How many valence electrons are present in a copper atom?

A copper atom has a total of 29 electrons, with its electron configuration being [Ar] 3d¹⁰ 4s¹. The valence electrons are those located in the outermost shell of an atom, which in copper's case are the electrons in the 4s and 3d subshells. Although there are 10 electrons in the 3d subshell, they are not typically considered valence electrons because they are inner electrons in a filled subshell.

For copper, the single electron in the 4s subshell is the one that participates in bonding and chemical reactions, making it the defining factor for counting valence electrons. Therefore, a copper atom effectively has one valence electron that can be involved in bonding, which is why the correct answer is that it has one valence electron.