Earth Materials. John O'Brien
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Название: Earth Materials

Автор: John O'Brien

Издательство: John Wiley & Sons Limited

Жанр: География

Серия:

isbn: 9781119512219

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СКАЧАТЬ 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.

Schematic illustration of 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.

      2.3.5 Transitional (hybrid) bonds

      Where electronegativity differences in transitional ionic–covalent bonds are smaller than 1.68, the bonds are primarily electron‐sharing covalent bonds. Where electronegativity differences are larger than 1.68, the bonds are primarily electron‐transfer ionic bonds. Calculations of electronegativity and bond type lead to some interesting conclusions. For example, when an oxygen atom with En = 3.44 bonds with another oxygen atom with En = 3.44 to form O2, the electronegativity difference (3.44 − 3.44 = 0.0) is zero and the resulting bond is 100% covalent. The valence electrons are completely shared by the two oxygen atoms. This will be the case whenever two highly electronegative, nonmetallic atoms of the same element bond together. On the other hand, when highly electronegative, nonmetallic atoms bond with strongly electropositive, metallic elements to form ionically bonded substances, the bond is never purely ionic. There is always at least a small degree of electron sharing and covalent bonding. For example, when sodium (Na) with En = 0.93 bonds with chlorine (Cl) with En = 3.6 to form sodium chloride (NaCl), the electronegativity difference (3.6 − 0.93 = 2.67) is 2.67 and the bond is only 83% ionic and 17% covalent. Although the valence electrons are largely transferred from sodium to chloride and the bond is primarily electrostatic (ionic), a degree of electron sharing (covalent bonding) exists. Even in this paradigm of ionic bonding, electron transfer is incomplete and a degree of electron sharing occurs. The bonding between silicon (Si) and oxygen (O), so important in silicate minerals, is very close to the perfect hybrid since the electronegativity difference is 3.44 − 1.90 = 1.54 and the bond is 45% ionic and 55% covalent.

Graph depicts the electronegativity difference and bond type in covalent–ionic bonds. Schematic illustration of triangular diagram representing the bond types of some common minerals.

      2.3.6 Van der Waals and hydrogen bonds