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The Solid State for AP Chemistry

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By — McGraw-Hill Professional
Updated on Feb 2, 2011

Practice problems for these concepts can be found at:

At the macroscopic level a solid is defined as a substance that has both a definite volume and a definite shape. At the microscopic level, solids may be one of two types—amorphous or crystalline. Amorphous solids lack extensive ordering of the particles. There is a lack of regularity of the structure. There may be small regions of order separated by large areas of disordered particles. They resemble liquids more than solids in this characteristic. Amorphous solids have no distinct, melting point. They simply get softer and softer as the temperature rises, leading to a decrease in viscosity. Glass, rubber, and charcoal are examples of amorphous solids.

Crystalline solids display a very regular ordering of the particles in a three-dimensional structure called the crystal lattice. In this crystal lattice there are repeating units called unit cells. Figure 12.1 shows the relationship of the unit cells to the crystal lattice.

Several types of unit cells are found in solids. The cubic system is the type most commonly appearing on the AP exam. Three types of unit cells are found in the cubic system:

  1. The simple cubic unit cell has particles located at the corners of a simple cube.
  2. The body-centered unit cell has particles located at the corners of the cube and in the center of the cube.
  3. The face-centered unit cell has particles at the corners and one in the center of each face of the cube, but not in the center of the cube itself.

Figure 12.2 shows three types of cubic unit cells.

Five types of crystalline solid are known:

  1. In atomic solids, individual atoms are held in place by London forces. The noble gases are the only atomic solids known to form.
  2. In molecular solids, lattices composed of molecules are held in place by London forces, dipole–dipole forces, and hydrogen bonding. Solid methane and water are examples of molecular solids.
  3. In ionic solids, lattices composed of ions are held together by the attraction of the opposite charges of the ions. These crystalline solids tend to be strong, with high melting points because of the strength of the intermolecular forces. NaCl and other salts are examples of ionic solids. Figure 12.3 shows the lattice structure of NaCl. Each sodium cation is surrounded by six chloride anions, and each chloride anion is surrounded by six sodium cations.
  4. The Solid State

  5. In metallic solids, metal atoms occupying the crystal lattice are held together by metallic bonding. In metallic bonding, the electrons of the atoms are delocalized and are free to move throughout the entire solid. This explains electrical and thermal conductivity, as well as many other properties of metals.
  6. The Solid State

  7. In covalent network solids, covalent bonds join atoms together in the crystal lattice, which is quite large. Graphite, diamond, and silicon dioxide (SiO2) are examples of network solids. The crystal is one giant molecule.

Practice problems for these concepts can be found at:

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