Properties of Metals
1. Luster or reflectiuiry. Freshly cleaned metallic surfaces are good reflectors of light. Most metals reflect all frequencies of visible light and appear silvery white under white light.
2. High electric conductivity. Metals such as copper and aluminum are used in electric cables becauseof their outstanding ability to conduct an electric current. When a small difference in voltage is maintained in a metal, a relatively large current flows through the metal. This flow of current represents the passage of electrons through the metal. The passage of current does not cause any change in the composition of the metal as it does when it passes through melted ionic substances or solutions.
3. High heat conductivity. Aluminum ware and copper plated pans have long been used by housewives as cooking utensils because of their outstanding ability to conduct heat. In contrast, most nonmetals (such as sulfur) and compounds (such as water) are poor conductors of heat and electricity. The best electric conductors are the best heat conductors. This implies that the two properties may be related to a common factor.
4. Workability. Most metals can be hammered into sheets (malleability), drawn into wires (ductility), or formed into various shapes without shattering. This is not true of most other pure substances.
5. Electron emission caused by heat or light. When metals are heated or when their surfaces are exposed to sufficiently short wavelengths of light, they emit electrons. Vacuum tubes and photoelectric tubes used in automatic door-openers and other electronic devices use this property of metals.
These features cannot be accounted for in terms of pure covalent bonds, ionic bonds, or van der Waals forces. The existence of such bonds in metallic crystals can be discounted on the basis of these observations:
1. Covalent bonds are directional with fixed lengths. Such bonds would resist the deformation which takes place when a stress is applied during the working of metals. Also, the electrons in covalent bonds are tightly bound and highly localized. Consequently, they do not readily conduct electricity or heat or reflect light.
2. Ionic bonds, although nondirectional, resist deformation while metallic bonds do not. When stress is applied to an ionic substance, ions of like charge come into contact, causing a high-energy, unstable situation that causes the shattering of the crystal. Substances containing ionic bonds do not conduct an appreciable electric current nor do they reflect much light. The electrons are strongly attracted by the nuclei of the respective ions and are not free to move throughout the crystal under the influence of applied voltage or of light energy.
3. van der Waals forces can be discounted as a significant factor because, of the relatively weak nature of these forces. The melting points of most metals are far too high to be explained in terms of the relatively weak van der Waals attractions. |
