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The electron configuration for chromium is $[Ar] \ 4s^13d^5$.
Which of the following is the BEST explanation for this irregular electron configuration?
A half-filled d orbital is slightly lower in energy than the 4s and fills in first.
The 4s orbital is at a lower energy than the 3d orbital and fills in first.
Chromium is a transition metal, and its electrons are loosely held.
d orbitals are dominant and "steal" electrons from orbitals close in energy.