Which quantum number defines the principal energy level (shell) in atomic structure?

• A. l
• B. n
• C. ms
• D. ml

Answer: (B)

The principal energy level is defined by the principal quantum number, n. This number is a positive integer (1, 2, 3, …) that sets the size and energy of the orbital—the higher the n, the larger the orbital and the higher the energy, roughly speaking. It also determines how many subshells sit inside that level, since each level can contain subshells with angular momentum values from 0 to n−1. The other quantum numbers are about finer details: the angular momentum quantum number l describes the orbital shape, the magnetic quantum number m_l sets the orbital’s orientation in space, and the spin quantum number m_s specifies the electron's spin. So n specifically marks the principal energy level or shell.