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: AX4Tetrahedral

Example

BrO3F, Perbromyl fluoride

Molecular Geometry: AX3E1Trigonal Pyramidal

Example

NF3, Nitrogen trifluoride

Molecular Geometry: AX2E2Bent/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:AX4E1See-saw

Example IF2O2-

Molecular Geometry:AX3E2T-structure

Example ClF3

Molecular Geometry:AX2E3Linear

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: AX6Octahedral

Example SF6

Molecular Geometry: AX4E1Pyramidal Planar

Example XeOF4

Molecular Geometry: AX4E2Square 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