Page 28 - Materials Chemistry, Second Edition

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2.2. Types of Bonding in Solids

material. As expected, these associations not only govern the behavior of a material
in the solid state, but also for the less ordered liquid phase. For example, the
hydrogen bonding interactions between neighboring water molecules within an ice
lattice are also important in the liquid phase, resulting in high surface tension and
finite viscosity. For the gaseous state, intermolecular forces have been overcome,
and no longer have an impact on its properties.

2.2.1. Ionic Solids

These solids are characterized by cationic and anionic species that are associated
through electrostatic interactions. All predominantly ionic salts possess crystalline
structures, as exhibited by common Group 1–17 or 2–17 binary salts such as NaCl
and CaCl 2 (Figure 2.1). The melting points of these solids are extremely high, as
very strong electrostatic attractions between counterions must be overcome.
Although oppositely charged ions have attractive interactions, like charges repel
one another. In the determination of the lattice energy, U, the sizes and charges of
the ions are most important (Eq. 1). That is, the lattice energy for MgO would be
much greater than BaO, since the ionic bonding is much stronger for the magnesium
salt due to its high charge/small size (large charge density). By contrast, the salt
MgN does not exist, even though Mg 3þ and N 3 would be very strongly attracted
through electrostatic interactions. The ionization energy required to produce the
trivalent magnesium ion is too prohibitive.

Figure 2.1. Ionic model for sodium chloride. This is a face-centered arrangement of chloride ions (white),
with sodium ions occupying the octahedral interstitial sites (red). The attractive electrostatic forces, a,
+
+
between adjacent Na and Cl ions, and repulsive forces, r, between Na ions are indicated.