Earth Materials. John O'Brien

      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.

Figure 2.10 (a) Ionic bonding develops between highly electronegative anions and highly electropositive cations. When neutral sodium (Na⁰) atoms (red) release an electron to become cations (Na⁺¹) their ionic radius decreases. When neutral chlorine (Cl⁰) atoms (blue) capture an electron to become anions (Cl⁻¹) their ionic radius increases. (b) Ions of opposite charge attract to form crystals such as sodium chloride (NaCl).

The most frequently cited example of ionic bonding is the bonding between sodium (Na⁺¹) and chloride (Cl⁻¹) ions in the mineral halite (NaCl) (Figure 2.10). As a column 1 (group IA) element, sodium is very metallic and electropositive, with a rather low electronegativity (0.93). Sodium has a strong tendency to give up one electron to achieve a stable electron configuration. On the other hand, chlorine, as a column 17 (group VIIA) element, is very nonmetallic, has a strong affinity for electrons and has a high electronegativity (3.5). It has a strong tendency to gain one electron to achieve a stable electron configuration. When sodium and chlorine atoms bond, the sodium atoms release one electron to become smaller sodium cations (Na⁺¹) with the “stable octet” electron configuration (neon), while the chlorine atoms capture one electron to become larger chloride anions (Cl⁻¹) with “stable octet” electron configurations. As we shall see, the “exchange” is incomplete. The two atoms are then joined together by the electrostatic attraction between particles of opposite charge to form the compound NaCl. In macroscopic mineral specimens of halite, many millions of sodium and chloride ions are bonded together, each by the electrostatic or ionic bond described above. Note that the numbers of chloride anions and sodium cations must be equal if the electric charges are to be balanced so that the mineral is electrically neutral. Other group IA (1) and group VIIA (17) elements bond ionically to produce minerals such as sylvite (KCl).

      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).

In ideal ionic bonds, ions can be modeled as spheres of specific ionic radii in contact with one another (Figure 2.10), as though they were ping‐pong balls or marbles in contact with each other. This approximates real situations because the attractive force between ions of opposite charge (Coulomb attraction) and the repulsive force (Born repulsion) between the negatively charged electron clouds are balanced when the two ions approximate spheres in contact (Figure 2.11). If they were moved farther apart, the electrostatic attraction between ions of opposite charge would move them closer together. If they were moved closer together, the repulsive forces between the negatively charged electron clouds would move them farther apart. It is when they behave as approximate spheres in contact that these attractive and repulsive forces are balanced.

      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

When nonmetallic atoms bond with other nonmetallic atoms they tend to form covalent bonds , also called electron‐sharing bonds. Because the elements involved are highly electronegative they each tend to attract electrons; neither gives them up easily. This is a little bit like a tug‐of‐war in which neither side can be moved so neither side ends up with sole possession of the electrons needed to achieve stable electron configurations. In simple models of covalent bonding, the atoms involved share valence (thus covalent) electrons (Figure 2.12). By sharing electrons, each atom gains the electrons necessary to achieve a more stable electron configuration in its highest principal quantum level.

Figure 2.11 Relationship between attractive and repulsive forces between ions produces a minimum net force when the near spherical surfaces of the ions are in contact.

Figure 2.12 Covalent bonding in oxygen