Copper (Cu). Diagram of the nuclear composition, electron configuration, chemical data, and valence orbitals of an atom of copper-64 (atomic number: 29), an isotope of this element. The nucleus consists of 29 protons (red) and 35 neutrons (orange). 29 electrons (white) successively occupy available electron shells (rings). Copper is a transition metal in group 11, period 4, and the d-block of the periodic table. It has a melting point of 1084 degrees Celsius. The trends across the transition metals are due to electrons filling an inner d-subshell (here, within the 3rd ring), shielding the outer (valence) electrons from the increasing nuclear charge.