Anomalous Electron Configurations:
Certain atoms appear to violate the orbital-filling rules:
- Chromium's electron configuration is [Ar]4s13d5 rather than the expected [Ar]4s23d4
- Copper's electron configuration is [Ar]4s13d10 rather than the expected [Ar]4s23d9
- There are other examples in the heavier d-block and f-block metals.
This anomalous behavior occurs when there are enough electrons to produce a half-filled set of degenerate orbitals (as in chromium) or to completely fill a d or f subshell (as in copper).