M11Q3: Types of Solids

Introduction

We now begin to look more closely at the solid phase of matter. In this section we will see that solids can have different structures and bonding motifs. These characteristics will then affect the physical properties of the solid.

Learning Objectives for Types of Solids and Unit Cell Definitions

| Key Concepts and Summary | Glossary | End of Section Exercises |

Crystalline & Amorphous Solids

When most liquids are cooled, they eventually freeze and form crystalline solids, 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 solids or noncrystalline solids (or, sometimes, glasses). The particles of such solids lack an ordered internal structure and are randomly arranged (Figure 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 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 example, candle waxes are amorphous solids composed of large hydrocarbon molecules. Some substances, such as silicon dioxide (shown in Figure 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 structures are shown. The first set contains numerous silicon and oxygen atoms bonded together without apparent order. The second set shows silicon and oxygen atoms bonded together in an orderly arrangement.
Figure 2. (a) Amorphous silica (SiO2), a type of manufactured glass, has a high degree of disorder in its structure. (b) Quartz, on the other hand, is a crystalline form of silica that has a very ordered structural arrangement.

Crystalline solids are generally classified according to 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: metallic, covalent network, ionic, and molecular.

Metallic Solids

Metallic solids such as crystals of copper, aluminum, and iron are formed by metal atoms (Figure 3). 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.

This figure shows large brown spheres arranged in a cube.
Figure 3. Copper is a metallic solid.

Covalent Network Solids

Covalent network solids 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 4. 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 pairs of images are shown. In the first pair, a square box containing a black atom bonded to four other black atoms is shown above a structure composed of many black atoms, each bonded to four other black atoms, where one of the upper atoms is labeled “carbon” and the whole structure is labeled “diamond.” In the second pair, a square box containing a white atom bonded to four red atoms is shown above a structure composed of many white atoms, each bonded to four red atoms, where one of the red atoms is labeled “oxygen” and one of the white atoms is labeled “silicon.” The whole structure is labeled “silicon dioxide.” In the third pair, a square box containing a blue atom bonded to four white atoms is shown above a structure composed of many blue atoms, each bonded to four white atoms, where one of the blue atoms is labeled “carbon” and one of the white atoms is labeled “silicon.” The whole structure is labeled “silicon carbide.” In the fourth pair, a square box containing six black atoms bonded into a ring is shown above a structure composed of many rings, arranged into sheets layered one atop the other, where one of the black atoms is labeled “carbon.” The whole structure is labeled “graphite.”
Figure 4. 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.

Ionic Solids

Ionic solids, 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 5). 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. Recall that you learned about these attractions back in Module 2 when we discussed lattice energy. 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.

This figure shows large purple spheres bonded to smaller green spheres in an alternating pattern. The spheres are arranged in a cube.
Figure 5. Sodium chloride is an ionic solid.

Molecular Solids

Molecular solids, such as ice, sucrose (table sugar), and iodine, as shown in Figure 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 H2, N2, O2, and F2, 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 images are shown and labeled “carbon dioxide” and “iodine.” The carbon dioxide structure is composed of molecules, each made up of one gray and two red atoms, stacked together into a cube. The image of iodine shows pairs of purple atoms arranged near one another, but not touching.
Figure 6. Carbon dioxide (CO2) consists of small, nonpolar molecules and forms a molecular solid with a melting point of −78 °C. Iodine (I2) 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 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 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 NaCl, Al2O3
metallic atoms of electropositive elements metallic bonds shiny, malleable, ductile, conducts heat and electricity well, variable hardness and melting temperature Cu, Fe, Ti, Pb, U
covalent network atoms of electronegative elements covalent bonds very hard, not conductive, very high melting points C (diamond), SiO2, SiC
molecular molecules (or atoms) IMFs variable hardness, variable brittleness, not conductive, low melting points H2O, CO2, I2, C12H22O11

Chemistry in Real Life: Graphene—Material of the Future

Carbon is an essential element in our world. The unique properties of carbon atoms allow the existence of carbon-based life forms such as ourselves. Carbon forms a huge variety of substances that we use on a daily basis, including those shown in Figure 7. You may be familiar with diamond and graphite, the two most common allotropes of carbon. (Allotropes are different structural forms of the same element.) Diamond is one of the hardest-known substances, whereas graphite is soft enough to be used as pencil lead. These very different properties stem from the different arrangements of the carbon atoms in the different allotropes.

