Applied Chemistry Mr. Gensits Class Notes 11/10/2014 Bonding in Metals Metals are made up of closely packed cations. The valence electrons of metal atoms can be modeled as a sea of electrons (i.e., the valence electrons are mobile and can drift freely from one part of the metal to another.) Metallic bonds consist of the attraction of the free-floating valence electrons to the positively charged metal ions. The sea-of-electrons model explains many physical properties of metals. (1) They are good conductors of electricity because electrons can move freely in them. (2) Metals are ductile and malleable because the sea of electrons shields (insulates) the positive metal ions and allows them to slide when a force is applied. Crystalline Structures of Metals Metal atoms are arranged in very compact and orderly patterns. For atoms of approximately the same size, there are three common closely packed arrangements. (1) Body-centered cubic Every atom has eight neighbors (except the atoms on the surface) Examples: Na, K, Fe, Cr, and W (2) Face-centered cubic Every atom has twelve neighbors. Examples: Cu, Ag, Au, Al, and Pb (3) Hexagonal Every atom also has twelve neighbors. Examples: Mg, Zn,and Cd Alloys – mixtures of two of more elements, at least one of which is a metal Alloys are important because their properties are often superior to those of their component elements. (1) Sterling Silver – 92.5% Ag, 7.5% Cu Harder and more durable than pure silver but still soft enough to be made into jewelry and tableware. (2) Bronze – usually composed of 7 parts Cu/1 part Sn Harder than copper and more easily cast. (3) Steels – iron and carbon (B, Cr, Mn, Mo, Ni, W, and V are commonly added) Wide range of useful properties, such as corrosion resistance, ductility, hardness, and toughness. Substitutional Alloys – the replacement atoms are approximately the same size as the original atoms. Interstitual Alloys – if the atomic sizes are quite different, the smaller atom can fit into the interstices (spaces) between the larger atoms. In steels the carbon atoms occupy spaces between iron atoms.