Chemical Bonding II: Molecular Geometry and Hybridization of Atomic Orbitals Chapter 10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Valence shell electron pair repulsion (VSEPR) model: Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs Theory: electron domains keep as far away as possible from one another 2 types of electron domains – Bonding domains: electron pairs that are involved in bonds between two atoms (single,double,triple bond==1 bond domain) – Nonbonding domains: contain electron pairs that are associated with a single atom (called Lone Pairs)

10.1 Molecules in which the central atom has no lone pairs No Lone Pairs on Central Atom Arrangement of electron domains is very symmetrical The molecular geometry is the same as the electron domain geometry

Steps to Assign Molecular Geometry Draw Lewis structure Count # of electron domains around the Central atom(treat double and triple bonds as single domain) Assign electron domain geometry Finally assign the molecular shape – based on the arrangement of the atoms – not the arrangement of the domains No Lone Pairs on Central Atom Arrangement of electron domains is very symmetrical The molecular geometry is the same as the electron domain geometry

Cl Be 2 atoms bonded to central atom 0 lone pairs on central atom 10.1 AB 2 20 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry linear B B

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 BF3

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 CH4

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 PCl 5

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 SF 6

Molecules in which the central atom has one or more lone Pairs Arrangement of electron domains is Not symmetrical (usually) The molecular geometry is different from the electron domain geometry

10.1

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

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 distorted tetrahedron SF4

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 distorted tetrahedron 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 distorted tetrahedron 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

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 ? SO O AB 2 E bent S F F F F AB 4 E distorted tetrahedron 10.1

Dipole Moments and Polar Molecules 10.2 H F electron rich region electron poor region    = Q x r Q is the charge r is the distance between charges 1 D = 3.36 x C m Predicting Polarity of Molecules: A measure of the polarity of a molecule The dipole moment can be determined experimentally Measure in Debye units Predict polarity by taking the vector sum of the bond dipoles

Molecules containing net dipole moments are called polar molecules. Otherwise they are called nonpolar molecules because they do not have net dipole moments. Diatomic molecules: Determined by the polarity of bond Polar molecules:HCl, CO,NO Nonpolar molecules: H2,F2,O2 Molecules with three or more atoms: determined by the polarity of the bond and the molecular geometry Dipole moment is a vector quantity, which has both magnitude and direction.

Symmetric molecules such as these are nonpolar because the bond dipoles cancel All of the basic shapes are symmetric, or balanced, if all the domains and groups attached to them are identical O C O C O Linear molecule Nodipolar moment bent molecule Net dipolar moment

Chloroform, CCl3H, is asymmetric. The vector sum of the bond dipoles is nonzero giving the molecule a net dipole. A molecule will be nonpolar if: (a) the bonds are nonpolar, or (b) there are no lone pairs in the valence shell of the central atom and all the atoms attached to the central atom are the same

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

Covalent Bonding Theories Valence bond (VB) theory – Bonding is an overlap of atomic orbitals Includes overlap of hybrid atomic orbitals Molecular orbital (MO) theory – Bonding happens when molecular orbitals are formed Molecular Geometry & Hybridization 10.3 Lewis structures and VSEPR do not tell us why electrons group into domains as they do How atoms form covalent bonds in molecules requires an understanding of how orbitals interact

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 Hybrid Orbitals 10.4

Formation of sp 3 Hybrid Orbitals

# 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? 1.Draw the Lewis structure of the molecule. 2.Count the number of lone pairs AND the number of atoms bonded to the central atom. Predict the overall arrangement of electron pairs using VSEPR model ( Table 10.1) 3.Deduce hybridization of central atom by matching the arrangement of the electron pairs with those of hybrid orbitals shown in Table

Hybrid orbitals # of hybrid shape orbitals sp 3 4 tetrahedral sp 2 3 trigonal planar sp 2linear sp 3 d 5 trigonal bipyramidal sp 3 d 2 6 octahedral

10.4

Hybridization in molecules containing double and triple bonds Two Kinds of covalent Bonds Sigma bond: bonding density is along the internuclear axis – head to head overlap of two hybrid orbitals – head to head overlap of p + hybrid any s character to the bond = sigma bond Pi bond: bonding density is above and below the internuclear axis – Sideway overlap p + p 10.5

Sigma bond

Pi bond

Multiple bonds Double bond: One sigma and one pi bond between the same atoms –Example: ethylene 10.5 Triple bond: One sigma and two pi bonds between the same atoms –Example: acetylene

Review of 2 Types of covalent bonds: 1. Sigma bond e-density (overlap region) is along the internuclear axis 2. Pi bond e-density (overlap region) is above and below the plane of the internuclear axis. Relative reactivity of bonds: Pi bonds are more reactive than sigma bonds Less energy is needed to break a pi bond than a sigma bond

Sigma (  ) and Pi Bonds (  ) Single bond1 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 = Summary: VB Theory framework of the molecule is determined by the arrangement of the sigma-bonds; Hybrid orbitals are used to form the sigma bonds and the lone pairs of electrons.

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 10.6 But experiment show that there are two unpaired electrons— paramagnetic. Valence bond theory

Bonds are formed from interaction of atomic orbitals to form molecular orbitals. MO theory takes the view that a molecule is similar to an atom The molecule has molecular orbitals that can be populated by electrons just like the atomic orbitals in atoms # of MOs formed = # of atomic orbitals combined Molecular Orbital (MO) Theory

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.(Electrons spread out as much as possible, with spins unpaired, over orbitals that have 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 -MOs follow the same filling rules as atomic orbitals

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