Chapter 10: Covalent Bond Theories

Valence Bond Theory

Molecular Orbital TheoryFirst a couple of jokes… (You’ll need a laugh this chapter!)

If a bear from Yosemite and a bear from Alaska fall into the water, which one dissolves faster?

Which is lower energy, hamburger or steak?

My name is Bond - Ionic Bond; taken not shared!

Valence Bond Theory – Covalent Bonds form from the __________ of atomic orbitals.

• Consider a BeF2 molecule:

F FBe

Valence Bond TheoryF

1s22s22p5

F

1s22s22p5

Be

1s22s2

One unpaired e- One unpaired e-

No unpaired electrons

How can Be form covalent bonds with F if it doesn’t have unpaired electrons???

Valence Bond Theory

Be

1s22s12p1

2 unpaired electrons

Promote one of the 2s e- to a 2p orbital

Be

1s22s2

no unpaired electrons

Valence Bond Theory

F

1s22s22p5

F

1s22s22p5

Be

1s22s12p1

One unpaired e- One unpaired e-

2 unpaired electrons

Predicted overlaps:2s (Be) – 2p (F)2p (Be) – 2 p (F)

Implies two different kinds of Be – F bonds!

Valence Bond Theory

• Both of the Be – F bonds in BeF2 are identical!

• The solution:– Mix the Be 2s orbital with one of the Be 2p

orbitals to form two hybrid orbitals• atomic orbitals formed by mixing 2 or more atomic

orbitals on an atom

Valence Bond Theory

• When Be forms covalent bonds with two F, each sp hybrid orbital on the Be atom overlaps with a p orbital located on a F atom.

Be

1s 2sp 2p

F

1s22s22p5

F

1s22s22p5

Valence Bond Theory

• The BeF2 molecule is linear.

• sp hybridization implies that the electron geometry around the central atom is linear.

The sp2 hybrid orbitals in BF3.

The sp3 hybrid orbitals in CH4.

The sp3 hybrid orbitals in NH3.

The sp3 hybrid orbitals in H2O.

The sp3d hybrid orbitals in PCl5.

The sp3d2 hybrid orbitals in SF6.

Show the orbital hybridization of SF4

Multiple BondsMultiple Bonds All of the bonds shown thus far have been single bonds - which result in sigma bonds Sigma bondSigma bond () A bond where the line of electron A bond where the line of electron density is concentrated symmetrically along the line density is concentrated symmetrically along the line connecting the two atoms. Results from connecting the two atoms. Results from head to head head to head overlapoverlap of orbitals of orbitals

Pi bondPi bond () A bond where the overlapping regions A bond where the overlapping regions exist above and below the internuclear axis.exist above and below the internuclear axis.

Pi bonds result from side to side overlap of atomic orbitals. Pi bonds cannot form from hybrid orbitals

A double bond consists of one sigma (hybrid) and one pure pi bond.

A triple bond consists of one hybrid sigma and 2 pure pi bonds

Example: HExample: H22C=CHC=CH22

Example: HExample: H22C=CHC=CH22

Only sigma bond pairs influence molecular geometry. What is the geometry around each central atom?

Multiple Bonding:

• Formaldehyde: H2CO

• Acetylene: C2H2

Identify sigma and pi bonds in the following molecules and name the orbital hybridization involved.

• Nitrogen gas

• Acetonitrile, CH3CN

The paramagnetic properties of O2

Using the valence bond theory, explain the paramagnetism of oxygen gas.

Molecular Orbital Theory –

• When 2 atomic orbitals overlap, two new molecular orbitals are formed:– Bonding molecular orbital

– Antibonding molecular orbital

Electrons are spread or delocalized over the whole molecule instead of remaining in atomic orbitals

Another theory for the formation of covalent bonds which explains phenomena for which the Valence Bond Theory can’t account .

Molecular Orbital Theory• Bonding molecular orbital

– Addition of overlapping atomic orbitals results in an area of increased e- density; increased probability e- will be found here.

– lower energy than atomic orbital

Bonding MO

Molecular Orbital Theory• Antibonding molecular orbital

– An area where the probability of finding an electron is reduced

• Results from subtracting one atomic orbital from the other

– higher energy than atomic orbital

Bonding MO

Antibonding MO

The one that is lower in energy is called the bonding orbital,

The one higher in energy is called an antibonding orbital.

These two new orbitals have different energies. 

BONDING

ANTBONDING

Molecular Orbital (MO) TheoryMolecular Orbital (MO) Theory

Molecular Orbital Theory

• The MO diagram for H2 molecule:

1s 1s

*1s

1sH atom H atom

H2 molecule

bonding electrons

Placing e-s in MO’s follows the Aufbau principle, Pauli’s and Hund’s Rule.

Molecular Orbital Theory

• The MO diagram for He2 molecule:

1s 1s

*1s

1sHe atom He atom

He2 molecule

bonding electrons

antibonding electrons

Adding electrons to antibonding molecular orbitals destabilizes the molecule.

Molecular Orbital Theory

• In MO Theory, the stability of a covalent bond can be related to its bond order:

• Bond order = (# bonding e- - # antibonding e-) 2– Old def.: # e- pairs / # links formed– Single bond: bond order = 1– Double bond: bond order = 2– Triple bond: bond order = 3

– Fractional bond orders also exist! (e.g. He2+1)

Energy level diagrams / molecular Energy level diagrams / molecular orbital diagramsorbital diagrams

Molecular Orbital Theory

• The bond order for H2 molecule:

Bond Order = (2 - 0) = 1

2

• The hydrogen atoms in an H2 molecule are held together by a single bond.

Molecular Orbital Theory

• The bond order for an He2 molecule:

Bond order = (2 - 2) = 0

2

• No bond exists between two He atoms.

Example: Example: LiLi22

Atomic orbitals combine to form molecular orbitals best when the atomic orbitals are of equal energy.

Electron Configurations for BElectron Configurations for B22 through Ne through Ne22

Second-Row Diatomic MoleculesSecond-Row Diatomic Molecules

Relative MO energy levels for Period 2 homonuclear diatomic molecules.

MO energy levels for O2, F2, and Ne2

MO energy levels for B2, C2, and N2

without 2s-2p mixing

with 2s-2p mixing

For B2, C2, and N2 the interaction is so strong that the 2p is pushed higher in energy than 2p orbitals

Molecular Orbital Theory

Example: Complete the MO diagram for N2. Calculate the bond order.

# valence electrons for each N atom =

Total # of valence electrons =

Molecular Orbital Theory

2s

2s

2p

2p

2p

2p

# bonding e- =

# antibonding e- =

Bond order =

Molecular Orbital Theory

Example: Complete the MO diagram for the O2 molecule. Determine the bond order.

# valence electrons for each O atom =

Total # of valence electrons =

Molecular Orbital Theory

2s

2s

2p

2p

2p

2p

# bonding e- =

# antibonding e- =

Bond order =

Isomers

1,1-dichloroethylene 1,2-dichloroethylene

cis-1,2-dichloroethylene trans-1,2-dichloroethylene