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Theories of Chemical Bonding

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Theories of Chemical Bonding Theories of Chemical Bonding Theories of bonding: explanations for chemical bond, Lewis dot structures and the following. Valance-bond (VB) theory Hybridization of atomic orbitals Multiple covalent bonds Molecular orbital (MO) theory Delocalized electrons Bonding in metals Practice mental reasoning and verbal explanation Theories of chemical bonding 1 Energy of Interaction Between Two H Atoms Energies of attraction and Potential repulsion as functions of energy distance between two H atoms are shown here. +346 kJ mol –1 The minimum of the antibonding attraction force occur at H–H bond length of 74 pm, at which, the antibonding distance orbital is +346 kJ mole–1 above 0, energy when H –346 kJ mol –1 atoms are far apart. H – H bond How does energy affectTheories the two of chemical-atom bonding system? 2 The Valence-bond Method Valence bond method considers the covalent bond as a result of overlap of atomic orbitals. Electrons stay in regions between the two atoms. Some bond examples s-s s-p s-d p-p p-d d-d H-H H-C H-Pd C-C Se-F Fe-Fe (?) Li-H H-N in Pd P-P H-F hydride But 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. How does valence-bond approach explain the formation of chemical Theoriesbonds? of chemical bonding 3 Hybridization of Atomic Orbitals The solutions of Schrodinger equation led to these atomic orbitals. 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, etc. However, overlap of these orbitals does not give a satisfactory explanation. In order to explain bonding, these orbitals are combined to form new set of orbitals – this method is called hybridization. During the lecture, these hybridized orbitals will be explained: sp 2 sp hybrid orbitals from mixing of a s and a p orbital sp 2 3 sp2 hybrid orbitals from mixing of a s and 2 p orbital sp3 fill in you explanation please sp3d 5 sp3d hybrid orbitals from mixing of a s and 3 p and a d orbital sp3d 2 ____________ Provide a description for hybridTheories orbitals of chemical sp, bonding sp2, sp3, sp3d, and sp3d2 4 The sp Hybrid Orbitals The sp hybrid orbitals: formation of two sp hybrid orbitals + + + - = + - + – + - = - + hybridization of s and p orbitals = 2 sp hybrid orbitals _ _ __ __ __ __ __ Two sp hybrid orbitlas => Two states of Be Theories of chemical bonding 5 Bonds with sp Hybrid Orbitals Formations of bonds in these molecules are discussed during the lecture. Be prepared to do the same by yourself. Cl–Be–Cl H–CC–H H–CN : O=C=O Double and triple bonds involve pi p bonding, and the the application of valence bond method to p bonds will be discussed. You are expected to be able to draw pictures to show the p bonding. Theories of chemical bonding 6 A p Bond Sigma (s) bond is symmetric about axis. Nodal plane 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 of 2 2p orbitals for overlap. the formation of p bond C 2s 2p 2p 2p sp2 sp2 sp2 2p Bonding of C2H4 Theories of chemical bonding 7 How are pi bonds formed? Triple Bonds in H-CC-H H-C-C-H: three s bonds due to overlapping of 1sH – spC; spC – spC; and spC – 1sH. Two p bonds in HCCH and HCN triple bonds are due to overlapping of p orbitals results. sp hybrid orbitals py over lap C in p bond 2s 2p 2p 2p H H sp sp 2p 2p px over lap Two nodal planes of p in p bond bonds are perpendicular to each other. Draw and describe how atomic orbitals overlap Theories of chemical bonding 8 to form all bonds in acetylene, H–CC–H Two p Bonds in H–CC–H A triple bond consists of a sigma and two pi bonds. Overlaps of two sets of p orbitals form of two p bonds. Theories of chemical bonding 9 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. During the lecture, I draw diagrams and explain the two s two p bonds in CO2. You are expected to be able to do the same, in a test. Resonance structures py over lap in p bond : O – C O : : O C – O : px over lap in p bond . Overlap p–p in s bonds Discuss the bonding of allene H2C=C=CH2 Theories of chemical bonding 10 O=C=O or H2C=C=CH2 See extra problems B17 in the handout Bonding in CO2 – another view 09_174 sigma bond (1 pair of electrons) pi bond (1 pair of electrons) O C O pi bond (1 pair of electrons) (a) Compare with H2C=C=CH2 O C O Theories of chemical bonding 11 (b) The sp2 Hybrid Orbitals Ground state and excited state electronic configuration of B _ _ _ __ _ __ __ The hybridization of a s and two p orbitals led to 3 sp2 hybrid orbitals for bonding. Compounds involving sp2 hybrid 2– orbitals: BF3, CO3 , H2CO, – H2C=CH2, NO3 , etc Theories of chemical bonding 12 Nov. 25 An example of using sp2 hybrid orbitals __ orbitals for bonding? Dipole moment = ____? Theories of chemical bonding 13 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. C 2s 2p 2p 2p sp2 sp2 sp2 2p Hybrid orbitals (sp2) for H–C and C–C s bond Overlap of p orbital for C–C p bond Theories of chemical bonding 14 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::, 4– 2– – SiO4 , SO4 , ClO4 , etc Theories of chemical bonding 15 C 2s 2p 2p 2p sp3 sp3 sp3 sp3 Theories of chemical bonding 16 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, TeCl4E, and BrF3E2. How many unshared electron pairs are present in TeCl4 and BrF3? What are their shapes? Theories of chemical bonding 17 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, AX4E2 AX3E3 and AX2E4 IOF5, IF5E, XeF4E2 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. Theories of chemical bonding 18 Molecules with more than one central atom Describe the structure of CH3NCO. Take a new look at slide 22 in Bonding Basics Draw the skeleton and add all valence electrons H3C – N – C – O Which Lewis dot structure is the most important (stable)? o NC–O 180 N–CO N = C = O Which 109o H–C 120o H–C structure is H–C more stable, H H H H H H and why? What hybridized orbitals are used for bonding in N and C? Why are the bond angles as indicated? No of s and p bonds = __, __? Give formal charges to all Theoriesatoms of in chemical all structures. bonding 19 Why Molecular Orbital (MO) Theory Lewis dot and valence bond theories do not always give satisfactory account for various properties of molecules. For example, the dot and VB theory does not explain the fact that O2 is paramagnetic and has a double bond. Dot and VB structures : O O : • O O • are unsatisfactory. MO theory, 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. There are other reasons, but it’s human nature to theorize. The theory is beautiful, and worth learning or teaching. Theories of chemical bonding 20 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 1sA 1sB The energy levels of these AO and MO are represented by the diagram here, with the math hidden. s Theories of chemical bonding AO MO AO21 + – + MO for H2–type molecules: H2 , H2, H2 , He2 Generalize the technique of LCAO Theories of chemical bonding 22 2 Electronic configuration, s , for H2 molecules Generalize the technique of LCAO Theories of chemical bonding 23 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 bondlength + 1 H2 1s (1s ) ½ 106 pm 2+ 2 H2, He2 1s 1 74, ~75 – + 2 H2 , He2 1s 1s* ½ ~106, 108 2– 2 2 H2 , He2 1s 1s* 0 not formed Describe the relationships of bondlength & bondorder and e-configurations; learn to reason Theories of chemical bonding 24 Sigma MOs Formed Using p AOs * Sigma MOs (s2p s2p ) can be formed using p AOs, similar to VB theory.
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