CHAPTER 10: BONDING AND MOLECULAR STRUCTURE: ORBITAL HYBRIDIZATION AND MOLECULAR ORBITALS
10.0 Objectives: • Describe the main features of valence bond theory and molecular orbital theory and their uses and limitations in describing covalent bonding. • Distinguish between sigma and pi bonding in terms of valence bond theory. • Use the concept of hybridization to rationalize shapes of certain covalent molecules. • Use molecular orbital theory and be able to classify electrons in bonding orbital, nonbonding orbitals, and antibonding orbitals to explain the properties of some molecules, like oxygen. • Understand the difference between a conductor, semiconductor, and an insulator.
Homework • HW #1 – 43, 45, 47, 49, 51, 53, 57
Theories of Chemical Bonding 1 -Valance-bond (VB) theory Hybridization of atomic orbitals 2 - Molecular orbital (MO) theory
VALENC BOND THEORY • Main idea – electrons are shared between atoms by overlapping atomic orbitals • Bonding Orbitals – molecular orbitals formed though overlap of atomic orbitals s-s s-p s-d p-p p-d d-d H-H H-F H-Pd C-C Se-F Fe-Fe (in Pd P-P hydride) • Sigma bond – covalent bond formed between the bonded atoms
Simple Explanation: • H2 HF F2
Modification of VB theory Overlapping of simple atomic orbitals does not explain all the features. Thus, we have to take another look, or do something about atomic orbitals – hybridization. Hybrid orbital – Orbital ofrmed through the mixing of two or more atomic orbitals
The sp3 Hybridized Orbitals Ground state and excited state electronic configuration of C The hybridization of a s and three p orbitals led to 4 sp3 hybrid orbitals for bonding. Compounds involving sp3 hybrid orbitals: CF4, CH4, : NH3, H2O::, SiO44–, SO42–, ClO4–, etc
The sp Hybrid Orbitals The sp hybrid orbitals: formation of two sp hybrid orbitals hybridization of s and p orbitals = 2 sp hybrid orbitals
The sp2 Hybrid Orbitals Carbon as sp2 The hybridization of a s and two p orbitals led to 3 sp2 hybrid orbitals for bonding. Compounds involving sp2 hybrid orbitals: BF3, CO32–, H2CO, H2C=CH2, NO3–, etc
The sp3d Hybrid Orbitals Hybridization of one s, three p, and a d orbitals results in 5 sp3d hybrid orbitals. The arrangement of these orbitals is a trigonal pyramid. Some structures due to these type of orbitals are PClF4, TeCl4, and BrF3.
The sp3d2 Hybrid Orbitals Hybridization of one s, three p, and two d orbitals results in 6 sp3d2 hybrid orbitals. The arrangement of these orbitals is an octahedron. Compounds using these type of orbitals are shown here. AX6, AX5E, AX4E2AX3E3 and AX2E4IOF5, IF5, XeF4 No known compounds of AX3E3 and AX2E4 are known or recognized, because they are predicted to have a T shape and linear shape respectively when the lone pairs of electrons are ignored.
Sigma bond – single bond between two atoms - May be hybridized pi bond – second or third bond between two atoms – usually occurring with p orbitals of atoms – NEVER HYBRIDIZED! Sigma and pi bonds
Hybridization of Atomic Orbitals The solutions of the Schrodinger equation led to these atomic orbitals: 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, etc. Orbital hybridization theorized by Linus Pauling to explain molecular geometries predicted and experimentally determined sp 2 sp hybrid orbitals from mixing of one s and one p orbital sp 23 sp2 hybrid orbitals from mixing of one s and two p orbitals sp3 4 sp3 hybrid orbitals from mixing of one s and three p orbitalssp3d 5 sp3d hybrid orbitals from mixing of one s and three p and one d orbital (expanded octet – AX5)sp3d 26 sp3d2hybrid orbitals from mixing of one s and three p and two d orbitals ( expanded octet – AX6)
Sigma and Pi Bonds H-C-C-H: three s (sigma) bonds due to overlapping of 1sH – spC; spC – spC; and spC – 1sH Two p (pi) bonds in HCCH and HCN triple bonds are due to overlapping of p orbitals sp hybrid orbitals py over lap C2s2p 2p2pspsp2p2p in p bond H H px over lap Two nodal planes of p bonds are perpendicular to each other. in p bond
A p Bond Sigma (s) bond is symmetric about axis. Pi (p) electron distribution above and below axis with a nodal plane, on which probability of finding electron is zero; p bond is not as strong as sigma - less overlap. Nodal plane Overlap of 2 2p orbitals for the formation of p bond C2s 2p 2p2psp2 sp2sp22p Bonding of C2H4
Bonding of CO2 For CO2, the C atom forms a s bond and a p bond with each of two O atoms. The two nodal planes of the two p bonds are also perpendicular.
