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Section 2: What Is a Solid?
As we first learned in Unit 2, solids are distinguished from liquids and gases by the strength of the forces that hold them together. The intermolecular attractive forces in solids are stronger than those in liquids or gases. They hold molecules close together, with very little space between them. Solid particles have low kinetic energy, so they cannot move freely, although they vibrate in place. Because their particles are closely packed, solids are hard to compress. And when scientists add a material to a solid to change its properties, the additive diffuses (is transported) through the solid very slowly. One common process that involves diffusion is galvanizing steel—applying a protective zinc coating to steel objects to keep them from rusting.
In most solids, the atoms or molecules that make up the substance are arranged in a regular order that repeats throughout the solid. These substances are called "crystalline solids." The building-block unit of a crystalline solid that repeats throughout the material (like a single brick in a brick wall) is called the "unit cell," and is usually a six-sided, three-dimensional shape whose faces have four straight sides. Each unit cell is packed with atoms, molecules, or ions of the substance. Familiar examples of crystalline solids include ice, table salt, and diamonds. Figure 13-3 shows the structure of unit cells for several salts.
Figure 13-3. Unit Cells for a Variety of Salts
Unit cells are the simplest repeating units in crystalline substances.
© Purdue, Bodner Research Group.
Crystalline solids have well-defined melting points because the same type of bonds hold particles together throughout the solid. So when the melting point is reached, the particles all respond in identical ways. Adding that maximum heat energy will uniformly disrupt the placement of the atoms, which then can move around. However, when some energy is added very quickly—for example, hitting a crystal with a hammer—this change isn't enough to overcome all of the bonds, so only some are broken. When crystals crack, they break apart along flat planes where their atomic bonding forces are the weakest.
Since the 1920s, scientists have used a technique called "X-ray crystallography" to analyze the structure of crystalline solids. This method was developed by British physicist William Henry Bragg (1862–1942) and his son William Lawrence Bragg (1890–1971), who recognized that when a beam of X-rays was aimed at a crystal, planes of atoms within the crystal would scatter the rays in patterns that could be used to map the crystal's internal structure. The Braggs shared a Nobel Prize in physics in 1915 for their work. Over the next several decades, other scientists applied diffraction to increasingly complicated substances. Dorothy Crowfoot Hodgkin (1910–1994), a British chemist, became a leading expert in the technique, which she used to determine the structures of insulin, penicillin, and vitamin B-12, an important nutrient. In 1964, Hodgkin became the third woman to receive the Nobel Prize in chemistry, following Marie Curie (1867–1934) in 1903, and again in 1911, and Irène Joliot-Curie (1897–1956) in 1935.
Solids whose particles are not arranged in an orderly structure are called "amorphous solids." In these materials, particles are arranged randomly, so they do not have the well-defined shapes that characterize crystalline structures. Amorphous solids may be compounds of molecules that do not combine neatly together, or may be composed of a single substance with large, complex molecules. Common examples include rubber and some types of plastic.
Figure 13-4. Silicon Dioxide in Crystalline and Amorphous Forms
When silicon dioxide is heated and then cooled rapidly, its ordered crystalline structure is unable to reform and it becomes glass, an amorphous solid.
© NDT Resource Center, http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/solidstate.htm, Center for NDE, Iowa State University.
Some substances can occur in both crystalline and amorphous forms. A familiar example is silicon dioxide (SiO2), which occurs naturally as quartz sand with a crystalline structure. When it is melted and quickly cooled, however, it forms glass, which is amorphous. (Figure 13-4) Glass is also rigid, but, unlike quartz, it breaks into random fragments because its structure does not contain well-defined planes. Many products are made today with safety glass, which has either been heat treated so that it will break into small, granular chunks or layered with plastic so that pieces will stick together if the glass breaks. Common uses for safety glass include car windows, skylights, and shower doors.
As this example shows, the properties of solid materials are shaped by the bonds that hold them together. In the next several sections, we will see how bond types affect the characteristics of four major categories of solids: ionic, covalent-network, molecular, and metallic.