Ionic compounds

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Super Conductivity

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                                  Super Conductivity   https://en.m.wikipedia.org/wiki/File:Periodic_table_with_superconducting_temperatures.jpg                                Metals are good conductors of electricity, and their conductivity increases as the temperature is lowered. In 1911 the Dutch scientist Heike Kamerlingh Oimes discovered that metals such as Hg and Pb became superconductors at temperatures near absoiute zero.  A superconductor has zero or almost zero electrical resistance: It can therefore carry an electric current without losing energy, and in principle the current can flow for ever. There is a critical temperature.  That which the resistance drops sharply and superconduction occurs. Later, Meissner and Ochsen feld found that some superconducting materials will not permit a magnetic ...
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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 di...
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Crystal structures of metals

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  Metallic elements usually have a close-packed structure with a coordination number of 12. There are two types of close packing depending on the arrangement of adjacent layers in the structure: cubic close packing ABCABC and hexagonal close packing ABAB (see Metallic bonds and metallic structures in Chapter 2). However, some metals have a body centred cubic type of structure (which fills the space slightly less efficiently) where there are eight nearest neighbours, with another six next-nearest neighbours about 15% further away. If this small difference in distance between nearest and next-nearest neighbours is disregarded, the coor-dination number for a body-centred cubic structure may be regarded loosely as 14. The mechanical properties of malleability and ductility depend on the ease with which adj~nt planes of atoms can glide over each other, to give an equivalent arrangement of spheres. These properties are also affected by physical imperfections such as grain boundaries and ...
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