Post-Laboratory Discussion

A systematic discussion on the types of bonding in various solids should take place after students have collected their data. Some generalizations can be drawn:

To a large degree, the physical properties of solids are determined by the type of bonding holding the atoms, molecules or ions in solid form. The bonding gives rise to ionic solids, molecular solids, metals, and covalent network solids; each with its set of characteristic physical properties. Thus, the type of solid can frequently be determined by studying its physical properties.

Ionic solids consist of oppositely-charged ions arranged in a crystal lattice in a way that allows each ion to be surrounded by either two, four, six, or eight oppositely-charged ions. This causes very strong attraction, giving typically hard solids with high melting points. Ionic solids are frequently soluble in water but insoluble in nonpolar, organic solvents. As solids, they do not conduct electricity, but do so both when fused (melted) or in water solution. However, not all ionic compounds are water soluble, e.g, calcium carbonate. In these cases attractions among ions in the lattice are too strong to be overcome by the attraction of polar water molecules.

Molecular solids are composed of molecules held together by relatively weak forces. Thus, these solids tend to be soft, easily melted, and more or less volatile. Molecular solids are likely to be insoluble in water but soluble in organic solvents. Neither the solids or their melted states conduct electricity, nor do any of their solutions. A few molecular solids such as common table sugar (sucrose), however, are water soluble, and thus must consist of polar molecules.

Covalent network solids are bonded by strong covalent forces into a three, two, or one dimensional network where every atom is linked to every other atom. The "molecules" are macro-sized so whatever size sample is being studied can be considered to be a molecule. Actually, the term molecule has little meaning in this regard. The strong multidimensional bonding produces solids with very high melting points that are generally hard, nonvolatile, and insoluble in all solvents. Covalent network solids are nonconductors of electricity with the exception of graphite, a two-dimensional network solid.

Metal atoms are bonded by metallic bonds. These bonds are usually strong (notable exceptions are mercury, a liquid at room temperature, and gallium which can melt in a person's hand) but not highly directional (as with covalent bonds). The nondirectional nature of the metallic bond is due to relatively mobile valence electrons and accounts for the malleability of metals. Most metals have 6, 8 or 12 nearest neighbors, but few valence electrons. Although the few valence electrons are free to move about, the atoms are not, since most metals are solids at room temperature and have definite shapes. So we deduce that metal atoms behave as if they are spherical and packed in regular arrays. There are almost flat planes (glide planes) between atom layers. Thus motion along the planes is easy. These characteristics give rise to the good electrical and heat conductivity of many metals as well as their malleability, nonvolatility, and shiny metallic luster. Although metals do not dissolve in water or organic solvents, they do form solid solutions in the form of alloys and amalgams with mercury.

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