LECTURE 10 Van der Waals bonding

LECTURE 10 Van der Waals bonding - Dr. P. Lucas U of A MSE...

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Unformatted text preview: Dr. P. Lucas U of A MSE 110 Metallic Bonding SUMMARY FROM LAST CLASS In the free electron gas model , the valence electrons form a gas of delocalized electrons surrounding the framework of cation cores. The attractive electrostatic forces between the positive cations and negative delocalized electrons constitute the metallic bonding. Due to the delocalized nature of the electrons in metallic bonds, all metals have high electrical conductivity. The valence electrons reside in extended molecular orbitals forming a quasi-continuous distribution of energy levels called an energy band . Alkalines have a half-filled s band while alkaline earth have overlapping p and s bands. A conducting electron must acquire kinetic energy and be promoted into an empty level of higher energy. Hence the energy band must be partially filled to allow for conduction. Free electrons can be excited by absorbing photons and relax by emitting the same photon, this create the shinny aspect of metals. Due to the delocalized and non-directional nature of metallic bonding, atomic displacements do not alter the bonding forces significantly. This gives rise to malleability and the ability to host many defects in the crystalline structure. Dr. P. Lucas U of A MSE 110 Primary and secondary bonding VAN DER WAALS BONDING These three types of bonds are strong and involve bonding energy ranging from 100-1000kJ/mole. Primary bonding correspond to ionic, covalent and metallic bonds. ionic covalent metallic Secondary bonding also called van der Waals bonding correspond to weak intermolecular or interatomic forces involving dipole interactions. Their bonding energy range from 0.1-10 kJ/mole. H 2 O Dr. P. Lucas U of A MSE 110 Molecular solids VAN DER WAALS BONDING • The weak bonding results in very soft material and low melting point....
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This note was uploaded on 10/06/2009 for the course MSE 110 taught by Professor Lucas during the Spring '08 term at Arizona.

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LECTURE 10 Van der Waals bonding - Dr. P. Lucas U of A MSE...

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