2VSEPR Theory Use VSEPR theory Valence-Shell-Electron-Pair Repulsion =the repulsion between electron pairs causes molecular shapes to adjust so that the valence-electron pairs stay as far apart as possible.Determined by number of valence electrons of the central atom3-D shape is a result of bonded pairs and lone pairs of electrons
3Bonding and Shape of Molecules Numberof BondsNumber ofUnshared PairsCovalentStructureShapeExamples2341AB2LinearTrigonal planarTetrahedralPyramidalBent or AngularBeCl2BF3CH4, SiCl4NH3, PCl3ONFAB3AB4AB3EAB2E
4Number of Unshared Pairs Number of BondsNumber of Unshared PairsCovalent StructureShapeExample2AB2E2Bent or AngularH2O5AB5Trigonal BipyramidalPF56AB6OctahedralSF6
5The VSEPR Model .. .. .. O S O S F N O C Linear Bent Trigonal planar pyramidalFPAB6FFSStudents often confuse electron-domain geometry with molecular geometry.You must stress that the molecular geometry is a consequence of the electron domain geometry.The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.FCTetrahedralTrigonal bipyramidalOctahedral
12Learning CheckDetermine the molecular geometry:A. CCl4B. H2CO
13Learning Check Determine the molecular geometry: A. CCl4 Tetrahedral B. H2CO
14Learning Check Determine the molecular geometry: A. CCl4 Tetrahedral B. H2CO Trigonal Planar
15Intermolecular Forces Forces of attraction between moleculesVery weak compared to ionic and metallic bonding.The strongest occur between polar molecules.
16Intermolecular Forces Dipole: One end slightly positive and one end slightly negative.Can show the polar nature with an arrow pointing to the more electronegative atom.OH H H Cl
17Just because a molecule has a polar bond does not make it a polar molecule. The bond polarities cancel if they are in opposite directions.Example: CO2
18Attractions Between Molecules The two weakest attractions between molecules.Dipole InteractionsWhen polar molecules are attracted to one another.Dispersions ForcesCaused by the motion of electronsWeakest of all interactions.
19Hydrogen BondsA strong attraction found in hydrogen-containing molecules.A hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom.ExampleWater