Chapter 11: Liquids and Solids

11.6 The Solid State of Matter

Learning Outcomes

  • Define and describe the bonding and properties of ionic, molecular, metallic, and covalent network crystalline solids
  • Describe the main types of crystalline solids: ionic solids, metallic solids, covalent network solids, and molecular solids
  • Explain the ways in which crystal defects can occur in a solid

When most liquids are cooled, they eventually freeze and form crystalline solid, solids in which the atoms, ions, or molecules are arranged in a definite repeating pattern. It is also possible for a liquid to freeze before its molecules become arranged in an orderly pattern. The resulting materials are called amorphous solid or noncrystalline solids (or, sometimes, glasses). The particles of such solids lack an ordered internal structure and are randomly arranged (Figure 11.6.1).

Two images are shown and labeled, from left to right, “Crystalline” and “Amorphous.” The crystalline diagram shows many circles drawn in rows and stacked together tightly. The amorphous diagram shows many circles spread slightly apart and in no organized pattern.
Figure 11.6.1. The entities of a solid phase may be arranged in a regular, repeating pattern (crystalline solids) or randomly (amorphous).

Metals and ionic compounds typically form ordered, crystalline solids. Substances that consist of large molecules, or a mixture of molecules whose movements are more restricted, often form amorphous solids. For examples, candle waxes are amorphous solids composed of large hydrocarbon molecules. Some substances, such as silicon dioxide (shown in Figure 11.6.2), can form either crystalline or amorphous solids, depending on the conditions under which it is produced. Also, amorphous solids may undergo a transition to the crystalline state under appropriate conditions.

Two sets of molecules are shown. The first set of molecules contains five identical, hexagonal rings composed of alternating red and maroon spheres single bonded together and with a red spheres extending outward from each maroon sphere. The second set of molecules shows four rings with twelve sides each that are joined together. Each ring is composed of alternating red and maroon spheres single bonded together and with a red spheres extending outward from each maroon sphere.
Figure 11.6.2. (a) Silicon dioxide, [latex]\ce{SiO2}[/latex], is abundant in nature as one of several crystalline forms of the mineral quartz. (b) Rapid cooling of molten [latex]\ce{SiO2}[/latex] yields an amorphous solid known as “fused silica”.

Crystalline solids are generally classified according the nature of the forces that hold its particles together. These forces are primarily responsible for the physical properties exhibited by the bulk solids. The following sections provide descriptions of the major types of crystalline solids: ionic, metallic, covalent network, and molecular.

Ionic Solids

This figure shows large purple spheres bonded to smaller green spheres in an alternating pattern. The spheres are arranged in a cube.
Figure 11.6.3. Sodium chloride is an ionic solid.

Ionic solid, such as sodium chloride and nickel oxide, are composed of positive and negative ions that are held together by electrostatic attractions, which can be quite strong (Figure 11.6.3). Many ionic crystals also have high melting points. This is due to the very strong attractions between the ions—in ionic compounds, the attractions between full charges are (much) larger than those between the partial charges in polar molecular compounds. This will be looked at in more detail in a later discussion of lattice energies. Although they are hard, they also tend to be brittle, and they shatter rather than bend. Ionic solids do not conduct electricity; however, they do conduct when molten or dissolved because their ions are free to move. Many simple compounds formed by the reaction of a metallic element with a nonmetallic element are ionic.

Metallic Solids

This figure shows large brown spheres arranged in a cube.
Figure 11.6.4. Copper is a metallic solid.

Metallic solid such as crystals of copper, aluminum, and iron are formed by metal atoms Figure 11.6.4. The structure of metallic crystals is often described as a uniform distribution of atomic nuclei within a “sea” of delocalized electrons. The atoms within such a metallic solid are held together by a unique force known as metallic bonding that gives rise to many useful and varied bulk properties. All exhibit high thermal and electrical conductivity, metallic luster, and malleability. Many are very hard and quite strong. Because of their malleability (the ability to deform under pressure or hammering), they do not shatter and, therefore, make useful construction materials. The melting points of the metals vary widely. Mercury is a liquid at room temperature, and the alkali metals melt below 200 °C. Several post-transition metals also have low melting points, whereas the transition metals melt at temperatures above 1000 °C. These differences reflect differences in strengths of metallic bonding among the metals.

Covalent Network Solid

Covalent network solid include crystals of diamond, silicon, some other nonmetals, and some covalent compounds such as silicon dioxide (sand) and silicon carbide (carborundum, the abrasive on sandpaper). Many minerals have networks of covalent bonds. The atoms in these solids are held together by a network of covalent bonds, as shown in Figure 11.6.5. To break or to melt a covalent network solid, covalent bonds must be broken. Because covalent bonds are relatively strong, covalent network solids are typically characterized by hardness, strength, and high melting points. For example, diamond is one of the hardest substances known and melts above 3500 °C.

Four substances are illustrated, each with a rendering of one molecule, and below that, a rendering of how the molecules stack together. First is diamond, packed densely; then silicone dioxide, orderly but with large gaps between moledules; then silicon carbide, then carbon, shown in four flat layers stacked together.
Figure 11.6.5. A covalent crystal contains a three-dimensional network of covalent bonds, as illustrated by the structures of diamond, silicon dioxide, silicon carbide, and graphite. Graphite is an exceptional example, composed of planar sheets of covalent crystals that are held together in layers by noncovalent forces. Unlike typical covalent solids, graphite is very soft and electrically conductive.

