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9 Molecular Geometry Solutions to Exercises : Br: Br Te Br: : Br: 9.21 Analyze/Plan. Draw the Lewis structure of each molecule and count the number of nonbonding (lone) electron pairs. Note that the question asks 'in the molecule' rather than just around the central atom. Solve. (a) (CH₃)₂S, 20 valence e⁻, 10 e⁻ pr, 2 nonbonding pairs H H H-C-S-C-H H H (b) HCN, 10 valence e⁻, pr, 1 nonbonding pair (c) 10 valence e⁻, 5 pr, 0 nonbonding pairs (d) CH₃F, 14 valence e⁻, pr, 3 nonbonding pairs H H-C-F: H 9.22 Analyze/Plan. See Table 9.1. Solve. (a) trigonal planar (b) tetrahedral (c) trigonal bipyramidal (d) octahedral 9.23 The electron-domain geometry indicated by VSEPR describes the arrangement of all bonding and nonbonding electron domains. The molecular geometry describes just the atomic positions. H₂O has the Lewis structure given below; there are four electron domains around oxygen so the electron-domain geometry is tetrahedral, but the molecular geometry of the three atoms is bent. H H H H Lewis structure electron-domain molecular geometry geometry We make this distinction because all electron domains must be considered when describing the atomic arrangement and bond angles in a molecule but the molecular geometry or shape is a description of just the atomic positions. 235