Definition of hyperfine structure: Many nuclei have spin I, accompanied by a magnetic moment. The interaction between the nuclear magnetic moment and the electrons causes the energy level to split. The nuclear magnetic moment is very small, and the splitting of energy levels is also very small. The hyperfine structure energy level is marked by the electron total angular momentum quantum number J, nuclear spin I, and the total angular momentum quantum number F including nuclear spin. The energy level interval follows the similar Lande interval rule. Many nuclei also have an electric quadrupole moment. The interaction of the nuclear quadrupole moment and the electric field gradient generated by the electrons at the nucleus causes a small change in energy, which is superimposed on the hyperfine structure caused by the magnetic moment, causing the splitting to deviate from the Lande interval rule. The hyperfine structure of the energy levels results in the hyperfine structure of the spectral lines.