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Tutorial topics: Introduction Viewing Basics Molecular Shapes (VSEPR) Conformations Ethane Butane Cycloalkanes Substituted Cyclohexane Sterochemistry SN2 Reaction SN1 Reaction Aromaticity

VSEPR Theory Valence Shell Electron Pair Repulsion   To jump to the "live" structures, click this button: In General Chemistry, we learned how to use VSEPR theory to predict the shapes of molecules. This theory predicts that bonding (sigma bonds only) and non-bonding electron pairs in a molecule will adopt a geometry in which the distance between the electron pairs is maximized from one another in order to minimize the repulsions. This will result in a molecular geometry with the lowest possible energy. The theory also allows us to predict which hybridization the central atom takes in bonding to other atoms. To start, we need to know the Lewis structure of a molecule. Then we count how many pairs of electrons (triple bonds are counted as one pair) are around the central atom. If there are two pairs of electrons, they must be positioned 180° apart from each other and the shape is therefore linear. Three pairs are best positioned 120° apart and the shape is thus trigonal planar. Here the shape is referred to include the non-bonding electron pairs. For the shape of a molecule without counting non-bonding electron pairs, make a normal prediction then look at the molecule without non-bonding electrons showing. Four pairs of electrons are best positioned as tetrahedral shape. Depending upon the number of non-bonding electron pairs, the shape of the molecule not counting non-bonding electron pairs can be: a) tetrahedral (no non-bonding pairs); b) trigonal pyramidal (one non-bonding pair); or c) "bent" or "V" (two non-bonding pairs). For five pairs of electrons, the shape is predicted to be trigonal bipyramidal. Last, the octahedral is the shape predicted for six pairs of electrons. In this tutorial, you will see some "live" molecules on your computer screen. You can rotate each one to see their shape. After that, you may want to test yourself on the following questions: 1. How many pairs of electrons (sigma bonds and non-bonding pairs) are around the central atom? 2. What is your predicted shape of the molecule including the electron pairs? What is the predicted shape of the molecule without the electron pairs? 3. What is the hybridization involved in the bonding of the central atom? When you are ready to check the answers, click on "Check the Answers" located near each molecule. Molecule Shape with non-bonding electron pair (if any) Shape without non-bonding electron pairs   "Live" Molecules   BeH2 Check Answer   BF3 Check Answer   CH4 Check Answer NH3 Check Answer   H2S Check Answer   PF5 Check Answer   BrF3 Check Answer   TeCl4 Check Answer   SF6 Check Answer   XeF4 Check Answer   XeF2 Check Answer Answers: (Click the arrow to go back to the top of the "live" examples) Molecule # of electron pairs Shapes with, and without non-bonding e pair Hybridization of central atom BeH2 2 linear, linear sp BF3 3 trigonal planar, trigonal planar sp2 CH4 4 tetrahedral, tetrahedral sp3 NH3 4 tetrahedral, trigonal pyramidal sp3 H2S 4 tetrahedral, bent sp3 PF5 5 trigonal bipyramidal, trigonal bipyramidal dsp3 BrF3 5 trigonal bipyramidal, T-shaped dsp3 TeCl4 5 trigonal bipyramidal, Seesaw dsp3 SF6 6 octahedral, octehedral d2sp3 XeF4 6 octahedral, square planar d2sp3 XeF2 5 trigonal bipyramidal, linear dsp3