How many valence electrons does a copper atom have?

A copper atom has a total of 29 electrons, with its electron configuration being [Ar] 4s² 3d¹⁰. When determining the number of valence electrons, only the electrons in the outermost shell are considered. In the case of copper, the outermost shell comprises the 4s and 3d orbitals. Although there are ten electrons in the 3d subshell, only the two electrons in the 4s subshell can be classified as valence electrons. Consequently, copper typically exhibits one valence electron when discussing its most common oxidation states, particularly when it forms compounds. This behavior aligns with copper's tendency to lose one electron to achieve stability during chemical reactions.