Bai Giang Chuong 11 - Type 2

8/6/2019 Bai Giang Chuong 11 - Type 2

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Introduction:

Orbital and bonding theories

- Valence Bond (VB) theory

- Molecular Orbital (MO) theory

→ All scientific models have limitations because theyare simplifications of reality. Each theory has a number

of advantages and use for different purposes.

→ Example: Magnetism property of O2 and N2 molecule



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11.1. Valence bond theory

11.1.1. Principle

In case of H2 molecule



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Principles of VB theory

• A covalent bond is formed when orbitals of two

atoms overlap.

Example:

- H2: bond s - s



3

• The overlap region is occupied by two electrons.

Opposing spins of the electron pair.

• Maximum overlap of bonding orbitals.

- HF: bond s - p



4

- F2: bond p - p

11.1.2. Some types of bonds

- Bond σ: overlap region follow line between 2 nuclear

atoms. (bond s-s; bond s-p; bond p-p)



5

- Bond ∏: overlap region follow line perpendicular to

line between 2 nuclear atoms ( bond p-p; bond p-d; bond

d-d).



6

C2H2



7

HCHO

⇒ Stable order: σ > ∏



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11.1.3. Hybridization

a. Problem: Normal overlap of orbitals cannot describe

all bonding in the molecules.

Example: BeCl2 17Cl: 1s2 2s2 2p6 3s2 3p5

4

Be: Stable state 1s

2

2s

2

Activated state 1s2 2s1 2p1

If normal overlap: Cl Be Cl

p (s, p) p

→ 1 bonding s-p and 1 bonding p-p



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In fact, bonds in the BeCl2 molecule are the same.

Reason ???

→ Solution: Hybridization - Linus Pauling



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b. Hybridization

- Definition: Hybridization is the combination of

different orbitals to new orbitals which have similar

shape and energy.

Initiation Mix Hybridization



11

- Characteristics:

+ The number of hybrid orbitals is equal with the

number of initial orbitals.

+ Hybrid orbitals belong one central atom of one

molecule or ion.



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c. sp hybridization: one orbital s and one orbital p.

Example BeCl2

- mix energy in the orbitals of Be

4

Be: Stable state 1s2

2s2


Stable state Activated state Hybrid state

2s

E

22

2s

sp1



13

- mix shape in the orbitals of Be

BeCl2



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d. sp2 hybridization: one orbital s and two orbital p

Example BCl3

- mix energy in the orbitals of B

5

B: Stable state 1s2

2s2

2p1



2p

2s2s

E

2p

sp2



15

- mix shape in the orbitals of B



16

BF3



17



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e. sp3 hybridization: one orbital s and three orbital p

Example CH4

- mix energy in the orbitals of C

6

C: Stable state 1s2

2s2

2p2



E

2s

2

2s

sp3

2p



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- mix shape in the orbitals of C

⇒ Stable order: sp3 > sp2 > sp1

Review

C

CH4



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Exercises

1. In the below molecules, which of the following

molecules have sp3 hybridization in the underline atom ?

a. BF3 b. H2O c. BeH2 d. NH3

2. Give examples about sp3 and sp2 hybridization of

carbon isomorphs in nature ?



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1. Results: H2O and NH3

2. sp3 and sp2 hybridization: diamond and graphite



22



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References:

sp3d hybridization in PCl5



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sp3d2 hybridization in SF6



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Chapter 11 – part 1 Review

1. Understand the differences between valence bond

theory and the other theory (ionic bonding model and

covalent bonding model)

2. Understand the orbital hybridization in the central

atom of one molecule or ion

Next part: Molecular Orbital theory and electron

delocalization

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Bai Giang Chuong 11 - Type 2