CHM 108 SUROVIEC SPRING 2014 Chapter 10 I. VSEPR Theory A simple model that we can use to look at molecular shapes is the VSEPR (valence shell electron pair repulsion) theory Theory is based on the idea that electron groups repel one another through columbic forces A. 2 electron groups - Linear BeCl 2 CO 2 B. 3 electron groups Trigonal Planar BF 3 CH 2 O C. 4 electron groups - tetrahedral CH 4 D. 5 and 6 electron groups Trigonal Bipyramidal PCl 5 Octahedral SF 6 10_T0102.JPG II. VSEPR Theory and Lone Pairs We just looked at molecules with all atoms bonded, what if there were lone pairs involved? A. Four electron groups with lone pairs NH 3 H 2 O B. 5 electron groups with lone pairs SF 4 BrF 3 C. 6 electron groups with lone pair BrF 5 XeF 4 Summary Geometry of a molecule is determine by number of electron groups on central atom Number of electron groups depends on correct Lewis structure Each of the following counts as a single electron group Geometry of electron groups is determined by minimizing their repulsion II. Molecular Shape and Polarity A. In Lewis structures covalent bonds look like the electrons are shared equally, but that is not always true Ex. HF and H 2 II. Molecular Shape and Polarity In a polyatomic molecule having a polar bond may or may not result in a polar molecule Ex. CO 2 vs. H 2 O II. Molecular Shape and Polarity B. Summary 1. Draw Lewis Structure 2. Determine the VSEPR stucture 2. Determine if there are any polar bonds 3. Determine if polar bonds add together to make a net dipole moment Ex. CH 4, NH 3, BF 3, CS 2 C. Polar, covalent and ionic Polar: unsymmetrical distribution of electrons Nonpolar: symmetrical distribution of electrons Ionic: complete transfer of electrons Increasing difference in electronegativity C. Classification of bonds by difference in electronegativity DifferenceBond Type 0Covalent 2 Ionic 0 < and