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How to Determine the Molecular Geometry for a Compound 1. Draw the Lewis Dot Structure for the compound. 2. Establish the AXE designation for the Dot Structure as follows: A # of X’s # of E’s = central atom = # of BONDED atoms to the central atom = # of LONE PAIRS of electrons on the central atom The numbers of X’s and E’s are then written as subscripts. DO NOT count Lone Pair electrons on the atoms which are connected to the central atom. ONLY the lone pairs of electrons on the central atom count as E’s. 3. The total number of X’s and E’s determine the ELECTRON PAIR geometry. (There are only 5 basic electron pair geometries) NOTES: 1. If you only have 2 atoms bonded together, there is NO central atom and NO AXE designation. Since two points make a straight line, this geometry is always LINEAR (bond angles of 180 degrees, sp hybridization) 2. If you have more than one central atom, determine the geometry on each atom separately. If the molecule is symmetrical, the geometry on each atom will be the same. If not, assign the geometry individually to each central atom. 3. If an atom has multiple bonds to a central atom, it is only counted ONCE. You count # of bonded atoms in this model, NOT # of bonds. How to Determine the Hybridization for a Compound 1. Draw the Lewis diagram for the compound. 2. The hybridization must follow the format spxdy (where x cannot be higher than 3 and y cannot be higher than 2 in this class) The total number of bonds to the central atom (X + E) must add up to the same number of letters in the hybridization. Ex.) AX3 = 3 bonds to the central atom so hybridization = sp2 (3 total letters) AX4E = 5 total bonds to central atom so hybridization = sp 3d (5 total letters) Molecular Geometry and Hybridization to Memorize!! Total Bonds (X + E) 2 (Linear) 3 (Trigonal Planar) 4 (Tetrahedral) 5 (Trigonal bipyramid) 6 (Octahedral) # bonded atoms (X) 2 # lone pairs (E) 0 AXE Notation Molecular Geometry Bond Angles AX2 Linear 120° 118° sp2 3 0 AX3 Trigonal Planar 2 1 AX2E Bent 180° HybridExample ization sp sp2 Polarity BeCl2 POLAR, for AX2 ONLY if X groups not identical. BCl3 POLAR, ONLY if X groups not identical. NO2- YES, always polar. CCl4 POLAR, ONLY if X groups not identical. 4 0 AX4 Tetrahedral 109.5° sp3 3 1 AX3E Trigonal Pyramid < 109.5° sp3 NH3 YES, always polar. 2 2 AX2E2 Bent < 109.5° sp3 H2O YES, always polar. 5 0 AX5 Trigonal Bipyramid 90°, 180°,120° sp3d AsCl5 POLAR, ONLY if X groups not identical. See-Saw (Distorted Tetrahedral) < 90°, 180°,120° sp3d SeCl4 YES, always polar. T-Shaped <90°,180° sp3d BrCl3 YES, always polar. Linear <180° sp3d XeF2 POLAR, ONLY if X groups not identical. AX6 Octahedral 90°, 180° sp3d2 TeBr6 POLAR, ONLY if X groups not identical. AX5E Square Pyramid < 90°,180° sp3d2 BrF5 YES, always polar. AX4E2 Square Planar < 90° sp3d2 XeF4 POLAR, ONLY if X groups not identical. 4 1 3 2 2 3 6 0 5 4 1 2 AX4E AX3E2 AX2E3