Three pairs of images are shown, each composed of a photo and a diagram. In the first pair, the photo shows a close-up view of a colorless, multi-faceted crystal and the diagram shows many gray spheres bonded together in a net-like structure. The caption below this pair reads “diamond.” In the second pair, the photo shows a rough textured, dark gray solid while the image shows four horizontal sheets, composed of interlocking black spheres, lying atop one another. This pair has a caption that reads “graphite.” The third pair shows a photo of twelve black hexagons on a yellow background where two of the hexagons are encircled by a gray border and a caption of “1.4 times 10, superscript negative 10, m, Distance between center of atoms” and an image of many black hexagons evenly arranged on a yellow background. The caption below this pair of images reads “Graphite surface.”
Figure 7. Diamond is extremely hard because of the strong bonding between carbon atoms in all directions. Graphite (in pencil lead) rubs off onto paper due to the weak attractions between the carbon layers. An image of a graphite surface shows the distance between the centers of adjacent carbon atoms. (credit left photo: modification of work by Steve Jurvetson; credit middle photo: modification of work by United States Geological Survey)

You may be less familiar with a recently discovered form of carbon: graphene. Graphene was first isolated in 2004 by using tape to peel off thinner and thinner layers from graphite. It is essentially a single sheet (one atom thick) of graphite. Graphene, illustrated in Figure 8, is not only strong and lightweight, but it is also an excellent conductor of electricity and heat. These properties may prove very useful in a wide range of applications, such as vastly improved computer chips and circuits, better batteries and solar cells, and stronger and lighter structural materials. The 2010 Nobel Prize in Physics was awarded to Andre Geim and Konstantin Novoselov for their pioneering work with graphene.

Four images are shown. In the upper image, labeled “Graphene sheet,” a box is drawn around a sheet of interconnected hexagonal rings. In the lower left image, a sphere is composed of hexagonal rings linked together and is labeled “Buckyball.” In the lower middle image, a tube is shown that is composed of many hexagonal rings joined together and is labeled “Nanotube.” In the lower right image, four horizontal sheets composed of joined, hexagonal rings is shown and labeled “Stacked sheets.”
Figure 8. Graphene sheets can be formed into buckyballs, nanotubes, and stacked layers.

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.

Glossary

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
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

Chemistry End of Section Exercises

  1. At very low temperatures oxygen, O2, freezes and forms a crystalline solid. Which best describes these crystals?
    1. ionic
    2. covalent network
    3. metallic
    4. amorphous
    5. molecular crystals
  2. Explain why ice, which is a crystalline solid, has a melting temperature of 0 °C, whereas butter, which is an amorphous solid, softens over a range of temperatures.
  3. Identify the type of crystalline solid (metallic, network covalent, ionic, or molecular) formed by each of the following substances:
    1. CaCl2
    2. SiC
    3. N2
    4. Fe
    5. C (graphite)
    6. CH3CH2CH2CH3
    7. HCl
    8. NH4NO3
    9. K3PO4
  4. Classify each substance in the table as either a metallic, ionic, molecular, or covalent network solid:
    Substance Appearance Melting Point Electrical Conductivity Solubility in Water
    X lustrous, malleable 1500 °C high insoluble
    Y soft, yellow 113 °C none insoluble
    Z hard, white 800 °C only if melted/dissolved soluble
  5. Classify each substance in the table as either a metallic, ionic, molecular, or covalent network solid:
    Substance Appearance Melting Point Electrical Conductivity Solubility in Water
    X brittle, white 800 °C only if melted/dissolved soluble
    Y shiny, malleable 1100 °C high insoluble
    Z hard, colorless 3550 °C none insoluble
  6. Substance A is shiny, conducts electricity well, and melts at 975 °C. Substance A is likely a(n):
    1. ionic solid
    2. metallic solid
    3. molecular solid
    4. covalent network solid
  7. Substance B is hard, does not conduct electricity, and melts at 1200 °C. Substance B is likely a(n):
    1. ionic solid
    2. metallic solid
    3. molecular solid
    4. covalent network solid

Answers to Chemistry End of Section Exercises

  1. E
  2. Ice has a crystalline structure stabilized by hydrogen bonding. These intermolecular forces are of comparable strength and thus require the same amount of energy to overcome. As a result, ice melts at a single temperature and not over a range of temperatures. The various, very large molecules that compose butter experience varied van der Waals attractions of various strengths that are overcome at various temperatures, and so the melting process occurs over a wide temperature range.
  3. (a) ionic;  (b) covalent network;  (c) molecular;  (d) metallic;  (e) covalent network
    (f) molecular;  (g) molecular;  (h) ionic;  (i) ionic
  4. X = metallic; Y = molecular; Z = ionic
  5. X = ionic; Y = metallic; Z = covalent network
  6. B
  7. D
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