The free electron theory of metals, which started with Drude (1902), has been able to explain quite successfully the following properties, called the metallic properties: 1. Good Electrical and Thermal Conductivity 2. Opaque and Lustrous 3. Good Ductility 4. Alloying Behaviour of Metals 5. High Melting Points.
As the free electrons in the electron cloud are not fixed in any one position, metals are good electrical conductors. Under the influence of an applied voltage, these valence electrons move (see the change in Fig. 1.9 (a) to 1.9 (b)), carry the electrical current if a circuit is complete.
The more loosely the valence electrons are held, more is the mobility and thus, better is the electrical conductivity. When a metal is heated, thermal vibrations (magnitude) of the ions increase, more structural disorders, like vacancies, etc., are created leading to disorder in periodicity. These factors diffract and scatter the electrons, consequently lower the conductivity.
Thus, the electrical conductivity decreases with the increase in the temperature of metals, Fig. 1.9 (c). At low temperatures, the amplitude of thermal vibrations are less, the defects are less, electrical conductivity increases. In some metals, as the formation of electron pairs takes place, and by having ordered motion at extremely low temperatures (below 20°K), the electrical conductivity becomes infinite.
This state is called superconductivity. Twenty three metals are known to be superconductive. Superconductivity occurs more readily in those metals which have low normal conductivity, having 3, 5 or 7 valence electrons. The reason of solid solutions (alloys) showing lower electrical conductivity as compared to their pure metals is that the local field around the solute atom is different from the field in other part of the material.
These local-field irregularities scatter the electrons and reduce the electrical conductivity, Fig 1.10.
Thus, brass has lower conductivity than pure copper. In a cold worked metal, such as cold drawn copper wires, defects like dislocations, vacancies, grain boundaries increase, which act as deflection sites for electron movements, decreasing thereby the electrical conductivity. That is why, cold drawn copper wires are used as electrical conductors only after annealing, which is able to remove these extra defects and recover the electrical conductivity.
Graphite is a crystalline form of carbon having hexagonal layered structure. One electron in the third dimension is not bonded and the flow of this electron under an outside voltage causes the current to flow to make graphite as an electrical conductor to be used as electrodes in furnace, although graphite is a non-metal.
The good thermal conductivity of metals is due to two main mechanisms of thermal conduction possible in them, – (i) by movements of electrons and, (ii) by phonons (or quantized elastic waves). Since electrons move rapidly through a metal, they also carry the heat from the hot end to the cold end during motion. Thus, major fraction of the thermal energy is conducted by electrons.
A direct comparison can be made between thermal conductivity and their electrical conductivity, Fig. 1.11.
As thermal conductivity of metals is primarily due to electronic contribution, there exists a relationship between it and electrical conductivity, which though is followed to a limited extent in many metals-
where, L is the Lorentz constant, k is the thermal conductivity, and σ is the electrical conductivity.
Property # 2. Opaque and Lustrous:
Metals are opaque and have a natural luster. As a light beam falls on the clean metal, the free electrons oscillate in the alternating electric field of the light, and the free electrons absorb the energy from the light photons, and do not allow it to pass through it. Thus, the metals are opaque.
Fig. 1.12 illustrates that the electron due to absorption of photon gets raised to higher energy level. When the electron falls back to its original energy level (as the electron is more stable at lowest energy level); it emits light waves as a reflected beam to make the metals look lustrous with high reflectivity.
In aluminium or silver, photons of almost identical wavelength are immediately emitted, i.e., virtually all of the visible spectrum is reflected, and these metals have a white, or silvery colour. In some cases, the amount of energy required to raise an electron to a higher energy level is exactly equal to the energy of certain colour of light.
When the white light strikes gold and copper, light rays corresponding to blue colour raise the energy level of the electrons. The metal reflects all the light except the blue colour. Of the three fundamental colours, blue, red and yellow, the reflected light imparts these metals reddish-yellow colour.
Property # 3. Good Ductility:
The characteristic of the metallic crystals of having positively charged ions surrounded by an electron cloud makes every positive ion equivalent. Whilst the positive ions will repel each other, they are held in equilibrium positions by forces of mutual attraction between each ion and the negatively charged ‘cloud’. The bond is not so strong and non-directional. Fig. 1.13 (a) illustrates a crystal of metal.
When a shear stress is applied to a plane of ions, this tends to move positive ions in one plane nearer to those in the next adjacent plane. While this process is taking place, the forces of repulsion between the ions will increase to a maximum at the elastic limit, but are then overcome so that one layer of ions moves with respect to its neighbour.
There is no disruption, however, at this stage, because of the mutual attraction between positive ions and the surrounding electron cloud. The plane along which movement occurs is called slip plane. The process is like slipping of cards in a deck of playing cards.
An ionic bonded crystal such as sodium chloride consists of positively charged, sodium ions and negatively charged chloride ions arranged in such a way that each ion is surrounded by those of opposite charge. When a stress is applied on this crystal to produce slip, and when such a movement is imagined to occur, an ion tries to slip from its original position having highly attractive forces from oppositively charged ions.
Increasing value of stress has to be applied to not only overcome these forces of attraction, but also to overcome the repelling force due to similar charged ions coming closer and then in contact. As like ions repel with such a large force, that the total stress level rises higher than its fracture stress, which causes a brittle fracture instead of causing plastic deformation.
Property # 4. Alloying Behaviour of Metals:
There is no saturation of the covalent bonding states in a metal, and this fact is responsible for the alloying behaviour of metals. Suppose two metals, copper and nickel are mixed together (they are made molten and allowed to solidify). Each atom reacts fairly unspecifically to other, because they are held together by the common free electron cloud to which both types of atoms have contributed their valence electrons.
Thus, solid solution alloys of different compositions can be made by replacing randomly atoms of one metal by those of other to produce substitutional solid solutions. As the metallic bond is not sensitive to particular metal atoms, it also causes to develop the ability of metals to be joined by welding and soldering as long as clean metal surfaces are brought in contact to form the bond.
Property # 5. High Melting Points:
The high melting points of transition metals (Fe, Ni, Co) is due to greater covalent character in the bonds of these metals. The non-directionality of metallic bond is also responsible for good mechanical properties of metals. Table 1.1 gives some characteristics of solids depending on the nature of bonding.
Table 1.1. Bonding Based Characteristics of Some Crystalline Solids