the atomic orbitals, and they can rotate. In the pi bound (that is a double bond), there are electrons moving on the molecule and it is not permitted the rotation on this type of bonds. Of the sigma bonds are capable of rotating while the pi bonds not.

the atomic mass is the bottom number on the element.

-steel jelly