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Molecular Shapes (geometry) and Forces of Attraction VSEPR Theory Valence Shell Electron Pair Repulsion theory predicts molecular shapes based on the idea that pairs of valence electrons surrounding an atom repel each other or The shape of a molecule is determined by the valence electrons surrounding the central atom. Unshared pairs of electrons (on the central atom) will cause atoms to bend. Five basic shapes: linear, bent, tetrahedral, trigonal pyramidal, and trigonal planar. Molecular Shapes (geometry) 1. LINEAR example: CO2 central atom ALL shared pairs NO unshared pairs bond angles 180° no bending! 2. BENT example: H2O central atom 2 shared pairs 2 unshared pairs bond angles 104.5° bends! 3. TETRAHEDRAL example: CH4 central atom 4 shared pairs 0 unshared pairs bond angles 109.5° “tetra” = four! 4. TRIGONAL PYRAMIDAL example: NH3 central atom 3 shared pairs 1 unshared pair bond angles 107.3° bends! 5. TRIGONAL PLANAR example: BH3 central atom 3 shared pairs 0 unshared pairs bond angles 120° triangle-shaped! remember… all covalent compounds are either polar or nonpolar…and that polar means oppositely charged ends! dipole = a polar bond or molecule Shape Affects Polarity. 1. POLAR Molecules all bent, trigonal pyramidal molecules any molecule that has unshared pairs linear, tetrahedral, and trigonal planar ONLY IF: dipoles do not cancel each other out no symmetry within the molecule 2. NONPOLAR Molecules linear, tetrahedral, and trigonal planar ONLY IF: dipoles cancel out there is symmetry within the molecule # # Shared Unshare Pairs d Pairs Molecular Shape Bond Angle (°) Linear 180 ALL 0 depends HF, CO2 Bent 104.5 2 2 polar SCl2, H2O 107 3 1 polar NH3 120 3 0 depends BF3 109.5 4 0 depends CH4 Trigonal Pyramidal Trigonal Planar Tetrahedral Polar or Examples Nonpolar Forces of Attraction Dispersion forces (one type of van der Waals forces) – the weakest kind of intermolecular attraction (caused by the motion of electrons) Dipole interactions (a second type of van der Waals forces) – a weak intermolecular force resulting from the attraction of oppositely charged regions of polar molecules Hydrogen Bonds – the strongest intermolecular force; hydrogen is bonded to FON (fluorine, oxygen and nitrogen).