notes_162_Exp_1_Notes_032708

notes_162_Exp_1_Notes_032708 - Experiment 1 Chemical Models...

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Experiment 1: Chemical Models and Magnetism Chemistry 162
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Part 1: Chemical Models Zumdahl, 5 th ed., Chapts. 12-14 • Objective: We will be using a molecular model kit to assist in our understanding of: – Lewis Dot Structures – Molecular Geometry • Electron-pair geometry and molecular shapes • Bond angles –Po lar i ty – Orbital hybridization
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Lewis Dot Structure Review • Before we can determine electron-pair geometry and molecular shape, we need to draw a Lewis Dot Structure for the molecule Steps for drawing Lewis Dot Structures 1. Sum the valence electrons from all the atoms. 2. Add or subtract electrons depending upon the overall charge on the molecule 3. Use a pair of electrons to form a bond between each pair of bound atoms 4. Arrange the remaining electrons to satisfy the duet rule for hydrogen and the octet rule for the second-row elements Adapted from Zumdahl, 5 th ed., pg. 613
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Lewis Dot Structure Examples • Consider: NH 3 Number of valence electrons Nitrogen 3 Hydrogens Total electrons 5 e - 3*(1 e - ) = 5 = 3 8 electrons + Step 1: Sum Valence Electrons Step 2: Add or subtract electrons to match overall charge Overall charge on NH 3 is 0, so no addition or subtraction of electrons is necessary.
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Lewis Dot Structure Examples Step 3: Connect pairs of atoms with an electron-pair bond NH H H Step 4: Fulfill the duet and octet rules Each bond represents 2 e - ’s 3*(2 e - ’s) = 6 e - ’s H H To finish this structure, we place the remaining two electrons on the nitrogen to complete its octet. 8 e - to satisfy octet 2 e - to satisfy duet
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Lewis Dot Structure Examples • Another example: CO 2 Number of valence electrons Carbon 2 Oxygens Total electrons 4 e - 2*(6 e - ) = 4 = 12 16 electrons + Step 1: Sum Valence Electrons Step 2: Add or subtract electrons to match overall charge Overall charge on CO 2 is 0, so no addition or subtraction of electrons is necessary.
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Lewis Dot Structure Examples Step 3: Connect pairs of atoms with an electron-pair bond CO O Step 4: Fulfill the duet and octet rules We’ve taken care of 4 e - ’s. Now for the last 12 e - ’s. O We start by fulfilling the oxygens’ octets: We now have a structure with 16 e - ’s, but carbon does not have a filled octet…
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CO 2 Example Continued Step 4: Fulfill the duet and octet rules To take care of carbon’s octet, form double bonds between the carbon and the two oxygens: CO O O 8 e - to satisfy octet
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VSEPR and Geometry • To determine electron-pair geometry and molecular shape, we use Valence Shell Electron Pair Repulsion Theory (VSEPR) • VSEPR: The structure around a given atom is determined principally by minimizing electron-pair repulsions (Zumdahl, pg. 627) Electron-pair geometry positions electron-pairs as far apart as possible.
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Electron-pair Geometry Linear Trigonal Planar Tetrahedral Trigonal Bipyramidal Octahedral Electron-pairs: 2 Electron-pairs: 3 Electron-pairs: 4 Electron-pairs: 5 Electron-pairs: 6
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Molecular Shape • Actual molecular shapes deviate from basic electron-pair geometries because lone electron-pairs take up more space than atoms.
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This note was uploaded on 05/08/2008 for the course CHEM 162 taught by Professor N. during the Spring '08 term at University of Washington.

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notes_162_Exp_1_Notes_032708 - Experiment 1 Chemical Models...

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