Why is 4s filled before 3d

However, from Scandium and onwards, the 3d orbital of transition metals actually becomes lower in energy than the 4s, which is why we write 3d before 4s in the configuration. In fact it has been determined that when building up Scandium, an electron first enters the 3d orbital, and the next two enter the 4s orbital due to the 3d orbital being more compact, which would result in higher energy and electron repulsion if the electrons were to enter the 3d orbital instead, and since the 4s isn't much higher in energy than the 3d, the atom is actually at a lower energy state when adding to 4s to avoid this electron repulsion.

Jump to. Citations Abstract It is demonstrated that the difference between the total energies of two electronic configurations can be expressed as an orbital energy difference, provided that the configurations differ only in the assignment of one electron and that each of the energies of the differentiating orbitals corresponds to an ionization energy in its own configuration.

Cited By. This article is cited by 21 publications. Valerie E. Fleischauer, Gaurab Ganguly, David H. Woen, Nikki J. Wolford, William J. Evans, Jochen Autschbach, Michael L.

Organometallics , 38 16 , Orbitals: Some Fiction and Some Facts. Journal of Chemical Education , 89 8 , Eugen Schwarz , Ronald L. Journal of Chemical Education , 87 4 , Eugen Schwarz. Chemical Reviews , 2 , Coote , Clemence Corminboeuf. Theory and practice of uncommon molecular electronic configurations. Historical Teaching of Atomic and Molecular Structure. Catalysis Letters , 1 , For atoms heavier than copper this effect dominates, and 4s electrons have higher energy than 3d electrons.

Raising l raises orbital energy. Higher l values result in orbitals with more nuclear nodes a node being a place where the probability of finding the electron is zero. We say high- l orbitals are "less penetrating" because their electrons have a lower probability of being found at or near the nucleus. That gives high- l orbitals like d orbitals more energy than low- l orbitals like s orbitals within the same shell.

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. Within a subshell, more unpaired spins means a lower overall energy.