Crystal_Structure - Crystal Structure Controls of Crystal...

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Crystal Structure Controls of Crystal Structure - largely from bonding - leads to convenient organization of crystal types: - Metallic, Covalent, Molecular, Ionic Metallic Bonding Atoms pack in regular arrangement to minimize open space - Hexagonal closest packing - Cubic closest packing Closest way to pack spheres of equal size - Assume spheres are in layers - Difference is the way the layers are stacked - In both cases, each sphere has 12 nearest neighbors Hexagonal closest packing - Every second layer aligned - ABABAB structure - Hexagonal symmetry Cubic Closest packing - Every third layer aligned - ABCABC structure - face-centered cubic lattice - Native metals good example - if same size and chemistry can form alloys, e.g. gold and silver Body centered cubic packing
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- e.g. Fe - atoms at nodes of body-centered cubic lattice - Arrangement leaves only 8 rather than 12 neighbors Resulting physical properties - metals often high density relative to ionic-bonded crystals - conduct electricity and malleable Covalent bonding Atoms need to be in specific arrangements - orbitals must overlap - Assume close packed arrangement - Typically hard and high melting points Molecular Crystals Discrete molecules packed in systematic way - molecules bonded internally with covalent bonds - molecules held together with van der Waals or Hydrogen bonds - e.g Graphite and Ice Ionic bonding
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This note was uploaded on 07/06/2011 for the course GLY 5245 taught by Professor Staff during the Spring '11 term at University of Florida.

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Crystal_Structure - Crystal Structure Controls of Crystal...

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