why the 3d came after 4s?
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According to the Aufbau principle, the 4s sublevel is filled before the 3d sublevel because the 4s is lower in energy. As the 3d sublevel becomes populated with electrons, the relative energies of the 4s and 3d fluctuate relative to one another and the 4s ends up higher in energy as the 3d sublevel fills.
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