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Chemical Bonding:Valence Bond Theory
“in a nutshell”
Chapter 10 Section 4 through 6of Jespersen 6th Ed)
Dr. C. Yau
Spring 2014
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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.
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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.
1s 1s
A B
2 separate H atoms
H2 molecule with covalent bond due to overlap of the 1s orbitals.
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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
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How are sp3 hybrid orbitals formed?
__ __ __ __ p s
If we take s and all three pwe form four sp3 hybrid orbitals.
__ __ __ __four sp3 hybrid 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.
109.5o
4 orbitals 4 orbitals
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How are sp2 hybrid orbitals formed?
__ __ __ __ p s
If we take s and just two p orbitals, we form three sp2 hybrid orbitals leaving one pure p untouched.
__ __ __three sp2 hybrid orbitals
These three sp2 hybrid orbitals are planar with angles of 120o.The un-hybridized p (pure p) are used in double and triple bonds.
__p
sp2
sp2
sp2
p
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How are sp hybrid orbitals formed?
__ __ __ __ p s
If we take s and just one p orbital, we form two sp hybrid orbitalsleaving two pure p untouched.
__ __ two sphybrid orbitals
These two sp hybrid orbitals are linear with angles of 180o.
__ __two p
sp sp
p
p
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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.
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Bonding Types• 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
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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.
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KNOW THIS WELL!
X X single bond = bond
X X double bond = bond + bond
X X triple bond = bond + two bonds
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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
C C C O
O
H C H
H
H....: :
1 2 3
9 bonds3 bonds
4
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Chemical Bonding in C2H6
H C C
H
H
H
H
H Each C is sp3.
H
C
H
H
CH
H
H
The C−C bond is a bond.
It is a “head-to-head” overlap.
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Chemical Bonding in C2H4
H C C
H
H
H Each C is sp2, with one pure p.
The C=C bond is made of bond and a bond.The bond consists of overlap of parallel p orbitals.
H
C
H H
C
H
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Chemical Bonding in C2H2
H C C H
Each C is sp, with two pure p.
The CC 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)
CH C H
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