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Chemical Bonding: Valence Bond Theory “in a nutshell” Chapter 10 Section 4 through 6 of Jespersen 6 th Ed). Dr. C. Yau Spring 2014. 1. 1. 1. VSEPR Theory. You had previously learned how to predict the molecular geometry of a species from examining its Lewis structure.

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Dr. C. Yau Spring 2014


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    1. Chemical Bonding:Valence Bond Theory“in a nutshell”Chapter 10 Section 4 through 6of Jespersen 6th Ed) Dr. C. Yau Spring 2014 1 1 1

    2. VSEPR Theory • You had previously learned how to predict the molecular geometry of a species from examining its Lewis structure. • It utilizes the concept of repulsion amongst the charge clouds of the central atoms. • This was the Valence Shell Electron Pair Repulsion Theory (VSEPR Theory). • It does not explain how a bond is formed and how bonding relates to the s, p, d orbitals. 2 2 2

    3. 1s 1s Valence Bond Theory (VB Theory) The VB Theory explains bonding as an overlap of valence orbitals. H2 bonds are due to the overlap of their 1s valence orbitals. A B 2 separate H atoms H2 molecule with covalent bond due to overlap of the 1s orbitals. 3 3 3

    4. Hybridization in VB Theory • Atomic orbitals are mixed to allow formation of bonds that have realistic bond angles • The newly mixed orbitals that result are called “hybrid orbitals” with specified shapes: Review: # charge clouds Hybridization Bond Angles 2 sp 180o 3 sp2 120o 4 sp3 109.5o 5 sp3d 120o & 90o 6 sp3d2 90o & 180o 4 4

    5. How are sp3 hybrid orbitals formed? 4 orbitals 4 orbitals __ __ __ __ p s __ __ __ __ four sp3 hybrid orbitals 109.5o If we take s and all threep we form foursp3hybrid orbitals. These hybrid orbitals are “degenerate.” That is, they are of the same E, higher than s but lower than p. Note that # orbitals is conserved. 5

    6. How are sp2 hybrid orbitals formed? __ p __ ____ __ p s __ __ __ three sp2 hybrid orbitals p sp2 sp2 If we take s and just twop orbitals, we form threesp2hybrid orbitals leaving one pure p untouched. sp2 These three sp2 hybrid orbitals are planar with angles of 120o. The un-hybridized p (pure p) are used in double and triple bonds. 6

    7. How are sp hybrid orbitals formed? __ __ two p ______ __ p s __ __ two sp hybrid orbitals p If we take s and just onep orbital, we form twosp hybrid orbitals leaving two pure p untouched. p sp sp These two sp hybrid orbitals are linear with angles of 180o. 7

    8. Hybrid Orbitals Involving d-Orbitals Hybrids from one s and three p can only make a maximum of 4 hybrid orbitals. When we run out of p orbitals, we start using d orbitals. Thus we have sp3d hybrid orbitals and sp3d2 hybrid orbitals (but only for elements larger than Ne, beyond Period 2). Remember that elements smaller than Ne do not have d-orbitals and therefore cannot have sp3d and sp3d2 hybrids. 8 8

    9. Two types of bonds result from orbital overlap: sigma () bonds from head-on overlap lie along the bond axis account for the first bond pi ()bonds pi bonds are perpendicular to bond axis account for the second and third bonds in a multiple bond Bonding Types 9 9

    10. Sigma and Pi Bonding Given the structural formula of a compound, be able to specify the hybridization of each atom, state the bond angles and determine the # of sigma and pi bonds. 10 10

    11. KNOW THIS WELL! X X single bond =  bond X X double bond =  bond +  bond X X triple bond =  bond + two  bonds 11 11

    12. 4 Always start with hybridization. Give the hybridization of each C and each O. Give the bond angles. How many  bonds are there? How many  bonds are there? Ans. C1 = sp C2 = sp C3 =sp2 C4 = sp3 O = sp3 C1-C2-C3 = 180o C2-C3-O = 120o C2-C3=O = 120o O-C4-H = 109.5o 1 2 3 9  bonds 3  bonds 12 12

    13. Each C is sp3. Chemical Bonding in C2H6 The C−C bond is a  bond. It is a “head-to-head” overlap.

    14. Each C is sp2, with one pure p. Chemical Bonding in C2H4 The C=C bond is made of  bond and a  bond. The  bond consists of overlap of parallel p orbitals.

    15. Each C is sp, with two pure p. Chemical Bonding in C2H2 The CC bond is made of one  bond and two  bond. The  bond consists of overlap of parallel p orbitals. Practice Exercise 10.20 & 10.21 on p. 444 & ques on p.467 #10.108 (ans at back of book)