Quantum theory aufbau principle

Further advances in the theory of the electronic structure of atoms reveal that when there are many electrons, the energies of states with the same principal quantum number but different angular momentum are different, so that, for example, in the carbon atom the energetic level of a 2s orbital is different from that of a 2p orbital. The electronic structures of atoms may be built by assigning two electrons with paired spins to each atomic orbital, starting from the lowest-energy ones. This aufbau principle naturally produces periodically similar electronic structures. Mendeleev... 

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