Q.Cu (belongs to period 4 and group 11)

Step 1. Note the period and group. Cu is in period 4 (n=4) and group 11, in the d-block.

Step 2. Predict the plain Aufbau configuration. Following the straightforward filling order through period 4's d-block (Sc to Zn), Cu (the ninth d-block element) would be expected to have configuration $[Ar]4s^23d^9$.

Step 3. Apply the extra-stability exception. A COMPLETELY-filled d subshell ($3d^{10}$) is significantly more stable than a $3d^9$ configuration, so one electron shifts from 4s into 3d — exactly the same kind of anomaly the chapter notes for chromium (section 7.3.3), but resulting here in a fully-filled rather than a half-filled d subshell. …

Predicting the plain-Aufbau configuration, then applying the filled/half-fill …

• Writing the 'expected' $4s^23d^9$ without checking for the known d-block stability exception.
• Confusing Cu's fully-filled-d anomaly with Cr's half-filled-d anomaly — the mechanism (extra …