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

The correct choice is the magnetic quantum number, which provides information about the orientation of an orbital within a given subshell. Each subshell has a specific number of orbitals that can occupy different positions in space, and the magnetic quantum number specifies these orientations relative to an external magnetic field.

For instance, in a p subshell, there are three orbitals corresponding to the three possible values of the magnetic quantum number: -1, 0, and +1. This indicates that the p orbitals can be oriented along different axes (typically x, y, and z) in three-dimensional space. Similarly, the d and f subshells have their own sets of orbital orientations as defined by their respective magnetic quantum numbers.

In contrast, the principal quantum number defines the energy level and size of the orbital, while the angular momentum quantum number describes the shape of the orbital. The spin quantum number refers to the intrinsic spin of the electrons within the orbital. Each of these quantum numbers plays a distinct role in the overall description of electron configurations, but only the magnetic quantum number is specifically responsible for orbital orientation.