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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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121212

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