Presentation on theme: "Molecular Geometry The properties of a compound are very much determined by the size and shape of its molecules. Molecular geometry specifies the positions."— Presentation transcript:
1Molecular GeometryThe 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.
2Molecular ModelsMolecular geometry can be represented in different ways.Perspective drawing:Ball and Stick:Space filling:
3The VSEPR ModelValence 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.
4Applying 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.
7The Trigonal Bipyramid There are two positions, axial and equatorial, and two bond angles - 90° and 120°.
8Some Basic Geometries = = = Mole Lewis EP Bond cule Structure Geometry AngleO C O=· ·CO22 -Linear180O S O=· ·SO23 -TrigonalPlanar120O S O=· ·O C O· ·O-2CO3-23 -TrigonalPlanar120
9Some Basic GeometriesMole Lewis EP Bond cule Structure Geometry AngleH C HHCH44 -Tetra-hedral109.5H N HH· ·NH34 -Tetra-hedral109.5
10Some Basic GeometriesMole Lewis EP Bond cule Structure Geometry AngleF S FF· ·TrigonalBipyra-midalSF45 -90,120F Xe FF· ·XeF46 -Octa-hedral90
11Distortions 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: H2O104.5
12Molecules With More than One Central Atom The geometry around each central atom must be determined.HC2H3O2:H C C O HHO· ·Trigonal planarTetrahedral
13Molecular 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:
14Molecular Dipole Moments For polyatomic molecules, the dipole moment is the geometric sum of all bond dipole moments.CO2 - NonpolarH2O - Polar
15Molecular Dipole Moments Are the following polar or nonpolar?BF· ·F B FF120BF3:TrigonalPlanarNonpolarCOHH C HO· ·120CH2O:TrigonalPlanarPolar
16Molecular Dipole Moments Are the following polar or nonpolar?CClCl C Cl· ·Cl109CCl4:TetrahedralNonpolarH C HHCl· ·CClHCH3Cl:TetrahedralPolar