Chromium, Z = 24
- Input
- Cr
- Expected output
- [Ar] 3d5 4s1
The rule would give [Ar] 4s2 3d4; one electron moves from 4s to 3d.
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electron configuration exceptions chromium copper
The Madelung diagram predicts chromium as [Ar] 4s2 3d4, but the measured chromium atom is [Ar] 3d5 4s1. There are 20 such elements up to hassium, and this page lists all of them, with the configuration NIST publishes and what the rule would give.
The rule would give [Ar] 4s2 3d4; one electron moves from 4s to 3d.
The rule would give [Ar] 4s2 3d9; 3d ends up full.
Same pattern as chromium, one row down.
Both 5s electrons move to 4d.
Analogous to copper.
Analogous to copper and silver, with 4f full.
Take the atomic number (the number of electrons in the neutral atom) and distribute them over the subshells in Madelung order, filling each to its maximum (s 2, p 6, d 10, f 14). For chlorine (Z = 17): 1s2 2s2 2p6 3s2 3p5, or [Ne] 3s2 3p5, with 2, 8 and 7 electrons in shells K, L and M.
20 between hydrogen and hassium, the elements whose ground state NIST has measured: Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr.
The correct answer is 4s1 3d5 (or 3d5 4s1), the real configuration. If the statement explicitly says to follow the diagram, 4s2 3d4. When in doubt, mention the exception and write both.
Start from the real configuration of the atom: Cr3+ removes the 4s1 and two electrons from 3d5 and ends up [Ar] 3d3; Cu+ removes the 4s1 and ends up [Ar] 3d10; Ag+ ends up [Kr] 4d10.
The small number in each box is the filling order and the large one is the number of electrons in the subshell. The arrows follow the diagonals.
Elements up to Z = 108 whose ground state departs from the Madelung filling order. Tap a symbol to open it.
See also: periodic table · molar mass · equation balancer
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