Here's why:

* Electron Configuration: Copper's electron configuration is [Ar] 3d¹⁰ 4s¹.

* Valence Electrons: Valence electrons are the electrons in the outermost energy level. In copper's case, the outermost energy level is the 4th level, and it has one electron in the 4s orbital.

Important Note: While copper has a full 3d subshell, it only has one valence electron. The 3d electrons are involved in bonding, but they are considered core electrons, not valence electrons.