Covalent Covalent Bonding: Bonding: orbitalsorbitals

Hybridization - Hybridization - The Blending of The Blending of OrbitalsOrbitals

=

=

+

+s orbital p orbital sp orbital

We have studied electron configuration notation and the sharing of electrons in the formation of covalent bonds.

Methane is a simple natural gas. Its molecule has a carbon atom at the center with four hydrogen atoms covalently bonded around it.

What Proof Exists for What Proof Exists for Hybridization?Hybridization?

Lets look at a molecule of methane, CH4.

What is the expected orbital notation of carbon in its ground state?

(Hint: How many unpaired electrons does this carbon atom have available for bonding?)

Can you see a problem with this?

Carbon ground state configuration

The first thought The first thought that chemists had that chemists had was that carbon was that carbon promotes one of its promotes one of its 2s2s electrons… electrons…

…to the empty 2p orbital.

Carbon’s Empty Carbon’s Empty OrbitalOrbital

The fourth bond is between a 2s electron from the carbon and the lone 1s hydrogen electron.

Such a bond would have slightly less energy than the other bonds in a methane molecule.

Unequal bond energy

In the case of methane, they call the hybridization sp3, meaning that an s orbital is combined with three p orbitals to create four equal hybrid orbitals.

These new orbitals have slightly MORE energy than the 2s orbital…

… and slightly LESS energy than the 2p orbitals.

sp3 Hybrid Orbitals

Here is another way to look at the sp3 hybridization and energy profile…

sp3 Hybrid Orbitals

While sp3 is the hybridization observed in methane, there are other types of hybridization that atoms undergo.

These include sp hybridization, in which one s orbital combines with a single p orbital.

This produces two hybrid orbitals, while leaving two normal p orbitals

sp Hybrid Orbitals

Another hybrid is the sp2, which combines two orbitals from a p sublevel with one orbital from an s sublevel.

One p orbital remains unchanged.

sp2 Hybrid Orbitals

1s

2s

3s

2p

3p

3d

SF6F F F

F F

F

Hybridization Involving “Hybridization Involving “dd” ” OrbitalsOrbitals

Beginning with elements in the third row, “d” orbitals may also hybridize

dsp3 = five hybrid orbitals of equal energy

d 2sp3 = six hybrid orbitals of equal energy

Hybridization and Molecular Hybridization and Molecular GeometryGeometry

FormsForms Overall StructureOverall Structure HybridizatioHybridization of “A”n of “A”

AX2 Linear sp

AX3, AX2E Trigonal Planar sp2

AX4, AX3E, AX2E2 Tetrahedral sp3

AX5, AX4E, AX3E2, AX2E3 Trigonal

bipyramidaldsp3

AX6, AX5E, AX4E2 Octahedral d2sp3

A = central atomA = central atom

X =X = atoms bonded to Aatoms bonded to A E = nonbonding electron pairs on AE = nonbonding electron pairs on A

σσBONDBONDSS

Sigma and Pi BondsSigma and Pi BondsSigma () bonds exist in the region directly between two bonded atoms.

Pi () bonds exist in the region above and below a line drawn between two bonded atoms.

Single bond 1 sigma bond

Double Bond 1 sigma, 1 pi bond

Triple Bond 1 sigma, 2 pi bonds

Sigma and Pi BondsSigma and Pi BondsSingle BondsSingle Bonds

C C

H

H

H

H

H

H

Ethane

1 bond

Sigma and Pi Bonds:Sigma and Pi Bonds:Double bondsDouble bonds

C C

H

H

H

H

Ethene

CC

H

H

H

H

1 bond

1 bond

Sigma and Pi BondsSigma and Pi BondsTriple BondsTriple Bonds

C C HH

CC HH

Ethyne

1 bond

1 bond

1 bond

CH4 - Methane

4 single bonds

Double bonds

Use unhybridized p orbitals to share electrons

Sigma Bond -  (σ) is a bond resulting from the head-on overlap of atomic orbitals.  The region of electron density is along and cylindrically around an imaginary line connecting the bonded atoms.

Pi Bond - (π) a bond resulting from side on overlap of atomic orbitals.  The regions of electron sharing are on opposite sides of an imaginary line connecting the bonded atoms and parallel to this line.

You can't have a Pi bond without first having a sigma bond.

Triple Bond = 1 sigma and 2 Pi bonds

The De-Localized Electron The De-Localized Electron ModelModel

Pi bonds () contribute to the delocalized model of electrons in bonding, and help explain resonance

H

H

H

H

H

H

H

H

H

H

H

H

Electron density from bonds can be distributed symmetrically all around the ring, above and below the plane.