Chemical Bonding II: Molecular Geometry and Hybridization of Atomic Orbitals Chapter 10

10.1

Valence shell electron pair repulsion (VSEPR) model: Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs. AB 2 20 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry 10.1 linear B B

Cl Be 2 atoms bonded to central atom 0 lone pairs on central atom 10.1

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar 10.1

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar 10.1 AB 4 40 tetrahedral

10.1

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar 10.1 AB 4 40 tetrahedral AB 5 50 trigonal bipyramidal trigonal bipyramidal

10.1

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar 10.1 AB 4 40 tetrahedral AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 6 60 octahedral

10.1

bonding-pair vs. bonding pair repulsion lone-pair vs. lone pair repulsion lone-pair vs. bonding pair repulsion >>

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar AB 2 E21 trigonal planar bent 10.1

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 E31 AB 4 40 tetrahedral trigonal pyramidal 10.1

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 4 40 tetrahedral 10.1 AB 3 E31tetrahedral trigonal pyramidal AB 2 E 2 22tetrahedral bent H O H

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR 10.1 AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 4 E41 trigonal bipyramidal See-Saw (distorted tetrahedron)

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR 10.1 AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 4 E41 trigonal bipyramidal See-Saw AB 3 E 2 32 trigonal bipyramidal T-shaped Cl F F F

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR 10.1 AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 4 E41 trigonal bipyramidal See-Saw AB 3 E 2 32 trigonal bipyramidal T-shaped AB 2 E 3 23 trigonal bipyramidal linear I I I

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR 10.1 AB 6 60 octahedral AB 5 E51 octahedral square pyramidal Br FF FF F

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR 10.1 AB 6 60 octahedral AB 5 E51 octahedral square pyramidal AB 4 E 2 42 octahedral square planar Xe FF FF

10.1

Predicting Molecular Geometry 1.Draw Lewis structure for molecule. 2.Count number of lone pairs on the central atom and number of atoms bonded to the central atom. 3.Use VSEPR to predict the geometry of the molecule. What are the molecular geometries of SO 2 and SF 4 ? (In SF 4, S uses an expanded octet of 10.) AB 2 Ebent S F F F F AB 4 E See-Saw (distorted Tetrahedron) 10.1 S O O S O O

Dipole Moments and Polar Molecules 10.2 H F electron rich region electron poor region  

10.2

Which of the following molecules have a dipole moment? H 2 O, CO 2, SO 2, and CH 4 O H H dipole moment polar molecule S O O CO O no dipole moment nonpolar molecule dipole moment polar molecule C H H HH no dipole moment nonpolar molecule

10.2 Chemistry In Action: Microwave Ovens

Bond Dissociation EnergyBond Length H2H2 F2F kJ/mole kJ/mole 74 pm 142 pm Valence bond theory – bonds are formed by sharing of e - from overlapping atomic orbitals. Overlap Of 2 1s 2 2p How does Lewis theory explain the bonds in H 2 and F 2 ? Sharing of two electrons between the two atoms. 10.3

Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals. 1.Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals. 2.Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process. 3.Covalent bonds are formed by: a.Overlap of hybrid orbitals with atomic orbitals b.Overlap of hybrid orbitals with other hybrid orbitals 10.4

Formation of sp 2 Hybrid Orbitals 10.4

Formation of sp Hybrid Orbitals 10.4

# of Lone Pairs + # of Bonded Atoms HybridizationExamples sp sp 2 sp 3 sp 3 d sp 3 d 2 BeCl 2 BF 3 CH 4, NH 3, H 2 O PCl 5 SF 6 How do I predict the hybridization of the central atom? Count the number of lone pairs AND the number of atoms bonded to the central atom 10.4

Sigma (  ) and Pi Bonds (  ) Single bond 1 sigma bond Double bond 1 sigma bond and 1 pi bond Triple bond 1 sigma bond and 2 pi bonds How many  and  bonds are in the acetic acid (vinegar) molecule CH 3 COOH? C H H CH O OH  bonds = = 7  bonds =

Sigma bond (  ) – electron density between the 2 atoms Pi bond (  ) – electron density above and below plane of nuclei of the bonding atoms 10.5

Molecular orbital theory – bonds are formed from interaction of atomic orbitals to form molecular orbitals. O O No unpaired e - Should be diamagnetic Experiments show O 2 is paramagnetic (has UNPAIRED e-) 10.6 MO Theory is NOT in the AP Chemistry Curriculum and will NOT be on the AP exam! This is just a quick overview!

Energy levels of bonding and antibonding molecular orbitals in hydrogen (H 2 ). A bonding molecular orbital has lower energy and greater stability than the atomic orbitals from which it was formed. An antibonding molecular orbital has higher energy and lower stability than the atomic orbitals from which it was formed. 10.6

1.The number of molecular orbitals (MOs) formed is always equal to the number of atomic orbitals combined. 2.The more stable the bonding MO, the less stable the corresponding antibonding MO. 3.The filling of MOs proceeds from low to high energies. 4.Each MO can accommodate up to two electrons. 5.Use Hund’s rule when adding electrons to MOs of the same energy. 6.The number of electrons in the MOs is equal to the sum of all the electrons on the bonding atoms Molecular Orbital (MO) Configurations

bond order = 1 2 Number of electrons in bonding MOs Number of electrons in antibonding MOs ( - ) 10.7 bond order ½10½

Delocalized molecular orbitals are not confined between two adjacent bonding atoms, but actually extend over three or more atoms. 10.8

10.6