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Unformatted text preview: 1 5th Lecture ClosestPacked Extended Structures Metallic radii Alloys Band theory Metallic bonding character Insulators Semiconductors So far we have discussed several different bonding models which pertain mostly to covalently bound molecules Lewis Structure Hybridization VSEPR MO Theory 2 There are two other kinds of bonding metallic and ionic Metallic and ionic bonding are mostly found in solid materials with extended structures Although some extended structures have covalently bound atoms Molecules have a definite countable number of atoms Extended structures have so many atoms we consider the number to be nearly infinite (remember the atoms are small) Molecules vs. Extended Structures With molecules you can draw in all the bonds Extended structures are difficult to draw, eventually at some point you will have to stop and you will be left with dangling bonds 3 Construction of an Extended Structure Most inorganic solids are crystalline Crystalline solids are described using a lattice A lattice is a 3dimensional system of points designating the center of the atoms The smallest repeating unit of the lattice is the unit cell A particular lattice can be generated by repeating the unit cell ( translating ) in all three dimensions to form an extended structure There are only 7 crystal systems There are only 14 Bravais Lattices Cubic primitive, facecentered, bodycentered Hexagonal primitive Trigonal primitive, rhombohedral Tetragonal primitive, bodycentered Orthorhombic primitive, facecentered, body centered Monoclinic primitive, facecentered Triclinic primitive 4 A lattice basically tells us how the atoms of a solid pack together in 3dimensions Most common kind of atom packing is closepacking Closepacking  when atom spheres come close together to minimize the spaces between them this results in a regular arrangement (it has a pattern) Take a rectangular box and fill it with oranges in a regular way to minimize the spaces Notice the hexagonal pattern 5 Then if we add in a second layer we will fill in the holes of the first Then there are two different ways to pack in the third layer to cover the holes Closest Packing One way of filling in the 3rd layer creates the same pattern as the bottom layer The other way creates a new kind of layer After the 3rd layer is placed, there is only one way to continue packing  repeat the motif ABAB = hexagonal closepacking, hcp ABCABC = cubicclosepacking, ccp (also called facecentered cubic) 6 Holes in Closest Packed Structures In both arrangements each sphere touches 12 others (CN = 12) 12 nearest neighbors Closest packed structures contain octahedral and tetrahedral holes (sites) Octahedral holes are larger In a closepacked array of atom A there is 1 oct hole for every atom A and there are twice as many td holes...
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This note was uploaded on 04/07/2008 for the course CHEM 311 taught by Professor Aitken during the Spring '08 term at Duquesne.
 Spring '08
 Aitken
 Organic chemistry, Inorganic Chemistry, Alloy, Mole

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