Since entropy is about how energy is shared, it is important to be able to identify places where energy can be put, or degrees of freedom. For example, a monoatomic atom moving in three dimensions in free space has three places to put energy: kinetic energy for motion in three independent directions, so it has three degrees of freedom. (because of quantum mechanics, a perfectly spherical atom cannot have any energy of rotation. ) a rigid diatomic molecule can have three kinetic energy degrees of freedom and two rotational degrees of freedom since it can rotate about two different axes. (rotation about the axis through the center line of the molecule doesn't have any rotational energy, again because of quantum mechanics and the fact that it doesn't change when it makes that rotation. ) both kinetic and potential energies can serve as degrees of freedom. To see more about these issues, see the reading example: degrees of freedom. For this problem assume that you can ignore gravity.