chp8_slides - CHP. 8 MOLECULAR SHAPE TOPICS: VSEPR (Valence...

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Unformatted text preview: CHP. 8 MOLECULAR SHAPE TOPICS: VSEPR (Valence Shell Electron Pair Re- pulsion) Model Common Shapes of Small Molecules 1. tetrahedral 2. pyramidal 3. trigonal planar 4. bent 5. linear Hybridization ( sp 3 , sp 2 , sp ) Polar Molecules 1 VSEPR (Valence Shell Electron Pair Repulsion) Model A method for predicting molecular shapes based on repulsion between electron pairs surrounding a cen- tral atom. These electron pairs can be shared (i.e. bonds) or unshared (i.e. lone pairs). The unshared pairs exert more repulsive force be- cause their charge density is concentrated near the central atom instead of being spread out between two atoms. To use VSEPR, 1. Draw a Lewis Dot Structure for the molecule starting with the central atom. 2. Count the total number of pairs of electrons about central atom . ( Whether a bond is sin- gle, double, or triple, count it as one shared pair ; the higher order bonding will not affect the shape. ) 3. Count the number of unshared pairs of elec- trons. 4. Deduce shape from repulsion between electron pairs. 2 Nitrogen and its Hydrogens Lets apply VSPER to NH + 4 , NH 3, NH- 2 , and NH 2- : If you draw the dot structure for NH + 4 , youll see the N-atom has four total pairs of electrons ( 0 un- shared ). These pairs repel each other equally,forcing the four H s to take up positions about the C as far apart from each other as possible. This puts them at the corners of a tetrahedron with the C at its center. (The H- N- H bond angle is 109 . 5 o .) If you draw the dot structure for NH 3 , youll see the N-atom has four total pairs of electrons ( 1 unshared ). The unshared pair repels the three shared pairs more forcefully resulting in a pyrami- dal shape. The unshared pair can be visualized with the C atom at the apex with the three H s form- ing the base of the pyramid. With the H s being forced towards one another by the unshared pair, the H- N- H bond angle is less than 109 . 5 o . If you draw the dot structure for NH- 2 , youll see the N-atom has four total pairs of electrons ( 2 un- shared ). The unshared pairs repel the two shared pairs yet more forcefully resulting in a bent shape, with the H- N- H bond angle yet smaller than that of NH 3 . NH 2- ( 4 total, 3 unshared pairs of e- ) is linear having only 2 atoms. 3 Boron and its Hydrogens Lets apply VSPER to BH 3 , BH- 2 , and BH 2- : If you draw the dot structure for BH 3 , youll see the B-atom has three total pairs of electrons ( 0 un- shared ). These pairs repel each other equally,forcing the three H s to take up positions about the B as far apart from each other as possible. This puts them at the corners of an equilateral triangle with the B atom at its center. The H- B- H bond angle is 120 o ....
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This note was uploaded on 10/19/2011 for the course PHY 2020 taught by Professor Staff during the Spring '08 term at University of Florida.

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chp8_slides - CHP. 8 MOLECULAR SHAPE TOPICS: VSEPR (Valence...

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