Definition of the principle of minimum energy
"The lower the energy of the system, the more stable it is", which is a universal law in nature. The arrangement of electrons outside the nucleus also follows this rule. When the multi-electron atom is in the ground state, the electrons outside the nucleus always occupy the lowest energy orbit as much as possible, and then follow the order of the atomic orbital approximate energy level diagram to higher energy. The distribution on the energy level is called the principle of minimum energy.