Multiple Choice
Which element has the ground state electron configuration 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1?
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10. Periodic Properties of the Elements
The Electron Configuration
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Which orbital-filling diagram represents the ground state electron configuration of chromium (Cr, atomic number 24)?
A
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^4
B
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^5
C
1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5
D
1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^4
Verified step by step guidance1
Recall that chromium (Cr) has an atomic number of 24, meaning it has 24 electrons to place in orbitals according to the Aufbau principle, Hund's rule, and the Pauli exclusion principle.
Write the expected electron configuration by filling orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, then 3d. Normally, this would give 4s^2 3d^4 for chromium.
Understand that chromium is an exception to the expected configuration because half-filled d subshells (3d^5) and half-filled s subshells (4s^1) provide extra stability due to electron exchange energy and symmetry.
Therefore, the ground state electron configuration of chromium is actually 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5, where one electron from the 4s orbital is promoted to the 3d orbital to achieve a half-filled d subshell.
To represent this in an orbital-filling diagram, place one electron in the 4s orbital and five unpaired electrons in the 3d orbitals, each occupying separate orbitals with parallel spins according to Hund's rule.
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