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The University of Michigan MSE 350-Fall 2007 Friday, September 14, 2007 ?Announcements ?Office Hours: Monday: after class (RG), 4-6 pm (LW); Tuesday: 10:30-11:30 am (RG) ?LW Office Hours: 2050 Dow; ASM Tutoring: 1st Floor UGLI ?Suggested Reading: SOM 2.3.4, 2.3.7 ?Homework #1: due Monday 9/17 ?Madelung Constant & Coordination # ?Strength of Covalent Bonds ?Covalent Bonding: Examples ?Ionic vs. Covalent Bonding ??Mixed? Bonding in Ceramics ?Metallic Bonding: Introduction (time-permitting) The University of Michigan MSE 350-Fall 2007 Madelung & Coordination # 4.172Al 2 O 3 2.40TiO 2 2.52CaF 2 1.638ZnS (cubic) 1.641ZnS (hex) 1.748NaCl 1.763CsCl / = #atoms/ formula unit StructureCoord. # The University of Michigan MSE 350-Fall 2007 Covalent bonding: why? Si: SiO 2 Why do the atoms want to ?share? orbital electrons? Which other elements tend to bond covalently? The University of Michigan MSE 350-Fall 2007 Strength of Covalent Bonds ?What determines the strength of a covalent bond? ?What are some consequence of directional bonding in covalent solids? ?What is the cohesive energy of a covalent solid? The University of Michigan MSE 350-Fall 2007 Covalent bonding: examples diamond: polyethylene The University of Michigan MSE 350-Fall 2007 Ionic vs. Covalent Bonding Ionicity Covalency Coordination # Atomic spacing Fraction Ionic Character % Ionic Character = 1 exp 1 4 A B () 2 = 1 exp 1 4 A B () 2 x100 The University of Michigan MSE 350-Fall 2007 Periodic Trends: Electronegativity http://www.webelements.com/webelements/elements/text/periodic-table/eneg.html The University of Michigan MSE 350-Fall 2007 ?Mixed? Bonding in Ceramics SiC Si 3 N 4 SiO 2 Al 2 O 3 MgO % covalent% ionic AtomsCompound The University of Michigan MSE 350-Fall 2007 Metallic Bonding I ??sea? of free conduction electrons which shield repulsion of ion cores and hold them together ?Delocalized electrons ?Non-directional bonding The University of Michigan MSE 350-Fall 2007 Metallic Bonding II ?Since a large # of electrons are free to move about (1-2 per atom), high electrical conductivity occurs ?Valence electrons of individual atoms become the conduction electrons of the metallic solid ?The energy to remove conduction electrons from a metallic solid must be greater than that to remove valence electrons from individual atoms ?NOTE: this is one of the simplest models for metallic bonding -- most metals have bonding between ?free- electron-like? and covalent The University of Michigan MSE 350-Fall 2007 Metallic Bonding: Cohesive Energy Rachel S. Goldman MSE350-mixed-14sep07.ppt