Which quantum number describes the size of the electron orbital and indicates the energy level?

• A. Spin quantum number
• B. Magnetic quantum number
• C. Azimuthal quantum number
• D. Principal quantum number

Answer: (D)

The principal quantum number n determines how big the orbital is and which energy level the electron occupies. As n increases, the electron’s average distance from the nucleus grows, so the orbital becomes larger. In atoms like hydrogen, energy levels are set mainly by n: higher n means the electron is less tightly bound and sits at a higher energy level (less negative energy). Other quantum numbers describe aspects other than size and energy: the spin quantum number only tells the electron’s spin direction, the magnetic quantum number relates to the orbital’s orientation in space, and the azimuthal quantum number describes the orbital’s shape and angular momentum. So, the one that governs both size and energy level is the principal quantum number.