Which quantum number indicates the orientation of an orbital in space?

The magnetic quantum number is the one that indicates the orientation of an orbital in space. In quantum mechanics, orbitals are three-dimensional regions around the nucleus where there is a high probability of finding electrons. The magnetic quantum number specifically describes the specific orientation of these orbitals relative to the three axes in three-dimensional space (x, y, and z).

For example, when considering the p orbitals, there are three different orientations (commonly designated as px, py, and pz), and the magnetic quantum number helps distinguish among these. It can take on values ranging from -l to +l, where l is the orbital quantum number that designates the shape of the orbital.

In contrast, the principal quantum number relates to the energy level and size of the orbital, while the orbital quantum number describes the shape of the orbital. The spin quantum number, on the other hand, relates to the intrinsic spin of the electron itself rather than the orientation of the orbital. Therefore, the correct choice addressing the orientation of an orbital is indeed the magnetic quantum number.