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Section 3: Ionic and Covalent-Network Solids
By contrasting two types of solids, ionic and covalent-network, we can easily see how centrally chemical bonds determine the characteristics of solid materials. Ionic solids are made up of ions held together by ionic bonds. All salts and most minerals are ionic solids. As we learned in Unit 5, ionic bonds form when electrons jump from the valence shell of one atom to another, creating negatively charged and positively charged ions. The ions attract each other and are held together by Coulombic forces.
Figure 13-5. Conceptual Design for a Molten Salt Reactor, Using Salt as Both Fuel and Coolant
Molten salt reactors take advantage of salts' high melting points, substituting salts for pressurized water as a coolant and sometimes as fuel.
© Wikimedia Commons, Public Domain. Author: U.S. Department of Energy, 2009.
Ionic bonds are strong, so it takes a lot of energy to break them. This property gives ionic solids high melting and boiling points. For example, sodium chloride melts at 800°C. The sodium and chloride ions that make up sodium chloride have charges of 1+ and 1-, respectively. The ions in magnesium oxide (MgO) have charges of 2+ and 2-, respectively, so their bonds are even stronger: Magnesium oxide's melting point is 2852°C. Ions cannot conduct electricity in their solid state, but can conduct electricity if they are dissolved in water or melted, thereby breaking their ionic bonds.
Because ionic solids have high melting points, nuclear engineers are studying them as a potential cooling material for nuclear reactors. Without a coolant, the heat generated by chain reactions inside nuclear fuel rods would melt the fuel, releasing radiation. Commercial reactors today use water as a coolant; but since water boils at 100°C, most reactors are pressurized to prevent the coolant from boiling (remember the role of pressure in phase changes from Unit 2). Reactors using molten salt as a coolant can operate at much higher temperatures and at near-atmospheric pressure, which reduces stress on the equipment.2 Figure 13-5 shows a conceptual design for a molten salt reactor.
Covalent-network solids are made up of many atoms held together in large, regular lattices by covalent bonds. These substances are extremely strong and have very high melting points because they contain so many covalent bonds, which are stronger than intermolecular forces. A covalent-network crystal is like one big molecule of the substance, since its units are held together in a continuous network of covalent bonds. Breaking the material is difficult because it involves breaking many chemical bonds. Examples include diamonds, silica, and graphite.
Figure 13-6. Bonding in Diamonds and Graphite
Diamonds and graphite are forms of carbon with very different properties due to their contrasting chemical structure.
© Science Media Group.
Many elements can be found as allotropes: different forms made up of the same element arranged in different structures. Diamonds and graphite are allotropes of carbon. (Figure 13-6) Each atom of a diamond is bonded to four other carbon atoms in a repeating three-dimensional structure, which makes diamonds so hard and durable that they are widely used in industry to cut other materials. In contrast, graphite's atoms are arranged in layers of flat six-sided rings, which are stacked and can slide freely past each other. This structure makes graphite a good lubricant. Cores of conventional writing pencils, widely referred to as "leads," are actually made of graphite mixed with a clay binder.
2For more information on molten salt reactors and other advanced designs, see M. Mitchell Waldrop, "Nuclear Energy: Radical Reactors," Nature, December 5, 2012. http://www.nature.com/news/nuclear-energy-radical-reactors-1.11957.