Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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from each atom.

      Other diatomic gases with covalent bonding mechanisms similar to oxygen include the column 17 (group VIIA) gases chlorine (Cl2), fluorine (F2), and iodine (I2) in which single electrons are shared between the two atoms to achieve a stable electron configuration. Another gas that possesses covalent bonds is nitrogen (N2) from column 15 (group V) where three electrons from each atom are shared to achieve a stable electron configuration. Nitrogen is the most abundant gas (>79% of the total) in Earth's lower atmosphere. In part because the two atoms in nitrogen and oxygen gas are held together by strong electron‐sharing bonds that yield stable electron configurations, these two molecules are the most abundant constituents of Earth's lower atmosphere.

Schematic illustration of covalent bonding (double lines) in a carbon tetrahedron with the central carbon atom bonded to four carbon atoms that occupy the corners of a tetrahedron (dashed lines).

      Source: Courtesy of Steve Dutch.

      Covalently bonded minerals are generally characterized by the following:

      1 Hard and brittle at room temperature.

      2 Insoluble in polar substances such as water.

      3 Crystallize from melts.

      4 Moderate to high melting temperatures.

      5 Absorb very little light, producing transparent to translucent minerals with light colors and vitreous to sub‐vitreous lusters in macroscopic crystals.

      2.3.4 Metallic bonds

      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