Molecular Geometry The properties of a compound are very much determined by the size and shape of its molecules. Molecular geometry specifies the positions of the atoms in terms of bond lengths and bond angles. We will focus on the molecular shape which depends on bond angles.
Molecular Models Molecular geometry can be represented in different ways. Perspective drawing: Ball and Stick: Space filling:
The VSEPR Model Valence Shell Electron Pair Repulsion is a simple but effective model for predicting molecular geometry. The first assumption of VSEPR is: A molecule adopts the geometry that min- imizes the repulsive force among a given number of electron pairs.
Applying VSEPR Draw the Lewis structure of the molecule. Count the number of electron pairs around the central atom. Multiple bonds count as one electron pair. The arrangement of electron pairs that min- imizes repulsion is called the electron-pair geometry. The arrangement of atoms is called the molecular geometry.
Electron-Pair Geometries #E.P.s Geometry 180 2 Linear 120 Trigonal Planar 3 109.5 4 Tetrahedral
Electron-Pair Geometries #E.P.s Geometry 90 Trigonal Bipyramidal 5 120 90 90 6 Octahedral
The Trigonal Bipyramid There are two positions, axial and equatorial, and two bond angles - 90° and 120°.
Some Basic Geometries = = = Mole- Lewis EP Bond cule Structure Geometry Angle O C O = · · CO2 2 - Linear 180 O S O = · · SO2 3 - Trigonal Planar 120 O S O = · · O C O · · O -2 CO3-2 3 - Trigonal Planar 120
Some Basic Geometries Mole- Lewis EP Bond cule Structure Geometry Angle H C H H CH4 4 - Tetra- hedral 109.5 H N H H · · NH3 4 - Tetra- hedral 109.5
Some Basic Geometries Mole- Lewis EP Bond cule Structure Geometry Angle F S F F · · Trigonal Bipyra- midal SF4 5 - 90, 120 F Xe F F · · XeF4 6 - Octa- hedral 90
Distortions from Ideal Geometry Further assumptions of VSEPR: Nonbonding EPs exert a greater repulsive force than bonding EPs. Multiple bonds exert a greater repulsive force than single bonds. Example: H2O 104.5
Molecules With More than One Central Atom The geometry around each central atom must be determined. HC2H3O2: H C C O H H O · · Trigonal planar Tetrahedral
Molecular Dipole Moments A molecule is polar if its centers of positive and negative charge are separated. Polarity causes a neutral molecule to interact with ions or other dipoles. Dipoles in an electric field:
Molecular Dipole Moments For polyatomic molecules, the dipole moment is the geometric sum of all bond dipole moments. CO2 - Nonpolar H2O - Polar
Molecular Dipole Moments Are the following polar or nonpolar? B F · · F B F F 120 BF3: Trigonal Planar Nonpolar C O H H C H O · · 120 CH2O: Trigonal Planar Polar
Molecular Dipole Moments Are the following polar or nonpolar? C Cl Cl C Cl · · Cl 109 CCl4: Tetrahedral Nonpolar H C H H Cl · · C Cl H CH3Cl: Tetrahedral Polar