Bonding in CO2 – another view Compare with H2C=C=CH2
Bonding of H2C=CH2 molecules Utilizing the sp2 hybrid orbitals, each C atom form two H–C s bonds for a total of 4 s H–C bonds. The C–C s bond is common to both C atoms. A C–C p bond is formed due to overlap of p orbitals from each of the C atoms. C2s 2p 2p2psp2 sp2sp22p Hybrid orbitals (sp2) for H–C and C–C s bond Overlap of p orbital for C–C p bond
Ex10.1 List the hybridization pattern of each central atom and the number of sigma and pi bonds in each structure for: CH3Cl CH2O HCN XeF4 CO2
(NOT ON CHAPTER 9 /10 TEST)Molecular Orbital Theory Lewis dot and valence bond theories do not always give satisfactory account for various properties of molecules. MO theory is different from VB in that MO theory considers the orbitals of the whole molecules. However the approach of linear-combination-of-atomic-orbitals (LCAO) is usually used. http://www.youtube.com/watch?feature=endscreen&v=KcGEev8qulA&NR=1
Principles of MO • Total number of molecular orbitals (MO)equals the total number of atomic orbitals (AO)contributed by the atoms that have combined • The bonding molecular orbital is lower in energy than the parent atomic orbitals, and the antibonding orbital is higher in energy than the parent atomic orbitals. • Electrons are assigned to orbitals of successively higher energy
The Molecular Orbital (MO) Theory For a molecule, there are certain orbitals each of which accommodates two electrons of opposite spin. The MO theory combines atomic orbitals (AO) to form MOs, & this method is called LCAO s* The two atoms in the H2 molecule may be represented by A and B. Their s orbitals 1sA and 1sB respectively, are used for two MOs: s* = 1sA – 1sB s = 1sA + 1sB The energy levels of these AO and MO are represented by the diagram here, with the math hidden. 1sA 1sB sMO AO AO
Electronic Configuration of H2-type Molecules From the previous theory, we can fill the M Os with electrons for the H2-type molecule: Molecule e-configuration Bond order bond length H2+ 1s(1s1) ½ 106 pm H2, He22+ 1s2 1 74, ~75 H2–, He2+ 1s2 1s* ½ ~106, 108H22–, He2 1s2 1s*2 0 not formed
Sigma MOs Formed Using p AOs Sigma MOs (s2p s2p* ) can be formed using p AOs, similar to VB theory. The gain in bonding orbital s2p (lower energy) is at the expense of the anti-bonding orbital s2p* (higher energy)
MO Diagram for F2 http://academicearth.org/lectures/molecular-orbital-theory-b
The O2+, O2, O2– , & F2+, F2, F2– Molecules For O=O_ _ O=O Paramagnetic , bond length indicates double bond, electronic configuration agrees F–F Electronic configuration agree with single bond.
MO Energy Level Diagram for Be2 – N2 Due to close energy levels of 2s and 2p, the MO energy level diagram for Be2 to N2 differs from those of O2 to F2. Reasons and explanation are given during the lecture. Hope you can do the same.
Benzene The benzene structure has fascinated scientists for centuries. It’s bonding is particularly interesting. The C atom utilizes sp2 hybrid AO in the sigma bonds, and the remaining p AO overlap forming a ring of p bonds. Sigma s bonds are represented by lines, and the p orbitals for the p bonds are shown by balloon-shape blobs. Note the + and – signs of the p orbitals. Thus, we represent it by + + + + + + – – – –
Delocalized electrons in Benzene and Ozone When p bonds are adjacent to each other or separated by on single C-C bonds, the p bonding electrons are delocalized. The delocalized electron path for benzene and ozone are shown here. These pictures represent p electron of the the structures formula contribute most to their structures. CO32– & have delocalized electrons O O O