Chapter 9 Lewis Theory-VSEPR Valence Bond Theory Molecular Orbital Theory
Problems with Lewis Theory Lewis theory generally predicts trends in properties, but does not give good numerical predictions. Lewis theory gives good first approximations of the bond angles in molecules, but usually cannot be used to get actual bond angles. Lewis theory cannot write one correct structure for many molecules where resonance is important. Lewis theory often does not predict the correct magnetic behavior of molecules.
Valence Bond Theory Linus Pauling and others applied the principles of quantum mechanics to molecules. They reasoned that bonds between atoms would occur when the atomic orbitals interacted to make new bonds. The types of interactions depend on whether the orbitals align along the axis between the nuclei, or outside the axis.
Orbital Interaction As two atoms approached, the half-filled valence atomic orbitals on each atom would interact to form molecular orbitals. The molecular orbitals would be more stable than the separate atomic orbitals because they would contain paired electrons shared by both atoms.
Orbital Diagram for the H S H Formation of H2S H 1s H S bond S 1s 3s 3p H S bond H
Predicts bond angle = 90 Actual bond angle = 92
Unhybridized C Orbitals Predict the Wrong Bonding & Geometry H 1s H 1s C 2s 2p
Valence Bond Theory - Main Concepts Valence electrons of the atoms in a molecule reside in quantum-mechanical atomic orbitals. The orbitals can be the standard s, p, d, and f orbitals, or they may be hybrid combinations of these. A chemical bond results when two of these atomic orbitals interact and there is a total of two electrons in a new molecular orbital. The shape of the molecule is determined by the geometry of the interacting orbitals.
Hybridization Hybridization is mixing different types of orbitals in the valence shell to make a new set of degenerate orbitals. # of new orbitals-----> 2, 3, 4, 5, 6 orbital designation---> sp, sp 2, sp 3, sp 3 d, sp 3 d 2 The same type of atom can have different types of hybridization: C,N = sp, sp 2, sp 3 The particular kind of hybridization that occurs is the one that yields the lowest overall energy for the molecule.
The sp Hybrid Orbitals in Gaseous BeCl2
Cl Be Cl
The sp 2 Hybrid Orbitals in BF3 F B F F
The sp 3 Hybrid Orbitals in CH4
The sp 3 Hybrid Orbitals in NH3
The sp 3 Hybrid Orbitals in H2O
Hybridization and VSEPR Theory All three central atoms are sp 3 hybridized!!
The sp 3 d Hybrid Orbitals in PCl5
The sp 3 d 2 Hybrid Orbitals in SF6
Carbon Hybridizations Unhybridized sp 3 hybridized 2s 2p 2 sp 3 Unhybridized 2s 2p sp 2 hybridized 2sp 2 2p Unhybridized 2s 2p sp hybridized 2sp 2p
Different Carbon Hybridizations Lead to Different Molecular Geometries sp 3 sp 2 sp electron density
sp 3 Hybridization Atom with four electron groups around it tetrahedral electron group geometry ~109.5 angles between hybrid orbitals tetrahedral molecular geometry for carbon trigonal pyramidal geometry for nitrogen bent geometry for oxygen Atom uses hybrid orbitals for all bonds & lone pairs
sp 3 Hybridized Atoms Place electrons into hybrid and unhybridized valence orbitals as if all the orbitals have equal energy Unhybridized atom sp 3 hybridized atom C 2s 2p 2sp 3 2s 2p 2sp 3 N 2s 2p 2sp 3 O
Bonding with Valence Bond Theory Bonding takes place between atoms when their atomic or hybrid orbitals interact ( overlap ). To interact, the orbitals must either be aligned along the axis between the atoms, or The orbitals must be parallel to each other and perpendicular to the interatomic axis.
Bonding in Methane
Ammonia Formation with sp 3 N
Water Formation with sp 3 O sp 3 H O 1s sp 3 H 1s
Types of Bonds Sigma (σ) bond - when the interacting atomic orbitals point along the axis connecting the two bonding nuclei Pi (π) bond - when the bonding atomic orbitals are parallel to each other and perpendicular to the axis connecting the two bonding nuclei. (Usually from overlap of two unhybridized p orbitals) The interaction between parallel orbitals is not as strong as between orbitals that point at each other; Therefore, σ bonds are stronger than π bonds.
Types of Bonds
sp 2 Hybridization Atom with three electron groups around it trigonal electron group planar system ~120 bond angles - flat C = trigonal planar molecular geometry N = bent molecular geometry O = linear geometry Atom uses hybrid orbitals for σ bonds and lone pairs Atom uses a nonhybridized p orbital for a π bond
sp 2 Hybridized Atoms Orbital Diagrams Unhybridized atom sp 2 hybridized atom C 2s 2p 2sp 2 2p 2s 2p 2sp 2 2p N 2s 2p 2sp 2 2p O
Formaldehyde, CH2O H H C O p C sp 2 C π σ p O sp 2 O σ σ 1s H 1s H
Bonding in Formaldehyde H H C O
Hybrid orbitals overlap to form σ bonds. Unhybridized p orbitals overlap to form a π bond.
