What is the condensed electron configuration for chromium (Z=24), and what explains the exception to the expected filling?

• A. [Ar] 3d5 4s1; a more stable half-filled 3d subshell and reduced electron–electron repulsion.
• B. [Ar] 3d4 4s2; the energy gap between 3d and 4s is large.
• C. [Ar] 3d6 4s2; fully filled 3d subshell is favored.
• D. [Ar] 4s2 3d4; energetic stability from 4s electrons.

Answer: (A)