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When metallic atoms bond with other metallic atoms, a metallic bond is formed. Because very metallic atoms have low first ionization energies, are highly electropositive and possess low electronegativities they do not tend to hold their valence electrons strongly. In such situations, each atom releases valence electrons to achieve a stable electron configuration. The positions of the valence electrons fluctuate or migrate between atoms. Metallic bonding is difficult to model, but is usually portrayed as positively charged partial atoms (nuclei plus the strongly held inner electrons) in a matrix or “gas” of “delocalized” valence electrons that are only temporarily associated with individual atoms (Figure 2.14). The weak attractive forces between positive partial atoms and valence electrons bond the atoms together. Unlike the strong electron‐sharing bonds of covalently bonded substances, or the frequently strong electrostatic bonds of ionically bonded substances, metallic bonds are rather weak, less permanent and easily broken and reformed. Because the valence electrons are not strongly held by any of the partial atoms, they are easily moved in response to stress or in response to an electric field or thermal gradient.

Excellent examples of metallic bonding exist in the native metals such as native gold (Au), native silver (Ag), and native copper (Cu). Such materials are excellent conductors of electricity and heat. When materials with metallic bonds are subjected to an electric potential or field, delocalized electrons flow toward the positive anode, which creates and maintains a strong electric current. Similarly, when a thermal gradient exists, thermal vibrations are transferred by delocalized electrons, making such materials excellent heat conductors. When metals are stressed, the weakly held electrons tend to flow, which helps to explain the ductile behavior that characterizes native copper, silver, gold, and other metallically bonded substances.

Figure 2.14 A model of metallic bonds with delocalized electrons (dark red) surrounding positive charge centers that consist of tightly held lower energy electrons (light red dots) surrounding individual nuclei (blue).

Minerals containing metallic bonds are generally characterized by the following features:

1 Fairly soft to moderately hard minerals.

2 Deform plastically; malleable and ductile.

3 Excellent electrical and thermal conductors.

4 Frequently high specific gravity.

5 Excellent absorbers and reflectors of light; so are commonly opaque with a metallic luster in macroscopic crystals.