Final answer:
The expected electron configuration for chromium is [Ar] 4s² 3d⁴, but its real configuration is [Ar] 4s¹ 3d⁵ due to the stability of a half-filled 3d subshell. Similar anomalies are observed with copper, which exhibits a fully filled d-manifold instead of the expected configuration.
Step-by-step explanation:
Expected vs. Real Electron Configuration of Chromium
The expected electron configuration of chromium (Cr, atomic number 24) according to the standard rules we apply (Aufbau principle, Pauli exclusion principle, and Hund's rule) would be [Ar] 4s² 3d⁴. However, chromium exhibits an anomalous electron configuration, which is actually [Ar] 4s¹ 3d⁵. This occurs because having a half-filled 3d subshell provides more stability as compared to the expected configuration due to the similar energy levels of the 4s and 3d orbitals and electron-electron repulsion considerations. Similar anomalous behavior is observed in copper (Cu), which instead of having an electron configuration of [Ar] 4s² 3d⁹, shows a configuration of [Ar] 4s¹ 3d¹⁰, having a fully filled d-manifold.
Rationalizing these deviations is based on the relative stability of half-filled and fully filled subshells. The small energy differences between the 4s and 3d orbitals and subtle electron repulsions contribute to these anomalous electron configurations.