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Lecture SEP 20 2011.docx

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CHEM 203
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Structure and Bonding Review - Bond Types Ionic Bonds Covalent Bonds Electron transfer Electrons shared Salt, e.g. NaCl e.g. C-N +--- (dipole moment) No Dipole Based upon ionization Difference in electron potential and electron affinity affinity Orgo is about polar reactions Lewis Structures – electron diagram of a covalent system - Geometry - Length of bonds are 1. Count electrons in the system (valence and any charges) 2. Draw the system – Central Atom → the one w/ the least electronegativity (never hydrogen) Draw bonds (2 electrons) from central atom to remaining atoms 3. Complete octets w/ lone pairs e.g. B, Al, and S, P, I can have expanded octets 4. Assign any formal charge (when applicable) Formal Charge = # valence electrons – Lone pair electrons – ½ bond electrons Remember ex. of Carbon, N, O, halogens e.g. CO2 2 O x 6 e- = 12 e- (valence) 1 C x 4 e- = 4 e- Total of 16 e- Ex. 2 H2SO4 4 O = 4 x 6 e- = 24 e- 2 H = 2 x 1 e- = 2 e- 1S = 1 x 6 e- = 6 e- 12 e- covalent; 20 e- If you can eliminate formal charges, do it. No formal charge = More stable species - Try N 3 Lewis Structures Lead to VSEPR VSEPR → Geometry → Shape w/ the most stability AX E R R A = Central Atom X = # Bonds (Covalent – Double and Triple count as one bond) E = # lone pairs on the central atom (only
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