Molecular Solid

Molecular solid, such as ice, sucrose (table sugar), and iodine, as shown in Figure 11.6.6, are composed of neutral molecules. The strengths of the attractive forces between the units present in different crystals vary widely, as indicated by the melting points of the crystals. Small symmetrical molecules (nonpolar molecules), such as [latex]\ce{H2}[/latex], [latex]\ce{N2}[/latex], [latex]\ce{O2}[/latex], and [latex]\ce{F2}[/latex], have weak attractive forces and form molecular solids with very low melting points (below −200 °C). Substances consisting of larger, nonpolar molecules have larger attractive forces and melt at higher temperatures. Molecular solids composed of molecules with permanent dipole moments (polar molecules) melt at still higher temperatures. Examples include ice (melting point, 0 °C) and table sugar (melting point, 185 °C).

Two drawings are shown. On the left, red and grey molecules are densely stacked in a 3-D drawing to represent carbon dioxide. On the right, purple molecules are scattered randomly to represent iodine.
Figure 11.6.6. Carbon dioxide ([latex]\ce{CO2}[/latex]) consists of small, nonpolar molecules and forms a molecular solid with a melting point of -78 °C. Iodine ([latex]\ce{I2}[/latex]) consists of larger, nonpolar molecules and forms a molecular solid that melts at 114 °C.

Properties of Solids

A crystalline solid, like those listed in Table 11.6.1, has a precise melting temperature because each atom or molecule of the same type is held in place with the same forces or energy. Thus, the attractions between the units that make up the crystal all have the same strength and all require the same amount of energy to be broken. The gradual softening of an amorphous material differs dramatically from the distinct melting of a crystalline solid. This results from the structural nonequivalence of the molecules in the amorphous solid. Some forces are weaker than others, and when an amorphous material is heated, the weakest intermolecular attractions break first. As the temperature is increased further, the stronger attractions are broken. Thus amorphous materials soften over a range of temperatures.

Table 11.6.1. Types of Crystalline Solids and Their Properties
Type of Solid Type of Particles Type of Attractions Properties Examples
ionic ions ionic bonds hard, brittle, conducts electricity as a liquid but not as a solid, high to very high melting points [latex]\ce{NaCl}[/latex], [latex]\ce{Al2O3}[/latex]
metallic atoms of electropositive elements metallic bonds shiny, malleable, ductile, conducts heat and electricity well, variable hardness and melting temperature [latex]\ce{Cu}[/latex], [latex]\ce{Fe}[/latex], [latex]\ce{Ti}[/latex], [latex]\ce{Pb}[/latex], [latex]\ce{U}[/latex]
covalent network atoms of electronegative elements covalent bonds very hard, not conductive, very high melting points [latex]\ce{C}[/latex] (diamond), [latex]\ce{SiO2}[/latex], [latex]\ce{SiC}[/latex]
molecular molecules (or atoms) IMFs variable hardness, variable brittleness, not conductive, low melting points [latex]\ce{H2O}[/latex], [latex]\ce{CO2}[/latex], [latex]\ce{I2}[/latex], [latex]\ce{C_{12}H_{22}O_{11}}[/latex]

Crystal Defects

In a crystalline solid, the atoms, ions, or molecules are arranged in a definite repeating pattern, but occasional defects may occur in the pattern. Several types of defects are known, as illustrated in Figure 11.6.9. Vacancies are defects that occur when positions that should contain atoms or ions are vacant. Less commonly, some atoms or ions in a crystal may occupy positions, called interstitial sites, located between the regular positions for atoms. Other distortions are found in impure crystals, as, for example, when the cations, anions, or molecules of the impurity are too large to fit into the regular positions without distorting the structure. Trace amounts of impurities are sometimes added to a crystal (a process known as doping) in order to create defects in the structure that yield desirable changes in its properties. For example, silicon crystals are doped with varying amounts of different elements to yield suitable electrical properties for their use in the manufacture of semiconductors and computer chips.

A diagram is shown in which one hundred and forty four spheres are arranged in a twelve by twelve square. A gap in the square is labeled “Vacancy” while one sphere that is a different color from all the rest is labeled “Interstitial impurity.” The top right corner of the square is disturbed and has a larger sphere inserted that is labeled “Substitution impurity atom.”
Figure 11.6.9. Types of crystal defects include vacancies, interstitial atoms, and substitutions impurities.

Key Concepts and Summary

Some substances form crystalline solids consisting of particles in a very organized structure; others form amorphous (noncrystalline) solids with an internal structure that is not ordered. The main types of crystalline solids are ionic solids, metallic solids, covalent network solids, and molecular solids. The properties of the different kinds of crystalline solids are due to the types of particles of which they consist, the arrangements of the particles, and the strengths of the attractions between them. Because their particles experience identical attractions, crystalline solids have distinct melting temperatures; the particles in amorphous solids experience a range of interactions, so they soften gradually and melt over a range of temperatures. Some crystalline solids have defects in the definite repeating pattern of their particles. These defects (which include vacancies, atoms or ions not in the regular positions, and impurities) change physical properties such as electrical conductivity, which is exploited in the silicon crystals used to manufacture computer chips.


amorphous solid: (also, noncrystalline solid) solid in which the particles lack an ordered internal structure

covalent network solid: solid whose particles are held together by covalent bonds

crystalline solid: solid in which the particles are arranged in a definite repeating pattern

interstitial sites: spaces between the regular particle positions in any array of atoms or ions

ionic solid: solid composed of positive and negative ions held together by strong electrostatic attractions

metallic solid: solid composed of metal atoms

molecular solid: solid composed of neutral molecules held together by intermolecular forces of attraction

vacancy: defect that occurs when a position that should contain an atom or ion is vacant

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