JEE Advance - Chemistry (1982 - No. 7)

Statement is FALSE. The outer electronic configuration of the ground state chromium atom is unusual compared to what might be expected based on a straightforward application of the Aufbau principle. The actual outer electronic configuration of chromium is $3d^{5} 4s^{1}$, not $3d^{4} 4s^{2}$.

This configuration arises because a half-filled d subshell ($3d^{5}$) is more stable than the configuration $3d^{4} 4s^{2}$. By promoting an electron from the 4s orbital to the 3d orbital to achieve a half-filled d subshell, chromium minimizes its energy. This is due to the increased exchange energy and symmetry of the half-filled d subshell, leading to greater stability. Thus, the correct outer electronic configuration of a ground state chromium atom is $3d^{5} 4s^{1}$.