9 Hybridization

7/28/2019 9 Hybridization

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Hybridization - The Blending of Orbitals

Poodle

+

+ Cocker Spaniel

=

=

=

=

+

+s orbital p orbital

Cockapoo

sp orbital


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We have studied electronconfiguration notation andthe sharing of electrons inthe formation of covalentbonds.

Methane is a simple natural

gas. Its molecule has acarbon atom at the centerwith four hydrogen atomscovalently bonded around it.

What Proof Exists for Hybridization?

Lets look at a molecule ofmethane, CH 4.


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What is the expected orbital notation of

carbon in its ground state?

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

Can you see a problem with this?

Carbon ground state configuration


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You should conclude that carbon only hasTWO electrons available for bonding. Thatis not not enough.

What is taking place in order that carbonmay form four bonds?

Carbons Bonding Problem


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The first thought thatchemists had was thatcarbon promotes one ofits 2s electrons

to the empty 2p orbital.

Carbons Empty Orbital


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However, they quickly recognized a problem with suchan arrangement

Three of the carbon-hydrogen bonds would involvean electron pair in which the carbon electron was a2p , matched with the lone 1s electron from ahydrogen atom.


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This would mean that threeof the bonds in a methanemolecule would be identical,because they would involveelectron pairs of equalenergy.

But what about the

fourth bond?


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The fourth bond is between a 2s electronfrom the carbon and the lone 1s hydrogen

electron.

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


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This bond would be slightly

different in character thanthe other three bonds inmethane.

This difference would bemeasurable to a chemistby determining the bondlength and bond energy.

But is this what they observe?


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In the case of methane, they call the hybridizationsp 3 , 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.

1s2sp 3 2sp 3 2sp 3 2sp 3


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While sp 3 is the hybridization observed in methane,there are other types of hybridization that atoms

undergo.These include sp hybridization, inwhich one s orbital combines witha single p orbital.

This produces two hybrid orbitals, while leaving twonormal p orbitals

sp Hybrid Orbitals


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Another hybrid is the sp 2 , which combines two orbitals

from a p sublevel with one orbital from an s sublevel.

One p orbitalremains unchanged.

sp2 Hybrid Orbitals


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An understanding of the derivation and depiction of these orbitals is beyond the scope of this course and the AP Exam. Current evidence suggests that hybridization involving d orbitals does not exist , and there is controversy about the need to teach any hybridization. Until there is agreement in the chemistry community, we will continue to include sp, sp 2 , and sp 3 hybridization in the current course.

Exclusion Warning


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Hybridization Involving d Orbitals

Beginning with elements in the third row, d orbitals may also hybridizedsp 3 = five hybrid orbitals of equal energy

d 2 sp 3 = six hybrid orbitals of equal energy


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Hybridization and Molecular Geometry

Forms Overall StructureHybridization

of A AX2 Linear sp

AX3, AX2E Trigonal Planar sp 2

AX4, AX3E, AX2E2 Tetrahedral sp 3

AX5, AX4E, AX3E2, AX2E3 Trigonal bipyramidal dsp 3 AX6, AX5E, AX4E2 Octahedral d 2 sp 3

A = central atomX = atoms bonded to AE = nonbonding electron pairs on A


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Sigma and Pi Bonds

Sigma ( ) bonds exist in the region directly betweentwo bonded atoms.

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

Single bond 1 sigma bond

Double Bond 1 sigma, 1 pi bond

Triple Bond 1 sigma, 2 pi bonds


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Sigma and Pi BondsSingle Bonds

C C

H

H

H

H

H

H

Ethane

1 bond


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Sigma and Pi Bonds: Double bonds

C C

H

H

H

H

Ethene

CC

H

H

H

H

1 bond

1 bond


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Sigma and Pi BondsTriple Bonds

C C HH

CC HH

Ethyne1 bond

1 bond

1 bond


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The De-Localized Electron ModelPi bonds ( ) contribute to the delocalized model ofelectrons in bonding, and help explain resonance

H

H

H

H

H

H

H

H

H

H

H

H

Electron density from bonds can be distributedsymmetrically all around the ring, above and belowthe plane.

9 Hybridization

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