Ethene, CH2CH2 H H C C H H p C sp 2 C π σ p C sp 2 C σ σ σ σ 1s H 1s H 1s H 1s H
Bonding in Ethene, C2H4 π π
Bonding in Ethene
CH2NH Orbital Diagram p C sp 2 C σ σ H H C π σ N H H C H σ N H p N sp 2 N 1s H 1s H 1s H
Bond Rotation Rotation around a σ bond does not require breaking the interaction between atomic orbitals. Rotation around a π bond requires the breaking of the interaction between atomic orbitals.
Restricted Rotation Around π-bonded Atoms in C2H2Cl2
Restricted Rotation Around π-bonded Atoms in C2H2Cl2 cis trans
Restricted Rotation Around π-bonded Atoms in C2H2Cl2 no net dipole
sp hybridization Atom with two electron groups linear shape 180 bond angle Atoms use hybrid orbitals for σ bonds or lone pairs Atom use nonhybridized p orbitals for π bonds
sp Hybridized Atoms Orbital Diagrams Unhybridized atom 2s 2p sp hybridized atom 2sp 2p C C 2s 2p 2sp 2p N
HCN Orbital Diagram H C N p C sp C 2π σ p N sp N σs 1s H
Bonding in HCN H C N H C N
HCCH (C2H2) Orbitals H C C H p C sp C 2π σ p C sp C s σ s σ 1s H 1s H
Bonding in C2H2
Bonding in HCCH H C C H H C C H
Bonding in C2H2
sp 3 d hybridization
sp 3 d hybridization
sp 3 d hybridization Unhybridized atom sp 3 d hybridized atom P 3s 3p 3d 3sp 3 d 3s 3p 3d 3sp 3 d S 4s 4p 4d 4sp 3 d (non-hybridizing d orbitals not shown) Br
sp 3 d hybridization
sp 3 d hybridization
sp 3 d hybridization
sp 3 d 2 hybridization
sp 3 d 2 Hybridized Atoms Orbital Diagrams Unhybridized atom sp 3 d 2 hybridized atom S 3s 3p 3d 3sp 3 d 2 Br 4s 4p 4d 4sp 3 d 2
sp 3 d 2 hybridization
sp 3 d 2 hybridization Atom with six electron groups around it octahedral electron geometry Square Pyramid, Square Planar 90 bond angles Use empty d orbitals from valence shell. d orbitals can be used to make π bonds.
Predicting Hybridization and Bonding Scheme 1. Start by drawing the Lewis structure 2. Use VSEPR Theory to predict the electron group geometry around each central atom. 3. Select the hybridization scheme that matches the electron group geometry. 4. Sketch the atomic and hybrid orbitals on the atoms in the molecule, showing overlap of the appropriate orbitals 5. Label the bonds as σ or π
Predict the hybridization and bonding scheme for CH2CH2 1. Start by drawing the Lewis structure 2. Use VSEPR Theory to predict the electron group geometry around each central atom The molecule has two interior atoms. Since each atom has three electron groups (one double bond and two single bonds), the electron geometry about each atom is trigonal planar.
Predict the hybridization and bonding scheme for CH2CH2 3. Select the hybridization scheme that matches the electron group geometry C1 = trigonal planar C1 = sp 2 C2 = trigonal planar C2 = sp 2 4. Sketch the atomic and hybrid orbitals on the atoms in the molecule, showing overlap of the appropriate orbitals continued
Predict the hybridization and bonding scheme for CH3CHO 1. Start by drawing the Lewis structure 2. Use VSEPR Theory to predict the electron group geometry around each central atom C1 = 4 electron areas C1= tetrahedral C2 = 3 electron areas C2 = trigonal planar
Predict the hybridization and bonding scheme for CH3CHO 3. Select the hybridization scheme that matches the electron group geometry C1 = tetrahedral C1 = sp 3 C2 = trigonal planar C2 = sp 2 4. Sketch the atomic and hybrid orbitals on the atoms in the molecule, showing overlap of the appropriate orbitals
Predict the hybridization and bonding scheme for CH3CHO Label the bonds as σ or π H H H C π C O H σ
Additional Examples Follow
Predict the hybridization of all the atoms in H3BO3 H = can t hybridize B = 3 electron groups = sp 2 O = 4 electron groups = sp 3
Predict the hybridization and bonding scheme of all the atoms in NClO O N C l N = 3 electron groups = sp 2 O = 3 electron groups = sp 2 Cl = 4 electron groups = sp 3 O N O N Cl Cl
SOF4 Orbital Diagram d S sp 3 d S π σ p O sp 2 O σ σ σ σ 2p F 2p F 2p F 2p F
sp 3 d hybridization Atom with five electron groups around it trigonal bipyramid electron geometry Seesaw, T Shape, Linear 120 & 90 bond angles Use empty d orbitals from valence shell d orbitals can be used to make π bonds