Copper is a Block D, Group 11, Period 4 element. The number of electrons in each of Copper's shells is 2, 8, 18, 1 and its electronic configuration is [Ar] 3d¹⁰ 4s¹. In its elemental form copper's CAS number is 7440-50-8. The copper atom has a radius of 127.8 .pm and it's Van der Waals radius is 140.pm. Copper is an essential trace element in animals and plants, but in excess copper is toxic. Due to its high electrical conductivity, large amounts of copper are used by the electrical industry for wire. Of all pure metals, only silver has a higher electrical conductivity. Copper is also resistant to corrosion caused by moisture, making it a widely used material in pipes, coins, and jewelry. Copper is often too soft for its applications, so it is incorporated in numerous alloys. For example, brass is a copper-zinc alloy, and bronze is a copper-tin alloy. Copper sulfate (CuSO 4·H2O), also known as blue vitrol, is the most well-known copper compound. It is used as an agricultural poison, an algicide, and as a pigment for inks. Cuprous chloride (CuCl) is a powder used to absorb carbon dioxide (CO2). Copper cyanide (CuCN) is often used in electroplating applications. Copper is available as metal and compounds with purities from 99% to 99.9999% (ACS grade to ultra-high purity); metals in the form of foil, sputtering target, and rod, and compounds as submicron and nanopowder. Copper was first discovered by Early Man. The origin of the word copper comes from the Latin word 'cuprium' which translates as "metal of Cyprus". Cyprus, a Mediterranean island, was known as an ancient source of mined copper. See Copper research below.