Expected configuration using Aufbau principle for $Z=24$ is $[Ar]3d^{4}4s^2$. However, Chromium ($Cr$) and Copper ($Cu$) show anomalous configurations due to the extra stability of half-filled and completely filled subshells.

• In $Cr$, an electron from the $4s$ orbital jumps to the $3d$ orbital to make $3d^5$ (exactly half-filled) and $4s^1$ (exactly half-filled).
• Correct configuration: $[Ar]3d^{5}4s^1$.