General Properties of metals

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General Properties of metals 

All metals have characteristic physical properties: 

1. They are exceptionally good conductors of electricity and heat. 

2. They have a characteristic metallic lustre - they are bright, shiny and highly reflective. 

3. They are malleable and ductile. 

4. Their crystal structures are almost always cubic close-packed, hexag-onal close-packed, or body-centred cubic. 

5. They form alloys readily.


Conductivity 

All metals are exceptionally good conductors of heat and electricity. Electrical conduction arises by the movement of electrons. This is in contrast to the movement of ions which is responsible for conduction in aqueous solution or fused melts of ionic compounds like sodium chloride, where sodium ions migrate to the cathode, and chloride ions migrate to the anode. In the solid state, ionic compounds may conduct to a very small extent (semiconduction) if defects, are present in the crystal. There is an enormous difference in the conductivity between metals and any other type of solid.


      Electrical conductivity of varlous solids







Most of the elements to the left of carbon in the periodic table are metals. A carbon atom has four outer electrons. If these are all used to form four bonds, the outer shell is complete and there are no electrons free to conduct electricity.

Elements to the left of carbon have fewer electrons, and so they must have vacant orbitals. Both the number of electrons present in the outer shell, and the presence of vacant orbitals in the valence shell, are important features in explaining the conductivity and bonding of metals. The conductivity of metals decreases with increasing temperature . Metals show some degree of paramagnetism. which indicates that they possess unpaired electrons.


Lustre 

Smooth surfaces of n1clals typically have a lustrous shiny appearance. All metals except copper and gold are silvery in colour. (Nole that when finely divided most metals appear dull grey or black.) The shininess is rather special, and is observed at all viewing angles, in contrast to the shininess of a few non-metallic elements such as sulphur and iodine which appear shiny when viewed at lowngles. Metals are used as mirrors because they reflect light at all angles. This is because of the 'free' electrons in the metal, which absorb energy from light and re-emit it when the electron drops back from its excited state to its original energy level. Since light of all wavelengths (colours) is absorbed, and is immediately re-emitted, practically all the light is reflected back - hence the lustre. The reddish and golden colours of copper and gold occur. because they absorb some colours more readily than others. Many metals emit electrons when exposed to light - the photoelectric effect. Some emit electrons when irradiated with short-wave radiation, and others emit electrons on heating (thermionic emission).












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