Which quantum number describes the shape of an orbital?

The azimuthal quantum number, also known as the angular momentum quantum number, is the quantum number that describes the shape of an orbital. It indicates the type of orbital – whether it is an s, p, d, or f orbital – each of which has a distinct shape.

For example:

• An s orbital (where the azimuthal quantum number is 0) is spherical in shape.

• A p orbital (where the azimuthal quantum number is 1) has a dumbbell shape.

• A d orbital (where the azimuthal quantum number is 2) has a more complex shape, often described as having a cloverleaf structure.

The principal quantum number defines the energy level and size of the orbital but does not give details about the shape. The magnetic quantum number describes the orientation of the orbital in space but does not pertain to the shape itself. The spin quantum number describes the intrinsic spin of electrons within the orbital, rather than the shape of the orbital. Hence, the azimuthal quantum number is the correct choice for describing the shape of an orbital.