Copper has an atomic number 29 and atomic mass of 63.55. It belongs to Group I-B transition metals. The melting point of copper is 1084.6�C. Copper occurs naturally in the cuprous (I, Cu+) and cupric (II, Cu2+) valence states. There is a single electron in the outer 4s orbital. The 3d10 orbital does not effectively shield this outer electron from the positive nuclear charge, and therefore the 4s1 electron is difficult to remove from the Cu atom (1). The first ionization potential is 7.72 eV and the second is 20.29 eV. Because the second ionization potential is much higher than the first, a variety of stable Cu+ species exist (2). The ionization state of copper depends on the physical environment, the solvent, and the concentration of ligands present. In solution, copper is present as Cu2+ or complexes of this ion. The cuprous ion Cu1+ is unstable in aqueous solutions at concentrations greater than 10-7 M (3). However, in wet soils, Cu1+ is moderately stable at typically expected conditions (10-6 to 10-7 M). Under such conditions, hydrated Cu1+ would be the dominant copper species (1). Copper can exist as two natural isotopes, 63Cu and 65Cu, with relative abundances of 69.09 and 30.91%, respectively (4). In the Earth's crust, copper is present as stable sulfides in minerals rather than silicates or oxides (3). The Cu1+ ion is present more commonly in minerals formed at considerable depth, whereas Cu2+ is present close to the Earth's surface (3).
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