Q.Write the ground-state electronic configuration of copper ($Z = 29$) and explain the anomaly in its filling.

Copper has $Z = 29$ electrons. The simple Aufbau order predicts $[\text{Ar}]\,3d^9\,4s^2$ (18 core electrons in $[\text{Ar}]$, plus 9 in $3d$ and 2 in $4s$). Experimentally, copper's actual ground-state configuration is

Here too, one electron that the simple rule would place in $4s$ instead moves into $3d$, completing the $3d$ subshell (two electrons in each of its five orbitals -- completely filled) while leaving $4s$ with only a single electron.

The reason is the same kind of extra stability seen for chromium, but now applied to a completely filled rather than a half-filled subshell: a completely filled $3d^{10}$ subshell has every orbital paired, giving a highly symmetric, spherically even distribution of charge (minimising electron-electron repulsion) and the maximum possible number of exchange interactions available to a fully occupied set of orbitals. This extra stabilisation outweighs the modest energy cost of leaving $4s$ with only one electron instead o …

Write the Aufbau-predicted configuration first, then compare with the known experimental exception, and explain it using …

Confusing copper's exception (completely filled $3d^{10}$) with chromium's (half-filled $3d^5$) -- they are two distinct stabilisation cases, not the same one; wr …