<text><span class="style10">etals (2 of 4)</span><span class="style7"></span><span class="style10">Conductivity</span><span class="style7">The conduction of heat and electricity that characterizes metals is due to their unique type of bonding. The solid metals behave as if they were composed of arrays of positively charged ions, with electrons free to move throughout the crystalline structure of the metal. This results in high electrical conductivity. The conduction of heat can also be seen in terms of the motion of electrons, which becomes faster as temperature rises. Since the electrons are mobile, the heat can be conducted readily through the solid.The majority of metals are good conductors of electricity, but germanium and tin (in the form stable below 19 deg C / 64 deg F) are semiconductors.</span><span class="style10">Mechanical strength</span><span class="style7">Many metals are used because of their strength. However, most pure metals are actually quite soft. In order to obtain a tough hard metal, something else has to be added. For instance, the earliest useful metal was not copper but bronze, which is copper alloyed with tin. Similarly, iron is never used in the pure state but as some form of steel.The softness of a pure metal results from a lack of perfection in the crystal frame work formed by its atoms (see diagram). Even when the most rigorous conditions are employed, it is impossible to grow any material in perfect crystalline form. There will always be some atoms in the wrong place or missing from their proper place. When solidification occurs fairly rapidly, as when a molten metal is cooled in a mold, even more defects occur. Under bending or shearing stresses, such defects can move and allow the metal to change shape easily. When the foreign atoms of an alloying element are present, they usually have a different size from those of the host and cannot easily fit into the crystal lattice. They therefore tend to site themselves where the lattice is irregular, i.e. where the defects are. The effect of this is to prevent the defects from moving, and so to increase the rigidity of the metal.</span></text>
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<text><span class="style10">etal deformation</span><span class="style7"> occurs as defects in the crystalline structure move under a shearing stress (red arrows).</span><span class="style10">Alloying.</span><span class="style7"> Impurity atoms (blue) are the wrong size to fit into the metal's crystal lattice. They therefore tend to site themselves at defective points in the lattice, where they become immobile and thus 'pin" the defects in place.</span></text>
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<text>ΓÇó ELECTROMAGNETISMΓÇó ELECTRICITY IN ACTIONΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó ELEMENTS AND THE PERIODIC TABLEΓÇó CHEMICAL BONDSΓÇó CHEMICAL REACTIONSΓÇó MINING, MINERALS AND METALSΓÇó IRON AND STEELΓÇó HUMAN PREHISTORY</text>
