The electron configuration of copper (Cu), with atomic number 29, is [Ar] 3d¹⁰ 4s¹ or 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹. 

Here's a more detailed explanation:

• Atomic Number:Copper (Cu) has 29 electrons. 

• Following the Aufbau principle, the expected configuration would be 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁹. 

• However, copper exhibits an exception to this rule. A full or half-filled d-orbital is more stable than a partially filled one. 

• Therefore, one electron moves from the 4s orbital to the 3d orbital, resulting in the configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹ or [Ar] 3d¹⁰ 4s¹.