Which quantum number defines the angular momentum (orbital) of an electron?

• A. ms
• B. l
• C. n
• D. ml

Answer: (B)

The azimuthal quantum number, l, determines the orbital angular momentum of the electron. Its value sets the shape of the orbital and fixes the magnitude of the orbital angular momentum, which is sqrt(l(l+1)) ħ. For a given energy level n, l can be 0 up to n−1. The other quantum numbers describe different aspects: ml fixes the orientation of the angular momentum, ms refers to the electron’s spin, and n sets the energy level and size of the orbital. So the quantity that defines the orbital angular momentum is l.