Final answer:
The electron configuration of Cu²+ ion is [Ar] 3d9.
Step-by-step explanation:
The electron configuration of Cu²+ ion is [Ar] 3d9.
The ground-state electron configuration of copper is [Ar] 3d10 4s1. However, when copper forms a Cu²+ ion, it loses two electrons. Since the 4s orbital is higher in energy than the 3d orbital, the electrons from the 4s orbital are removed first. Thus, the electron configuration of Cu²+ ion is [Ar] 3d9.