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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
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• The overlap region is occupied by two electrons.
Opposing spins of the electron pair.
• Maximum overlap of bonding orbitals.
- HF: bond s - p
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- 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)
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- Bond ∏: overlap region follow line perpendicular to
line between 2 nuclear atoms ( bond p-p; bond p-d; bond
d-d).
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C2H2
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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
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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
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- 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
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Be: Stable state 1s2
2s2
Activated state 1s2 2s1 2p1
Stable state Activated state Hybrid state
2s
E
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2s
sp1
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- 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
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B: Stable state 1s2
2s2
2p1
Activated state 1s2 2s1 2p2
Stable state Activated state Hybrid state
2p
2s2s
E
2p
sp2
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- mix shape in the orbitals of B
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BF3
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e. sp3 hybridization: one orbital s and three orbital p
Example CH4
- mix energy in the orbitals of C
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C: Stable state 1s2
2s2
2p2
Activated state 1s2 2s1 2p3
Stable state Activated state Hybrid state
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
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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