10 1 Chapter 10 Chemical Bonding II Molecular Shapes Valence Bond Theory and Molecular Orbital Theory Four Models for Describing Bonding in Molecules Scientific Model VSEPR Consider the shapes of these molecules CO2 SO2 H2O BCl3 ClF3 NH3 10 2 VSEPR stands for The Main Idea Behind VSEPR 1 Draw the for the molecule or ion in order to determine the number of VSEPR electron pairs Each bond single double or triple counts as VSEPR pair or Each lone non bonding pair counts as VSEPR pair or 2 Determine the overall shape which will allow for maximum separation of electron pairs and bonds in order to 3 Name the molecular geometry based on the position of the not based on the positions of Two additional points 1 Lone pairs are more than bonded pairs 2 Multiple bonds usually require than single bonds Know these 5 basic geometries 10 3 Important Know this table of VSEPR geometries e pairs e groups 2 e pair geometry hybridization bonding pairs nonbonding pairs molecular geometry linear 180 sp 2 0 linear 180 examples Y Y X CO2 Y X 3 trigonal planar 120 sp2 3 0 trigonal planar 120 Y 2 1 bent 120 Y Y BF3 Y NO2 X Y 4 tetrahedral 109 5 sp3 4 0 tetrahedral 109 5 3 1 trigonal pyramidal 109 5 2 2 bent 109 5 Y X Y Y Y Y Y Y CH4 X Y NH3 X H2O Y Y Y 5 trigonal bipyramidal 90 120 sp3d 5 0 X Y Y trigonal bipyramidal 90 120 PCl5 Y Y Y 4 1 X Y seesaw 90 and 120 SF4 Y X 3 2 Y Y T shaped 90 ClF3 Y X 2 3 Y linear 180 XeF2 Y Y Y 6 octahedral 90 sp3d2 6 0 X Y Y Y octahedral 90 SF6 Y Y Y 5 1 02 Y Y square pyramidal 90 BrF5 Y Y 4 X square planar 90 X Y Y XeF4 10 4 Determine the molecular geometry of CH4 NH3 H2O O3 SCl2 SO2 SF4 BrF5 XeF4 CO2 10 5 Polar and Nonpolar Molecules 1 A forms between two atoms with different Examples 2 A is one with an unequal distribution of as a result of its and 3 A polar molecule has 4 In order to determine if a molecule is polar you must know its 5 If a molecule is polar then it has a with the symbol 6 Two ways to determine if a molecule is polar 7 Examples H2 HCl CO2 H2O NH3 10 6 CCl4 CHCl3 8 One useful generalization molecules are Six perfectly symmetric shapes 9 Another useful generalization hydrocarbons are Valence Bond Theory and Hybridization The main idea behind Valence Bond Theory is that Question Why do two hydrogen atoms combine to form a bond H H H H Model 1 Lewis Structures and Octet Rule Model 2 Valence Bond Theory So this model says that bonds form when atomic orbitals 10 7 Question How would these two models describe the bonding in ammonia 3H N NH3 Model 1 Lewis Structures and Octet Rule Model 2 Valence Bond Theory using simple atomic orbitals If simple atomic orbitals are used then bond angles of are predicted But NH3 is actually Whenever the overlap of simple atomic orbitals does not adequately explain certain bond properties such as equivalency of bonds bond angles etc the concept of is invoked The process of corresponds to a mixing or combining of orbitals an imaginary process accomplished mathematically resulting in new orbitals called Hybridization Give electron configuration of a ground state C atom 3 sp hybridization E 10 8 2 sp hybridization E sp hybridization E In a similar way mixing one s orbital three p orbitals and two d orbitals results in equivalent orbitals Know this Table VSEPR e pairs e pair geometry hybridization notes 2 linear 180 2 and 2 on C ex C2H2 3 trigonal planar 120 3 and 1 on C ex C2H4 4 tetrahedral 109 5 4 bonds on C ex CH4 5 trigonal bipyramidal 90 120 ex PCl5 6 octahedral 90 ex SF6 10 9 sigma bond a bond in which the electron density is concentrated pi bond a bond in which the electron density is concentrated single bond double bond triple bond Why is a bond is stronger than a bond Molecular Orbital Theory 1 Summary of Molecular Orbital MO Theory Quantum Mechanics says Electrons in exist in allowed energy states called Electrons in exist in allowed energy states called Think of MOs this way they are formed from a Two important types of MOs Determining MO electron configuration is analogous to determining AO electron configuration 2 Atomic Orbitals AOs and Molecular Orbitals MOs Two equivalent AOs will combine to form 2 MOs 10 10 The Bonding MO is in energy than the AOs The antibonding MO is in energy than the AOs and it has a between the two atoms One way to understand this AOs and MOs are wave functions QM says e have wave characteristics Two AOs can combine constructively wave reinforcement to form a MO Or they can combine destructively wave cancellation to form an MO Example H H H H H H H E 3 H E Bonding MOs are designated and MOs Antibonding MOs are designated and MOs pronounced sigma star and pi star molecular orbitals 4 Two s orbitals will combine to form 5 Two head to head p orbitals will combine to form 6 Two parallel p orbitals will combine to form 7 The general order of filling is 1s 1s 2s 2s 2px 2py 2pz 2px 2py 2pz 10 11 8 MO theory explains why O2 is paramagnetic 2 unpaired electrons O E Important Terms 1 HOMO 2 LUMO 3 HOMO LUMO gap 4 Bond Order 5 Paramagnetic 6 Diamagnetic 7 Node O O O E 10 12 H2 MO electron configuration Bond Order Magnetic Character He2 MO electron configuration Bond Order Magnetic Character He2 MO electron configuration Bond Order Magnetic Character B2 MO electron configuration Bond Order Magnetic Character O2 MO electron configuration Bond Order Magnetic Character