According to the rule, the electrons are added progressively to the various orbitals in the order of increasing energies starting with the orbital of the lowest energy. Increasing order of energies of various orbitals is:

     1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, . . .   

It may not be out of place to mention here that certain configurations like Cr (Z = 24), Cu (Z = 29) etc do not obey this rule.

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