The element copper has 11 valence electrons, distributed amongst the outermost d and s orbitals. These electrons are the ones that engage in chemical reactions that involve photons corresponding to visible light. The inner electrons require far higher energies to liberate, and do not figure in chemical reactions.
Copper's valence shell configuration is responsible for it's premier conductivity among metals. It's thermal conductivity is second only to silver, because the single atom in it's outermost valence s orbital interacts readily to fill the shell with the relatively stable configuration of two electrons. Copper has 29 isotopes, which have different amounts of valence electrons.