(i) The first quantum number describes the electronic shell, or energy level, of an atom. Principal quantum number (n) describes the energy of an electron and the probable distances of the electron from the nucleus. The principal quantum number designates the principal electron shell.
(ii) The angular quantum number (l). The second quantum number, known as the angular or orbital quantum number, describes the sub-shell and gives the magnitude of the orbital angular momentum.
(iii) Magnetic quantum number (m) describes the orbital in a sub shell.
