Which quantum number is symbolized by m and indicates the orientation of an orbital around the nucleus?

• A. Magnetic quantum number
• B. Angular momentum number
• C. Spin quantum number
• D. Principal quantum number

Answer: (A)

Orientation of an orbital in space is described by the magnetic quantum number, m_l. For a given angular momentum l, m_l can take values from -l to +l, giving 2l+1 possible orientations. These correspond to the different orbitals within a subshell (for example, a p subshell has three orbitals with m_l = -1, 0, +1). The other quantum numbers serve different roles: the principal quantum number n sets energy level and size, the angular momentum quantum number l identifies the subshell type and general shape, and the spin quantum number m_s describes the electron’s intrinsic spin. So the quantum number that indicates the orientation of an orbital around the nucleus is the magnetic quantum number.