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The Aufbau principle is the building up principle that combines both Pauli Exclusion principle and Hunds rule. Pauli Exclusion principle states that there can be a max of two electrons per orbital and that the two electrons must be spin paired. Hunds rule states that the electrons should be placed one per orbital first in a parallel manner. The 4s subshell only has lower energy if the 3d subshell is not filled yet. Once the 3d subshell has electrons within it, then it has lower energy than the 4s.
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