Which electron configuration represents chromium as an exception to the expected order?

Which electron configuration represents chromium as an exception to the expected order?

• A. [Ar] 4s2 3d4
• B. [Ar] 4s2 3d5
• C. [Ar] 4s1 3d5
• D. [Ar] 4s2 3d10

Answer: (C)

A key idea here is that stability can be gained from half-filled or fully filled d subshells, which can override the simple order in which orbitals are filled. For chromium, the naive fill would place both electrons in 4s before filling 3d, giving [Ar] 4s2 3d4. But having five electrons in the 3d subshell creates a half-filled 3d set, which is unusually stable due to exchange interactions and reduced repulsion among the d electrons. To achieve this extra stability, one electron from the 4s is promoted into the 3d subshell, yielding [Ar] 4s1 3d5. This arrangement is energetically favored over the expected [Ar] 4s2 3d4, so it represents chromium’s exception to the straightforward filling order. (For context, copper shows a related pattern with [Ar] 4s1 3d10, reinforcing how these stability factors influence transition metal configurations.)

A key idea here is that stability can be gained from half-filled or fully filled d subshells, which can override the simple order in which orbitals are filled. For chromium, the naive fill would place both electrons in 4s before filling 3d, giving [Ar] 4s2 3d4. But having five electrons in the 3d subshell creates a half-filled 3d set, which is unusually stable due to exchange interactions and reduced repulsion among the d electrons. To achieve this extra stability, one electron from the 4s is promoted into the 3d subshell, yielding [Ar] 4s1 3d5. This arrangement is energetically favored over the expected [Ar] 4s2 3d4, so it represents chromium’s exception to the straightforward filling order. (For context, copper shows a related pattern with [Ar] 4s1 3d10, reinforcing how these stability factors influence transition metal configurations.)