C121 11-3 lec25GL

C121 11-3 lec25GL - Lecture 25 November 7, 2011 Bonding:...

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Lecture 25 November 7, 2011 Bonding: Specific Models Last Day: I Localized Electron Bonding Models (i) Lewis Structures Rules, Formal Charge,Exceptions, Resonance (ii) Valence Shell Electron Pair Repulsion Theory Today: I Localized Electron Bonding Models (ii) Valence Shell Electron Pair Repulsion Theory Examples Molecular Shape and Polarity (iii) Hybrid Atomic Orbitals
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Determining Molecular Shapes Using VSEPR Theory 1. Draw valid Lewis structure. 2. Determine the number of electron pairs around the central atom # of e - pairs = # lone pairs + # bonded atoms * treat multiple bonds as equivalent to a single e - pair 3. Choose parent shape based on the total number of electron pairs to give the arrangement of the bonds and lone pairs around central atom.
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4. Place lone pairs in appropriate sites first and bonded atoms in the remaining sites. 5. Name the molecular shape or structure by considering the lone pairs to be invisible and describing the shape adopted by the bonded atoms only.
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Parent Shapes Minimizing Electron Repulsion 180˚ 120˚ 90˚ 90˚ 120˚ 90˚ 109.5˚ Linear 2 Trigonal Planar 3 Tetrahedral 4 Trigonal Bipyramidal 5 Octahedral 6
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VSEPR Examples: What are (i) the arrangements of bonds and lone pairs (i.e. parent shapes) and (ii) molecular shapes of the following: OF 2 H 2 CO SeF 4
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KrF 4 CH 3 NCO
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The Effect of Molecular Shape on Molecular Polarity • recall bond polarity arises from differences in electronegativity δ + δ - ie. H Cl polar bond polar molecule • if we sum all the bond dipole moments in a molecule we can determine the net dipole moment of the molecule • if Σ μ bonds = 0 (ie. μ cancel out) molecule non-polar • if Σ μ bonds ≠ 0 (ie. μ do not cancel out) molecule polar μ
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To find molecular polarity: (i) Find the molecular shape with VSEPR (ii) Draw the direction of each of the bond dipoles
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This note was uploaded on 12/10/2011 for the course CHEM 121 taught by Professor Leech during the Fall '11 term at Simon Fraser.

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C121 11-3 lec25GL - Lecture 25 November 7, 2011 Bonding:...

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