Chemistry LESSON: VSEPR Theory 1 Principle: Electron pairs around a central atom tend to be oriented to be as far apart as possible. Draw Lewis Structures On test, students only have 80 seconds to answer. Know the basic models formed by each group. BF3 CF4 PF5 SF6 BeF2 Although alternatives are popular: The five basic molecular shapes: BF2 BeF3; SO3 AX2 linear (180o) AX3 trigonal planar (120o) Effect of lone pair repulsion: CH4: tetrahedral (109.5o) PF3 SF4 CH4 PF5 SF6 AX4 tetrahedral (109.5o) trigonal bipyramidal (90o / 120o sides) octahedral (90o) NH3: trigonal pyramidal (107o) H2O: bent (104.5o) (but NH4+ is tetrahedral; AX4) Multiple Bonds: Geometry depends on: (1) A double bond acts like a single bond {O=C=O AX2 = linear} (2) The number of unshared pairs (E) around the central atom Example: o C2H6: (tetrahedral @ each carbon) o C2H4: (116o between hydrogens; 122o between carbons) b/c double bond occupies slightly more space than single bond, so a bit more repulsion. H-C=C slightly > 120o, H-C-H slightly < 120o o C2H2: 180o between carbons Document1 (2/8/2016) Chemistry Molecular Modeling VSEPR & Molecular Modeling p. 2 Atom....................Color carbon..................black oxygen .................. red nitrogen ...............blue hydrogen ...........white halogens ............green Draw Lewis Structures & Make Models for: H2 Lewis Structure CH4 NH3 H2O VSEPR Shape (e.g., AX2) Geometry Bond Angle (modification from lone pairs) CH3OH Lewis Structure VSEPR Shape (e.g., AX2) Geometry Bond Angle (modification from lone pairs) H3C-CHOH-CH3 Chemistry VSEPR & Molecular Modeling p. 3 Hydrocarbons – Alkane Series C2H6 C2H4 C2H2 Lewis Structure VSEPR Shape (e.g., AX2) Geometry Bond Angle (modification from lone pairs) C2 doesn’t exist. Why? Hydrocarbons – Isomers (same chemical formulas, different structures) C2H2Cl2 C3H8O (two isomers) (two isomers) Lewis Structure VSEPR Shapes (e.g., AX2) Geometry Bond Angle (modification from lone pairs) C3H6 (straight chain & ring) Chemistry VSEPR & Molecular Modeling p. 4 Example of Isomers: One flavor in basil (anethole) is an isomer of another chemical found in anise (estragole).