Molecular Geometry and Bonding Theory Chapter 9 AP Chemistry

Molecular Geometry Molecular Geometry- general shape of a molecule as determined by the relative position of the nuclei. The geometry and size of a molecule helps to determine it’s chemical behavior. VSEPR- Valence-Shell-Electron-Pair-Repulsion model- predicts geometry based upon e’s around the central atom.

Principles of the VSEPR theory Electrons are kept as far away from one another as possible – minimizing e pair repulsions Electron pairs are considered as being bonding or non-bonding (lone pairs) A multiple bond counts as a single bonding pair Electron pair geometry is described by the regions of e’s around the central atom Molecular geometry is a consequence of electron pair geometry.

Predicting Structures VSEPR Derive the Lewis Structure to the form AXmEn A = central atom X = atoms bonded to the central atom E = lone pairs of e’s on the central atom M = # of bonded atoms N = # of lone pairs

Electron Geometry Linear Example Electron and Molecular Geometry Linear Bond angles 180 Sp Hybrid Non-polar

More Examples AX2

Electron Geom. Trigonal Planar Molecular Geom. Trigonal Planar bond angles exactly 120 non-polar Bent bond angles aprox 120 polar molecule Sp2 hybridized

Electron Geom.Trigonal Planar Molecular Geometry: AX3 (3BP or 2BP + 1LP) ex. 3bp NO3-                                  <>

Examples Cont. 3bp BF3 <>                     <>

Examples Cont. 2bp + 1 lp Ex O3 (ozone)

Electron Geom. Tetrahedral Molecular Geom. Tetrahedral bond angles exactly 109.5 non-polar molecule, Trigonal pyramidal bond angles aprox. 109.5 polar molecule, Bent bond angles aprox. 109.5 polar molecule Sp3 hybridized

Molecular Geometry: AX4 [4BP or (3BP + 1LP) or (2BP + 2LP)] Electron Geometry: AX4 Tetrahedral

Example BrO3F, Perbromyl fluoride

Molecular Geometry: AX3E1 Trigonal Pyramidal

Example NF3, Nitrogen trifluoride

Molecular Geometry: AX2E2 Bent/Angular

Example H2O, Water, ClOF, Chlorosyl fluoride

Electron Geom. Trigonal Bypyramidal Mol. Geom. Trigonal Bypyramidal – bond angles ax. exactly 108 deg. eq. exactly 120 deg. Non-polar, See-Saw – bond angles ax. aprox. 108 deg. eq. aprox. 120 deg. Polar, T-shaped - bond angles ax. aprox. 108 deg. No –eq, Linear – bond angle exactly 180 non-polar. All sp3d hybridized

Mol geo. Trigonal bipramidal AX5 [5BP or (4BP + 1 LP) or (3BP + 2LP) or (2BP + 3LP)] 5bp

Example PCl5(gas phase), Phosphorous pentachloride

Molecular Geometry: AX4E1 See-saw Example IF2O2-

Molecular Geometry: AX3E2 T-structure Example ClF3

Molecular Geometry: AX2E3 Linear Examole XeF2

Electron Geometry Octahedrial Mol geo. Octahedral – bond angle exactly 90 - non-polar, square pyramidal – bond angle aprox 90- polar, square planar- bond angle exactly 90 non-polar Sp3d2 hybridized

Molecular Geometry: AX6 Octahedral Example SF6

Molecular Geometry: AX4E1 Pyramidal Planar Example XeOF4

Molecular Geometry: AX4E2 Square Planar Example XeF4

Bond Angles Non-bonding pairs of e’s take up more space (att. by one nucli) than bonded e pairs Double and triple bonds take up more space than single bonds (more e’s) Volume occupied lone pairs > triple bonds > double bonds > single bonds

Forces Non-bonding pairs exert repulsive forces on adjacent e pairs and compress angles Multiple bonds also exert repulsive forces and compress angles