Step 1: Start from neutral Cu.
Cu atom: [Ar] \(3d^{10}4s^{1}\).
Step 2: Remove electrons for Cu$^{2+$.}
Electrons are removed first from \(4s\) then \(3d\):
Cu$^{2+}$: \([Ar]\,3d^{9}\).
Step 3: Conclusion.
Therefore, the configuration is [Ar] \(3d^{9}\).