Section 03

 Malleability and cohesive force 


The mechanical properties of metals are that they are typically malleable and ductile. This snows that there is not much resistance to deformation of the structure. but that a large cohesive force holds the structure together.















The cohesive force may be measured as the heat of atomization. Some numerical values of △H° , the heats Of atomization at 25°C, are given in Table. The heats of atomization (cohesive energy) decrease on descending a group in the periodic table Li-Na-K-Rb-Cs, showing that they are inversely proportional to the internudeat distatice. The cohesion energy increases across the periodic table from Group I to Group II to Group III. This suggests that the strefigth of metallic bonding is related to the number of valency electrons. the cohesive energy increases at first on crossing the transition series Sc-Ti-V as the number of unpaired d electrons increases. Continuing across the transition series the number of electrons per atom involved in metallic bonding eventually falls, as the d electrons become paired, reaching a minimum at Zn. The melting points and to an even greater extent the boiling points of the metals follow the trends in the cohesive energies. The cohesive energies vary over an appreciable range, and they approach the magnitude of the lattice energy which holds ionic crystals together. 

The cohesive energies are much larger than the weak van der Waals forces which hold discrete covalent molecules together in the solid state. There are two rules about the cohesive energy and structure of metals (or alloys), and these are examined below: 

Rule 1. The bonding energy of a metal depends on the average number of unpaired electrons available for bonding on each atom. 

Rule 2. The crystal structure adopted depends on the number of s and porbitals on each atom that are involved with bonding. 

Consider the first rule - Group I metals have the outer electronic configuration ns1, and so have one electron for bonding. In the ground state (lowest energy), Group II elements have the electronic configuration ns2, but if the atom is excited, an outer electron is promoted, giving the configuration ns1, np1, with two unpaired electrons, which can form two bonds. Similarly Group III elements in the ground state have the configuration ns2, np1, but when excited to ns1, np2, they can use three electrons for metallic bonding.


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