what is aufbau's principle?
Aufbau's Principle states that the orbitals are filled in the increasing order of their
energies (in the ground state). Following the (n + l) rule, the orbitals
in the increasing order of energy are arranged as
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f