7.10 Valence Bond Theory

Chm 123Chapter 7

7.10 – Valence Bond Theory

When a covalent bond is formed, there is shared electron density between the nuclei of the bonded atom

The simultaneous attraction of the shared electron density for both nuclei holds the atoms together, forming a covalent bond

Valence Bond Theory: A quantum mechanical model which shows how electron pairs are shared in a covalent bond.

Bond forms between two atoms when the following conditions are met:

•Covalent bonds are formed by overlap of atomic orbitals, each of which contains one electron of opposite spin.

•Each of the bonded atoms maintains its own atomic orbitals, but the electron pair in the overlapping orbitals is shared by both atoms.

•The greater the amount of overlap, the stronger the bond.

In some cases, atoms use “simple” atomic orbital (e.g., 1s, 2s, 2p, etc.) to form bonds. In other case, they use a “mixture” of simple atomic orbitals known as “hybrid” atomic orbitals.

Two special names for covalent bonds of Organic molecules

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Sigma (σ) bonds

Created when “head on” overlap occurs of orbital

Pi (π) bonds

Created when “side on” overlaps occurs of orbital (p orbitals)

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Hybrid orbital – orbitals are used to describe bonding that is obtained by taking combinations of atomic orbitals of the isolated atoms.

1. When forming hybrid orbitals, the number of hybrid orbitals formed equals the number of orbitals mathematically combined or “mixed”. For example, if an s orbitalis combined with a p orbital the result is two “sp” hybrid orbitals.

1. Hybrid orbitals have orientations around the central atom that correspond to the electron-domain geometry predicted by the VSEPR Theory.

How can the bonding in CH4 be explained?

sp3 Hybridization

-Mixing 1s and all 3p atomic orbital

-Four sp3 hybridized orbitals equal in size, energy and shape

-Responsible for sigma bond ( single bond)

-Tetrahedral shape

Consider methane (CH4)

3-D representation of methane (CH4)

The sp2 Hybridization

-Mixing 2s and two 2p atomic orbitals

-Three sp2 hybridized orbitals equal in size, energy and shape

-Responsible for σ bond ( single bond)

-One π bond (double bond)

-Trigonal planar shape

Consider ethane (C2H4)

3-D representation of ethane (C2H4)

The sp Hybridization

-Mixing 2s and one 2p atomic orbitals

-Two sp hybridized orbitals equal in size, energy and shape

-one σ bond ( single bond)

-Two π bond (triple bond)

-Linear

Consider ethyne (C2H2)

3-D representation of ethyne (C2H2)

Bonding to O and N

Like Carbon, O and N can participate in single bond and multiple bonds compose of σ and π

*Note: the lone pair or non bonding e- pair occupies space just as bonded atom

Five types of hybrid are shown below

Examples:

What hybrid orbitals orbitals would be expected for the central atom in each of the following?

a)SF2b)ClO3-

c)BrF3d)ClF4+

d)Predict the hybridization, geometry, and bond angle for the carbon and oxygen atoms in acetaldehyde (CH3CHO)

e)Assign hybridization for all carbon atoms and identify the angle at C-C-C. Then draw bonding pictures for the σ and π framework.

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