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    • Activity 1: Physical Properties and
    Chemical Bonding in Solids

    Introduction

    In this activity you will investigate selected properties of four types of solids-ionic solids, molecular solids, covalent network solids, and metals. The properties studied will allow you to distinguish among the four types of solid. These properties will also allow you to determine the type of bonding in an unknown solid.

    Recall that ionic solids have ions located at regular, repeating crystal lattice sites with bonding among ions being primarily electrostatic. This type of bonding is essentially nondirectional; ions arrange themselves in sites to form a crystal structure that maximizes attractions and minimizes repulsions. The electrostatic forces are strong, giving rise to large lattice energies-it takes a large quantity of energy to disrupt the crystal.

    By contrast, covalent molecular solids have molecules held in place in a crystal lattice either by weak dispersion forces or dipole-dipole forces. These forces allow the crystal to be disrupted with a much smaller energy input than is the case for ionic solids.

    Covalent network solids contain only covalent bonds, but the bonding is quite different between adjacent atoms in the crystal. These bonds consist of primary covalent bonds-strong chemical bonds. This kind of bonding in one, two or three dimensions gives rise to strong structures as in quartz, mica, and asbestos. It takes a very large energy input to disrupt the crystal lattice, leading to very high melting points.

    Bonding in metals is quite different from that of other classes. Metallic bonding is electrostatic as in ionic solids, but the attraction is between valence electrons and the positively-charged metal atom kernels. This creates nondirectional bonding; electrons are not strongly associated with any one particular atomic kernel. Thus the electrons are rather mobile, accounting for the properties of metals.

    Since substances used in the activity are fairly typical of their classes, it is possible to generalize and correlate the structures and properties of the substances with the bonding involved.


    Purpose

    To investigate some physical properties of solids containing ionic bonds, covalent bonds, van der Waals forces, and metallic bonds to learn how to discriminate among them on the basis of properties correlated with their bonding forces.


    Safety

    1. Wear protective goggles throughout the laboratory activity.

    2. Lauric acid is generally regarded as safe; however, as with all substances, exercise care while handling it.

    3. Cyclohexane is highly flammable and is toxic by ingestion, inhalation, and absorption through the skin. Cyclohexane should be kept at least three meters from any open flame.

    4. The electrical conductivity apparatus is a potential source of electric shock.

    5. Volatile substances should be used in a fume hood.

    6. Place the used cyclohexane in the container provided. Place the used lauric acid and other solids in the designated container(s).


    Chemical Bonding (BOND)
    Page 6

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