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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      2.3.2 Ionic (electrostatic) bonds

      When very metallic atoms bond with very nonmetallic atoms, an ionic bond, also called an electrostatic bond, is formed. Because the very metallic atoms (e.g., columns 1 and 2) are electropositive, they have a strong tendency to give up one or more electrons to achieve a stable configuration in their highest principal quantum level. In doing so, they become positively charged cations, whose charge is equal to the number of electrons each has lost. At the same time, very nonmetallic atoms (columns 16 and 17) are electronegative and have a strong tendency to gain one or more electrons in order to achieve a stable configuration in their highest principal quantum level. In doing so, they become negatively charged anions, with a charge equal to the number of electrons each has gained. When very metallic and very nonmetallic atoms bond, the metallic atoms give up or donate their valence electrons to the nonmetallic atoms that capture them. It is like a tug‐of‐war in which the electronegative side always wins the battle for electrons. In the electron exchange process, the atoms of both elements develop stable noble element electron configurations while becoming ions of opposite charge. Because particles of opposite charge attract, the cations and anions are held together by the electrostatic attraction between them that results from their opposite charges. Larger clusters of ions form as additional ions exchange electrons and are bonded and crystals begin to grow.

Schematic illustration of ionic bonding develops between highly electronegative anions and highly electropositive cations.

      Ionic bonds also form when group IIA and group VIA elements combine. In the mineral periclase (MgO), magnesium (Mg+2) and oxygen (O−2) ions are bonded together to form MgO. In this case, electropositive, metallic magnesium atoms from group IIA tend to donate two valence electrons to become stable, smaller divalent magnesium cations (Mg+2) while highly electronegative, nonmetallic oxygen atoms from group VIA capture two valence electrons to become stable, larger divalent oxygen anions (O−2). The two oppositely charged ions are then held together by virtue of their opposite charges by an electrostatic or ionic bond. Once again, the number of magnesium cations (Mg+2) and oxygen anions (O−2) in periclase (MgO) must be the same if electrical neutrality is to be conserved. A slightly more complicated example of ionic bonding involves the formation of the mineral fluorite (CaF2). In this case, electropositive, metallic calcium atoms from class IIA release two electrons to become stable divalent cations (Ca+2). At the same time, two nonmetallic, strongly electronegative fluorine atoms from class VIIA each accept one of these electrons to become stable univalent anions (F−1). Pairs of F−1 anions bond to each Ca+2 cation to form ionic bonds in electrically neutral fluorite (CaF2).

      Bonding mechanisms play an essential role in contributing to material properties. Crystals with ionic bonds are generally characterized by the following:

      1 Variable hardness that increases with increasing electrostatic bonding forces

      2 Brittle at room temperatures.

      3 Quite soluble in polar substances (such as water).

      4 Intermediate melting temperatures.

      5 Absorb relatively small amounts of light, producing translucent to transparent minerals with light colors and vitreous to sub‐vitreous luster in macroscopic crystals.

      2.3.3 Covalent (electron‐sharing) bonds

Schematic illustration of covalent bonding in oxygen (O2) by the sharing of two electrons from each atom.