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Molecular Geometry MOLECULAR GEOMETRY VSEPR stands for: Valence Shell Electron Pair Repulsion theory. • Most important factor in determining the shape of the molecule is the repulsion between electron pairs. Molecules form their shapes to minimize the repulsions of the electrons. *They get as far away from each other as possible! VSEPR Charts • Use the Lewis Structure to determine the geometry of the molecule. • Focus only on the CENTRAL atom for all molecules! Count the number of electron regions. • Look for REGIONS WHERE ELECTRONS ARE FOUND rather than number of bonds. (Ex: Double and Triple bonds count as 1 region) Lets look at Methane (CH4). The molecule is really three dimensional, not planar and flat. H C H H H When models are constructed, they show the bonds, but not the lone pairs of electrons. .. 109.5° (109.5°) 109.5° (107°) .. .. 109.5° (104.5°) Some Common Geometries Structure Determination by VSEPR Water, H2O • 1) Looking at the central atom, •• how many regions are •• there? H O H The electron pair geometry is TETRAHEDRAL 2 bond pairs 2 lone pairs • 2) How many bonded atoms? How many lone pairs? The molecular geometry is BENT. Structure Determination by VSEPR Ammonia, NH3 The electron pair geometry is Tetrahedral , and the Geometry name is TRIGONAL PYRAMID.