technotes.qxp 10/27/2008 8:02 AM Page 1
Tech Notes
Metallic Bonding metal is defined as any In metallic bonding, each positive and negative ions. element that tends to lose atom in the metal In the ionic bond, the electrons from the outer contributes its valence electrons are shared by an shells of its atoms. The electrons to the formation of electropositive ion (cation) resulting positive ions are an electron cloud that and an electronegative ion held together in a crystalline surrounds the positively (anion). The electropositive structure by the cloud of charged metal ions, as ion gives up its valence these free electrons in what illustrated in the diagram. electrons, while the is known as the metallic Hence, the valence electronegative ion bond. The metallic bond electrons are shared by all captures them to produce Ayields three physical of the atoms. The positively ions having full electron characteristics typical of charged ions repel each orbitals or suborbitals. As a solid metals: other uniformly, so they consequence, no free • Metals are good arrange themselves into a electrons are available to conductors of electricity, regular pattern that is held conduct electricity. Ionic • Metals are good together by the negatively bonds are formed between conductors of heat, and charged electron cloud. electropositive metals and • Metals have a lustrous Since the negative electron electronegative nonmetals. appearance. cloud surrounds each of the positive ions that make up Covalent bonds are In addition, most metals are the orderly three- formed by many elements malleable, ductile, and dimensional crystal that have three or more generally of higher density structure, strong electronic valence electrons. The than other elemental attraction holds the metal atoms are bound into substances. together. crystal structures by forces arising from the Bonding types Because the valence sharing of electrons. Bonding in solids may be electrons are no longer In covalently bonded classified as either primary attached to specific positive ceramics, the bonding or secondary. Primary ions and are free to travel between atoms is specific bonding includes metallic, among the positive ions, and directional, ionic, and covalent bonds. metals exhibit high involving the exchange of Although metallic bonding electrical and thermal electron charge between This information is from may be predominant, conductivity. pairs of atoms. High Elements of Metallurgy and other types of bonding, hardness and low electrical Engineering Alloys, by F.C. in particular covalent Ionic bonding is a result of conductivity are Campbell. It is available at bonding, may also be electrical attraction characteristic of covalent www.asminternational.org. present. between alternately placed bonds.
Metallic bond Covalent bond Ionic bond
Primary bonding mechanisms include metallic, ionic, and covalent. ADVANCED MATERIALS & PROCESSES/NOVEMBER 2008 133