www.hschemsolutions.com Valence Bond Theory Chemical Bonding 4.8 • Combines Lewis’ theory of filling octets by sharing pairs of electrons with the electron configuration of atomic orbitals. • Valence Bond Theory states that bonding occurs when atomic orbital overlap. Valence Bond Theory Hybrid Orbital Theory Multiple Bonds Building BF3 with Valence Bond Theory B: Building BF3 with Valence Bond Theory B: 1s 2s 2p 1s 2s 2p 1s 2s 2p F: Boron enters an excited state where an electron from the 2s orbital is promoted to the 2py orbital. Valence Bond Theory has problems with the shape 2py F 2px B F 2s Valence Bond Theory Views bonding as an overlapping of atomic orbitals. • Valence Bond Theory says 90o and a random angle for the fluorine bonded to the s-orbital. • VSEPR theory says 120o Solution (Hybrid Orbitals) Gets the bond angle wrong F Problems with Valence Bond Theory and BF3 Problem (the bond angle is wrong) • When the electron is promoted, the 2s, 2px, and 2py orbitals of boron morph into three separate sp2 hybrid orbitals that are identical in shape and size. © 2009 High School Chem Solutions. All rights reserved. 1 www.hschemsolutions.com sp2 Hybrid Orbitals sp2 Hybrid Orbitals sp2 F s sp2 + + p = p sp2 sp2 sp2 F F sp2 The 2s and two 2p orbitals morph into three identical sp2 hybrid orbitals sp3 Hybrid Orbitals (e.g. CH4) H: sp3 Hybrid Orbitals (e.g. CH4) H: 1s 1s C: C: 1s 2s 2p 1s 2s 2p Hybridization of Carbon C: 1s sp3 sp3 Hybrid Orbitals (e.g. CH4) H Bond Angle = 109.5o sp3 H sp3 sp3 H sp3 H Valence Bond Theory and Lone Pairs (e.g. H2O) Building H2O with Valence Bond Theory H: H: 1s 1s O: 1s 2s 2p Oxygen could accept one electron from one Hydrogen in its py orbital and another from the other Hydrogen in its pz orbital. © 2009 High School Chem Solutions. All rights reserved. 2 www.hschemsolutions.com Problems with Valence Bond Theory in the H2O Example Valence Bond Theory and Lone Pairs (e.g. H2O) • Valence Bond Theory views bonding as an overlapping of atomic orbitals. H py pz py px pz px Valence Bond Theory get the angle wrong again! H • The bond angle is wrong. • Valence Bond Theory predicts 90o • VSEPR Theory predicts 104.5o • The orbital shape must be wrong. • Shared electrons are not spending enough time with the Hydrogen. sp3 Hybrid Orbitals (H2O) Hybrid Orbital Theory and Lone Pairs (e.g. H2O) H: H: 1s sp3 1s sp3 sp3 O: 1s sp3 Electrons are not promoted here, but every orbital in the n = 2 energy level becomes hybridized. Hybrid Orbital Theory and 5 Charge Clouds! (e.g. PCl5) Four sp3 hybrid orbitals are formed sp3 H Bond Angle = 104.5o H sp3d Hybrid Orbitals Atomic Orbitals P: .. :Cl .. .. :Cl: P :Cl: .. :Cl: .. .. Cl: .. 3s 3p Promotion P: 3s 3p Hybridization P: sp3d 3d 3d © 2009 High School Chem Solutions. All rights reserved. 3 www.hschemsolutions.com Hybrid Orbital Theory and 6 Charge Clouds! (e.g. SF6) sp3d2 Hybrid Orbitals Atomic Orbitals S: .. :F: .. :F .. .. F: .. S :F: .. :F: .. :F: .. 3s 3p Promotion S: 3s 3p Hybridization S: sp3d2 Summary Charge Clouds 2 3 4 5 6 3d 3d Double Bonds (e.g. C2H4) Hybridization sp sp2 sp3 sp3d sp3d2 H H C C H H Each carbon has 3 charge clouds. sp2 hybrid orbitals Double Bond (C2H4) Double Bonds (e.g. C2H4) Atomic Orbitals C: 2s Promotion C: 2s Hybridization C: sp2 p H 2p sp2 sp2 sp2 H σ Bond 2p 2p H sp2 sp2 sp2 p H π Bond © 2009 High School Chem Solutions. All rights reserved. 4 www.hschemsolutions.com Triple Bonds (e.g. C2H2) Triple Bonds (e.g. C2H2) Atomic Orbitals C: H C C H Each carbon has two charge clouds. sp hybrid orbitals 2s Promotion C: 2s Hybridization C: sp 2p 2p 2p Triple Bond (C2H2) p sp p σ Bond sp sp p p sp Two π Bonds © 2009 High School Chem Solutions. All rights reserved. 5
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