BONDING.docx - BONDING METALLIC BONDING As the name implies...

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BONDING M ETALLIC B ONDING As the name implies, metallic bonding is the predominant bond mechanism for metals. It is also referred to as electronic bonding , from the fact that the valence electrons (electrons from unfilled shells) are freely shared by all the atoms in the structure. Mutual electrostatic repulsion of the negative charges of the electrons keeps their distribution statistically uniform throughout the structure. At any given time, each atom has enough electrons grouped around it to satisfy its need for a full outer shell. It is the mutual attraction of all the nuclei in the structure to this same cloud of shared electrons that results in the metallic bond. Because the valence electrons in a metal uniformly distribute themselves and because all the atoms In a pure metal are of the same size, close-packed structures result. Such close-packed structures contain many dislocations and slip planes along which movement can occur during mechanical loading, producing the ductility that we are so accustomed to for metals. Pure metals typically have very high ductility and can undergo 40–60% elongation prior to rupturing. Highly alloyed metals such as superalloys also have close-packed structures, but the different-size alloying atoms disrupt slip along planes and movement of dislocations and decrease the ductility. Superalloys typically have 5– 20% elongation. The free movement of electrons through the structure of a metal results in high electrical conductivity under the influence of an electrical field and high thermal conductivity when exposed to a heat source. Metallic bonding occurs for elements to the left and in the interior of the periodic table. Alkali metals such as sodium (Na) and potassium (K) are bonded by outer s electrons and have low bond energy. These metals have low strength and low melting temperatures and are not overly stable. Transition metals such as chromium (Cr), iron (Fe), and tungsten (W) are bonded by inner electrons and have much higher bond strengths. Transition metals thus have higher strength and higher melting temperatures and are more stable. The metallic bond derives from the attraction between the cations and the free electrons and, as would be expected, repulsive components of force develop when cations are brought into close proximity. However, the bonding forces in metallic structures are spatially non-directed, and we can readily simulate the packing and space-filling characteristics of the atoms with modelling systems based on equal-sized spheres (polystyrene balls, even soap bubbles). Other properties such as ductility, thermal conductivity and the transmittance of electromagnetic radiation are also directly influenced by the non-directionality and high electron mobility of the metallic bond.
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