For atoms heavier than copper this effect dominates, and 4s electrons have higher energy than 3d electrons.
This effect causes 4s orbitals to have lower energy than 3d orbitals for elements lighter than copper. (Although for hydrogen, the unoccupied 4s and 3d orbitals have nearly identical energies).
Author: Fred Senese email@example.com
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/electrons/faq/print-4s-3d.shtml