What is the actual ground-state electron configuration for copper (Cu)?

Copper is anomalous because it’s ground-state electron configuration isn’t what you’d expect it to be. The actual configuration is due to the fact that it’s more beneficial to fill the 3d orbital and leave a half-full 4s orbital than to have a full 4s orbital and a not-quite-full 3d orbital.

Copper’s actual ground-state electron configuration is 1s22s22p63s23p64s13d10. In noble gas form (which is considerably easier to read), it’s [Ar] 4s13d10.