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UNC-Chapel Hill CHEM 251 - Part 2. Ionic Crystal Structures

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CHEM 251 1st Edition Lecture 14 Outline of Last Lecture I. Clicker QuestionII. Ionic vs. Covalent BondingIII. Ionic CompoundsIV. Fajans’ RulesV. Charge DensityOutline of Current Lecture VI. Clicker QuestionVII. Ionic LatticesVIII. Packing StructureIX. Application-Photovoltaic ElectricityCurrent LectureX. Clicker Questiona. Lowering melting point: MgCl2 or CuCl2?i. Non-noble gas configurations are more covalentb. Ionic radius of Li+ is 76 pm and Br- is 196 pm. What is preferred coordination number of Li+ in LiBr?i. r+/r- = .39 => coordination number 4XI. Ionic Latticesa. Assumed to be hard spheresb. Try to maximize interactionsi. Ionic bonds = non-directional: geometry determined solely by ionic radii and geometrical constraints of packing spheres ii. Bonds are electrostatic in nature; involve bringing charges close together rather than relying on orbital overlapc. Cation/anion ratio must match compositiond. What determines ionic lattice packing? Ionic radiusi. Size of central cation, more or less anions can fit aroundThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.XII. Packing Structuresa. Coordination Number 8i. Cubic1. Identical to cubic structure for metals2. Stoichiometry is 1:1ii. Fluorite1. Unequal stoichiometry causes central vacancy2. Unit cell larger than simple cubiciii. anti-fluorite1. Unequal stoichiometry causes vertex vacancy2. Switch positions of cation and anionb. Coordination Number 6i. Sodium Chloride/Rock Salt Packing1. Total 4 NaCl molecules in the unit cellii. Rutilec. Coordination Number 4i. Sphalerite=cubic close packedii. Wurtzite=hexagonal close packed iii. ZnS can adopt either formiv.d. Coordination Number 12i. Perovskite – CaTiO31.e. Multicomponenti. Hydroxyapetite – Ca10(OH)2(PO4)6 1. Major component of bones and teeth2.XIII. Application-Photovoltaic Cellsa. Converts solar energy into electron flow resulting in electric powerb. Rely on semiconductorsc.d.e. Typesi. Silicon1. Network covalent bonding2. Classic and most dominant3. 20% efficiencyii. Gallium Arsenide1. Efficiency >25%2. Higher electron mobility than siliconiii. Perovskite1. Mix of organic small molecules and ionic lattice2. In two years, up to 15%


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UNC-Chapel Hill CHEM 251 - Part 2. Ionic Crystal Structures

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