Chromium and copper adopt half-filled or fully filled d subshells for extra stability, giving 4s1 configurations.
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Chromium and copper deviate from the Aufbau order by adopting configurations with a half-filled or fully filled d subshell. Chromium is [Ar] 3d5 4s1 and copper is [Ar] 3d10 4s1, rather than the predicted 3d4 4s2 and 3d9 4s2. Half-filled and filled d subshells are unusually stable because of exchange energy, which is greatest when electrons have parallel spins in different orbitals. The 3d and 4s energies are also close, so the balance tips easily. These exceptions matter for the chemistry of the two metals.
Key ideas: Cr is [Ar] 3d5 4s1 (not 3d4 4s2) and Cu is [Ar] 3d10 4s1, because half-filled and fully filled d subshells have extra stability from exchange energy and small 3d-4s gap. Similar cases: Mo, Ag, Au and Pd ([Kr] 4d10). Both elements still lose 4s first in forming cations.
Common mistakes and connections: Students apply these exceptions to all elements or to ions; only the neutral atom is affected. Another mistake is explaining only by symmetry, while exchange energy and orbital energy proximity are the real causes. Many other exceptions exist in 4d, 5d and f blocks (such as Gd and Pt). This connects to transition metal oxidation states.
Copper takes 3d10 4s1 rather than 3d9 4s2 because a full d subshell is extra stable, leaving just one electron in the outer shell. The interactive lesson shows this as a live animation or chart.
Chromium has six unpaired electrons in its 3d5 4s1 arrangement, which contributes to its strong magnetic and bonding behaviour.
Chromium and copper adopt half-filled or fully filled d subshells for extra stability, giving 4s1 configurations.
Exceptional Configurations of Cr and Cu is a intermediate topic in Atomic Structure & Periodicity. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Atomic Structure & Periodicity first: Madelung Rule, Electron Configuration, Noble Gas Shorthand.
The next topics in Atomic Structure & Periodicity are Valence Electrons and Core Electrons.
Yes. The interactive lesson includes a self-check question on Exceptional Configurations of Cr and Cu, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Exceptional Configurations of Cr and Cu is topic 67 of 115 in Atomic Structure & Periodicity.