The observed electron configuration of copper (Cu) is [Ar] 3d^10 4s^1.
Copper has an atomic number of 29, which means it has 29 electrons. The electron configuration describes the distribution of these electrons in the various energy levels and orbitals around the nucleus.
The noble gas configuration of argon (Ar) is [Ne] 3s^2 3p^6. Copper's electron configuration begins by filling the orbitals up to the 3d sublevel with 10 electrons, following the Aufbau principle and Hund's rule. This gives us [Ar] 3d^10.
After the 3d sublevel is filled, one electron occupies the 4s orbital, resulting in the observed electron configuration of copper as [Ar] 3d^10 4s^1. It is important to note that while the 4s orbital is filled before the 3d orbital in most elements, copper is an exception due to the relatively higher stability of a fully-filled or half-filled d sublevel.