Aufbau's principle states that an electron in an atom is filled according to the energy of orbitals, orbital with minimum energy is first filled, and so on.
so the order of filling of electrons in orbitals are 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p 8s.
so, the 4f subshell is filled after the complete filling of electrons in 6s.
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