Which of the following electron configurations represents the transition element Chromium $_{24}Cr$?

Step 1: Analyze the question You need the ground-state electron configuration of chromium (atomic number 24), so it has 24 electrons.

Step 2: The concept Normally, electrons fill orbitals in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d. Chromium is a well-known exception. A half-filled d subshell (3d⁵) is especially stable because of symmetrical electron distribution and exchange energy. So one electron moves from 4s to 3d:

• Expected: [Ar] 4s² 3d⁴
• Actual: [Ar] 4s¹ 3d⁵

Step 3: Solution Fill the electrons up to argon (18 electrons): 1s² 2s² 2p⁶ 3s² 3p⁶, which is 2 + 2 + 6 + 2 + 6 = 18

The remaining 6 electrons go to 4s¹ 3d⁵, giving 18 + 1 + 5 = 24 ✔

Full configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵

Step 4: Check the options

• A: 4s² 3d⁴ has the correct total of 24 but is the "expected" configuration, not chromium's actual one. ✘
• B: 3d⁶ with no 4s gives 24 electrons but is wrong. ✘
• C: it is missing 3p⁶, so the total is only 18 electrons. ✘
• D: 4s¹ 3d⁵ has the correct total and the half-filled d subshell. ✔

Answer: D

Tip: Remember the two famous exceptions in the first transition series: Cr (4s¹ 3d⁵) and Cu (4s¹ 3d¹⁰). Both gain stability from a half-filled or fully filled d subshell. Whenever you see Cr or Cu in an exam, check for this.