valence bonding theory

7/31/2019 Valence Bonding Theory

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Valence Bond Theory


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How do bonds form?

The valence bond model or atomic orbital modelwas developed by Linus Pauling in order toexplain how atoms come together and formmolecules.

The model theorizes that a covalent bond formswhen two orbitals overlap to produce a newcombined orbital containing two electrons ofopposite spin.

This overlapping results in a decrease in theenergy of the atoms forming the bond.

The shared electron pair is most likely to be foundin the space between the two nuclei of the atoms

forming the bonds.


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Example H2

H H

1s1s

Overlapping of the1s orbitals

Covalent BondH-H

1s

The newly combined orbital will contain an electron

pair with opposite spin just like a filled atomic orbital.


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Example HF

In hydrogen fluoride the 1s orbital of the H will overlap

with the half-filled 2p orbital of the F forming a covalentbond.

H F

2p1s

+

Overlapping of the1s and 2p orbitals

+

Covalent BondH-F

+


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Other Points on the Valence Bond Theory

This theory can also be applied to moleculeswith more than two atoms such as water.

Each covalent bond results in a new

combined orbital with two oppositelyspinning electrons.

In order for atoms to bond according to thevalence bond model, the orbitals must havean unpaired electron.


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Covalent Bonding: Orbitals

Hybridization

The mixing of atomic orbitals to formspecial orbitals for bonding.

The atoms are responding as needed togive the minimum energy for themolecule.


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Figure 9.5. An Energy-Level Diagram Showing the

Formation of Four sp3 Orbitals


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Figure 9.2. The Valence Orbitals on a Free Carbon

Atom: 2s, 2px, 2py, and 2pz


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Figure 9.3. The Formation ofsp3 Hybrid Orbitals


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Figure 9.6. Tetrahedral Set of Four sp3 Orbitals


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Figure 9.7. The Nitrogen Atom in Ammonia is sp3 Hybridized


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Figure 9.9. An Orbital Energy-Level Diagram for sp2 Hybridization


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Figure 9.8. The Hybridization of the s,px, andpy Atomic Orbitals


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A sigma () bond centers along the

internuclear axis. end-to-end overlap

of orbitals

A pi () bond occupies the space above

and below the internuclear axis. side-to-side overlap of orbitals

CC

H H

HH


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Figure 9.12. Sigma and Pi Bonding


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Figure 9.10. An sp2 Hybridized C Atom


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Figure 9.11. The Bonds in Ethylene


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Figure 9.13. The Orbitals for C2H4


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Figure 9.16. The Orbital Energy-Level Diagram for the

Formation ofsp Hybrid Orbitals on Carbon


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Figure 9.14. When One s Orbital and Onep Orbital are

Hybridized, a Set of Two sp Orbitals Oriented at 180Degrees Results


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Figure 9.17. The Orbitals of an sp Hybridized Carbon Atom


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Figure 9.18. The Orbital Arrangement

for an sp2

Hybridized Oxygen Atom


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Figure 9.15. The Hybrid Orbitals in the CO2 Molecule


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Figure 9.19. The Orbitals for CO2


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Figure 9.20. The Orbitals for N2


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Figure 9.21. A Set ofdsp3

Hybrid Orbitals on a Phosphorus Atom


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Figure 9.23. An Octahedral Set ofd2sp3 Orbitals on a Sulfur Atom


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Figure 9.24. The Relationship of the Number of Effective Pairs,

Their Spatial Arrangement, and the Hybrid Orbital Set Required


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Figure 9.46. A Benzene Ring


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Figure 9.47. The Sigma System for Benzene


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Figure 9.48. The Pi System for Benzene


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The Localized Electron Model

Three Steps:

Draw the Lewis structure(s)

Determine the arrangement of electron

pairs (VSEPR model).

Specify the necessary hybrid orbitals.


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Figure 9.45. The Resonance Structures for O3 and NO3-


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Paramagnetism

unpaired electrons

attracted to induced magnetic field

much stronger than diamagnetism

valence bonding theory

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