1. Chapter 10: Covalent Bond Theories
Valence Bond Theory
Molecular Orbital Theory
First 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!

2. Valence Bond Theory – Covalent Bonds form from the __________ of atomic orbitals.
Consider a BeF2 molecule: Be F F

3. Valence Bond Theory F 1s22s22p5 F 1s22s22p5 One unpaired e-
How can Be form covalent bonds with F if it doesn’t have unpaired electrons??? Be
1s22s2
No unpaired electrons

4. Valence Bond Theory Promote one of the 2s e- to a 2p orbital Be
1s22s2
no unpaired electrons Be
1s22s12p1
2 unpaired electrons

5. Valence Bond Theory F 1s22s22p5 F 1s22s22p5 One unpaired e-Be
1s22s12p1
Predicted overlaps:
2s (Be) – 2p (F)
2p (Be) – 2 p (F)
Implies two different kinds of Be – F bonds!
2 unpaired electrons

6. 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

7. Valence Bond Theory F
1s22s22p5 F
1s22s22p5 Be 1s
2sp 2p
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.

8. Valence Bond Theory The BeF2 molecule is linear.
sp hybridization implies that the electron geometry around the central atom is linear.

9. The sp2 hybrid orbitals in BF3.

10. The sp3 hybrid orbitals in CH4.

11. The sp3 hybrid orbitals in NH3.

12. The sp3 hybrid orbitals in H2O.

13. The sp3d hybrid orbitals in PCl5.

14. The sp3d2 hybrid orbitals in SF6.

15. Show the orbital hybridization of SF4

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

18. Pi bond (p)  A bond where the overlapping regions 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

19. Example: H2C=CH2

20. Example: H2C=CH2
Only sigma bond pairs influence molecular geometry. What is the geometry around each central atom?

21. Multiple Bonding: Formaldehyde: H2CO Acetylene: C2H2
Identify sigma and pi bonds in the following molecules and name the orbital hybridization involved.
Formaldehyde: H2CO
Acetylene: C2H2

22. Nitrogen gas
Acetonitrile, CH3CN

23. The paramagnetic properties of O2
Using the valence bond theory, explain the paramagnetism of oxygen gas.

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

25. 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

26. 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
Antibonding MO
Bonding MO

28. Molecular Orbital (MO) Theory
ANTBONDING
These two new orbitals have different energies. 
BONDING
The one that is lower in energy is called the bonding orbital,
The one higher in energy is called an antibonding orbital.

29. Molecular Orbital Theory
The MO diagram for H2 molecule:
Placing e-s in MO’s follows the Aufbau principle, Pauli’s and Hund’s Rule.
s*1s 1s 1s
H atom
s1s
H atom
bonding electrons
H2 molecule

30. Molecular Orbital Theory
The MO diagram for He2 molecule:
antibonding electrons
s*1s 1s 1s
He atom
s1s
He atom
bonding electrons
He2 molecule
Adding electrons to antibonding molecular orbitals destabilizes the molecule.

31. 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)

32. Energy level diagrams / molecular orbital diagrams

33. 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.

34. Molecular Orbital Theory
The bond order for an He2 molecule:
Bond order = (2 - 2) = 0 2
No bond exists between two He atoms.

35. Example: Li2
Atomic orbitals combine to form molecular orbitals best when the atomic orbitals are of equal energy.

37. Second-Row Diatomic Molecules
Electron Configurations for B2 through Ne2

38. Relative MO energy levels for Period 2 homonuclear diatomic molecules.
without 2s-2p mixing
with 2s-2p mixing
MO energy levels for O2, F2, and Ne2
MO energy levels for B2, C2, and N2

39. For B2, C2, and N2 the interaction is so strong that the s2p is pushed higher in energy than p2p orbitals

40. Molecular Orbital Theory
Example: Complete the MO diagram for N2. Calculate the bond order.
# valence electrons for each N atom =
Total # of valence electrons =

41. Molecular Orbital Theory
s*2p
# bonding e- =
# antibonding e- =
Bond order =
p*2p
s2p
p2p
s*2s
s2s

42. 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 =

43. Molecular Orbital Theory
s*2p
# bonding e- =
# antibonding e- =
Bond order =
p*2p
p2p
s2p
s*2s
s2s

44. Isomers 1,1-dichloroethylene 1,2-dichloroethylene
cis-1,2-dichloroethylene trans-1,2-dichloroethylene