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Section 5: Metallic Solids
Metallic solids are made up entirely of metal atoms arranged in crystal lattices. They are joined together by metallic bonds, which are similar to covalent bonds in that they involve shared electrons. However, valence electrons in metals are shared among all the atoms in a crystal, not just between two individual atoms. The atoms in the crystal become positive ions surrounded by a sea of electrons, and the interaction between these ions and the valence electrons binds the entire crystal together. Figure 13-9 shows this arrangement for atoms in a block of aluminum, which has three valence electrons: The valence electrons merge into an electron cloud around Al 3+ cations.
Figure 13-9. Metallic Bonding in Aluminum
Aluminum atoms in a crystal lattice of positively charged ions and negative electrons.
© NDT Resource Center, http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/metallic.htm, Center for NDE, Iowa State University.
Metals generally are solids at room temperature, except for mercury, but their melting points vary widely. Generally, the hardest metals are the ones at the center of the periodic table, as metals with more valence electrons for binding are harder and have higher melting points. The softest are the alkali metals, with only one valence electron: Sodium, potassium, rubidium, and cesium all have melting points below 100°C and are easy to cut with a knife. At the other extreme, tungsten melts at 3422°C and is 70 percent denser than lead. But metals with more electrons, such as silver and zinc, start to become softer again because they only have one or two electrons in their valence shell's s orbital.
Metals have other properties that make them extremely useful materials. They are less brittle than ionic or covalent-network solids: Their atoms can slide past each other without breaking their metallic bonds because they are surrounded by a sea of electrons. Most metals are ductile (easily stretched into wires) and malleable (easily shaped and formed by hammering or pressure). They also conduct electricity well, because free electrons flow readily between metal atoms. Copper is widely used in electrical wiring because it excels in many of these qualities. It is highly ductile, so it can be drawn into thin wires; has high tensile strength, so considerable force is required to break it; and has high electrical conductivity, so it transmits electricity with little resistance. (Resistance converts a fraction of the current passing through the wire into waste heat that does not reach the destination point.)
Well before chemistry became a science, it was widely known that metals could be made even more useful by combining them in an alloy. An alloy contains more than one element, including at least one metal, and has metallic properties. Modern solid-state chemistry is producing many highly sophisticated alloys, which are discussed in Section 9 of this unit.