<P> Copper is essential to all living organisms as a trace dietary mineral because it is a key constituent of the respiratory enzyme complex cytochrome c oxidase . In molluscs and crustaceans, copper is a constituent of the blood pigment hemocyanin, replaced by the iron - complexed hemoglobin in fish and other vertebrates . In humans, copper is found mainly in the liver, muscle, and bone . The adult body contains between 1.4 and 2.1 mg of copper per kilogram of body weight . </P> <P> Copper, silver, and gold are in group 11 of the periodic table; these three metals have one s - orbital electron on top of a filled d - electron shell and are characterized by high ductility, and electrical and thermal conductivity . The filled d - shells in these elements contribute little to interatomic interactions, which are dominated by the s - electrons through metallic bonds . Unlike metals with incomplete d - shells, metallic bonds in copper are lacking a covalent character and are relatively weak . This observation explains the low hardness and high ductility of single crystals of copper . At the macroscopic scale, introduction of extended defects to the crystal lattice, such as grain boundaries, hinders flow of the material under applied stress, thereby increasing its hardness . For this reason, copper is usually supplied in a fine - grained polycrystalline form, which has greater strength than monocrystalline forms . </P> <P> The softness of copper partly explains its high electrical conductivity (59.6 × 10 S / m) and high thermal conductivity, second highest (second only to silver) among pure metals at room temperature . This is because the resistivity to electron transport in metals at room temperature originates primarily from scattering of electrons on thermal vibrations of the lattice, which are relatively weak in a soft metal . The maximum permissible current density of copper in open air is approximately 3.1 × 10 A / m of cross-sectional area, above which it begins to heat excessively . </P> <P> Copper is one of a few metallic elements with a natural color other than gray or silver . Pure copper is orange - red and acquires a reddish tarnish when exposed to air . The characteristic color of copper results from the electronic transitions between the filled 3d and half - empty 4s atomic shells--the energy difference between these shells corresponds to orange light . </P>

When was copper added to the periodic table