The Crystalline Structure of Metals

All chemical elements can exist as either solids, liquids or gases depending upon the prevailing conditions of temperature and pressure. Thus, at atmospheric pressure, oxygen liquifies at — 183°C and solidifies at —219°C. Similarly, at atmospheric pressure, the metal zinc melts at 419°C and boils at 9070C. In the gaseous state particles are in a state of constant motion and the pressure exerted by the gas is due to the impact of these particles with the walls of the container. As the temperature of the gas is increased, the velocity of the particles is increased and so the pressure exerted by the gas increases, assuming that the container does not allow the gas to expand. If, however, the gas is allowed to expand the distance apart of the particles increases and so the potential energy increases. Engineers will understand the term potential energy as being that energy possessed by a body by virtue of its ability to do work. Similarly, matter possesses potential energy by virtue of its state. As the distance apart of particles increases, so the potential energy increases. In fact this is a simple application of the First Law of Thermodynamics which states that if a quantity of heat bQ is supplied to a system, part of that heat energy may be used to increase the internal energy of the system by an amount bU and part to perform external work by an amount bW. Thus:
SQ = SU+ SW
In this case bW is the work done by the gas as it expands against some external pressure. Whatever changes occur, energy is conserved. In a gas such as oxygen the particles referred to are molecules, each of which consists of two covalently-bonded atoms but in a metallic gas these particles consist of single atoms since insufficient valence electrons are available for metallic atoms to be covalently bonded. Each atom has its own complement of electrons and in the gaseous state the metallic bond does not exist.
On condensation to a liquid the atoms come into contact with each other to form bonds (Fig. 3.1), but there is still no orderly arrangement of the atoms, though a large amount of potential energy is given up in the form of latent heat. When solidification takes place there is a further discharge of latent heat, and the potential energy falls even lower as the atoms take up orderly positions in some geometrical pattern which constitutes a crystal structure. The rigidity and cohesion of the structure is then due to the operation of the metallic bond as suggested in (1.76).