Materials containing metals such as copper, aluminum, gold and silver consist of metallic bonds, while materials with transition metals such as iron and nickel are composed of both covalent bonds and metallic bonds.
Metallic bonds are strong electrostatic attractions that bind metal atoms. Since metals have low ionization energy the electrons can move freely. Therefore, in a metal bond, there is a sea of delocalized valence electrons surrounding the positive atomic nuclei. These electrons are attracted toward the positively charged atomic nuclei forming a metal lattice. Due to this arrangement, metals have unique properties such as shiny metallic luster, electrical and thermal conductivity, malleability, ductility, high tensile strength, hardness and opaqueness. Also, since the strength of metallic bonds is intermediate to that of ionic bonds and covalent bonds, metals have melting points and boiling points in between that of the ionic and covalent compounds. The low volatility and high density of metals can also be attributed to the strong attractive forces. Zinc, cadmium and mercury, which are Group-7 metals, are considered exceptions to these arrangements. Metallic bonds are non-directional bonds. These bonds are also weak because of the simultaneous attraction of valence electrons to a large number of atomic nuclei.