Lecture Notes for Wednesday, September 5, 2001

PROPERTIES OF SOLIDS

The physical properties of crystals are determined by the kinds of particles they are composed of, and the attractive forces that occur between those particles. Some examples of the variety of physical properties which we observed in solids are listed below;

1. Diamond is an extremely hard substance

2. Graphite is a very good lubricant

3. Both dry ice (solid carbon dioxide) and ice are soft

4. Metals are good conductors

All of these properties depend on;

1. The kind of intermolecular forces

2. Strength of the intermolecular forces

We can divide crystals into types, based on the type of intermolecular attractive forces, which exhibit similar properties. (see Table 11.6 on page 402.)

I. Extended Covalent

(Show diamond example)

Extended covalent (covalent network solids) solids consist of atoms which are held together in large networks containing extended covalent bonds. Quartz is an example of a three dimensional network of covalent bonds. (See pages 873 - 879 in BLB.) Quartz consists of SiO2 groups which covalently bond to other SiO2 groups in all three directions. Notice that all the silicon atom are tetrahedral bonded to oxygen atoms. In diamond (see Figure 11.40(a) on page 403), which is one form of elemental carbon, all the carbon atoms are tetrahedrally bonded in a three dimensional network of covalent bonds. Graphite (see Figure 11.40(b) on page 403) consists of sheets of a hexagonal bonded, sp² hybridized, carbon atoms. The fact that these sheets are layered on top of each other allows for slippage of layers relative to each other making graphite an excellent lubricant.

Extended covalent solids of this type are very hard and high melting. Some properties of the solids mentioned above are;

1. High melting, typically above 1000 degrees C. Diamond melts at 3700 degrees C and boils at 4200 degrees C. Graphite sublimes at 3650 degrees C. Quartz melts at 1713 degrees C.

2. Insoluble in most common solvents

3. Poor electrical conductors. (Graphite is an exception.)

II. Molecular

Molecular solids consist of molecules which are held together by London dispersion, dipole-dipole or hydrogen bonding forces. The solid contains ordered arrangements of atoms or molecules which organized, relative to each other, in an orderly three dimensional pattern. Solids of this type are soft and low melting.

III. Metallic

Metallic solids are though to be made up of positive nuclei orderly arranged with the electrons spread out over the positive nuclei (see Figure 9.2C on page 332). The electrons are not thought of as belonging to a particular nuclei, but to the crystal as a whole. The freedom of these electrons to flow accounts for the high electrical and thermal conductivity of metals. The loosely bound character of the electrons, and their ability to vibrate easily, accounts for the characteristic metallic luster of metals.

IV. Ionic

Ionic solids consist of cations and anions distributed throughout the crystal in an orderly three dimensional pattern. Ionic solids are more complicated in their structure, but can be thought of as an orderly pattern of one ion, generally the anion, with cations positioned in 'holes' between the anions. The occupation of these 'holes' depends on the formula of the ionic compound. Ionic solids are characterized as hard yet brittle substances which are high melting. High melting because the electrostatic attractions of the ionic bonds are stronger than the intermolecular forces for molecular solids.