1. Let’s Focus on Valence Bond Theory
As we saw earlier, a single bond formed between two atoms is formed with orbital overlap.

2. Let’s Focus on Valence Bond Theory
Polyatomic molecules form via hybrid orbitals.
Carbon, for example, yields four sp3 hybrid orbitals, allowing a molecule like CH4 to have a tetrahedral shape.

3. Hybrid Orbitals
As we have discussed before, once you know the electron-domain geometry, you know the hybridization state of the atom.

4. Sigma Bonds (σ) And Pi Bonds (π)
Valence Bond Theory designates two bond types arising from orbital overlap and hybrid orbitals.
Sigma Bonds (σ)
And
Pi Bonds (π)

5. Sigma () Bonds Sigma bonds are characterized by Head-to-head overlap.
Cylindrical symmetry of electron density about the internuclear axis. In other words, electron density concentrated between the two bonded nuclei.

6. Pi () Bonds Pi bonds are characterized by Side-to-side overlap.
Electron density above and below the internuclear axis.

7. Single Bonds
Single bonds are always  bonds, because  overlap is greater, resulting in a stronger bond and more energy lowering.

8. Multiple Bonds
In a multiple bond one of the bonds is a  bond and the rest are  bonds.

9. Multiple Bonds
In a molecule like formaldehyde (shown at left) an sp2 orbital on carbon overlaps in  fashion with the corresponding orbital on the oxygen.
The unhybridized p orbitals overlap in  fashion.

10. Multiple Bonds
In triple bonds, as in acetylene, two sp orbitals form a  bond between the carbons, and two pairs of p orbitals overlap in  fashion to form the two  bonds.

11. Delocalized Electrons: Resonance
When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion.

12. Delocalized Electrons: Resonance
In reality, each of the four atoms in the nitrate ion has a p orbital.
The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen.

13. Delocalized Electrons: Resonance
This means the  electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion and thus accounting for the equal bond lengths observed throughout the structure.

14. Resonance
The organic molecule benzene has six  bonds and a p orbital on each carbon atom.

15. Resonance
In reality the  electrons in benzene are not localized, but delocalized.
The even distribution of the  electrons in benzene makes the molecule unusually stable.

16. Example Consider acetonitrile:
Predict the bond angles around each carbon atom
Give the hybridization at each of the carbon atoms
Determine the number of σ and π bonds in the molecule

17. Example
Which of the following molecules or ions will exhibit delocalized bonding? SO3, SO32-, H2CO, O3, NH4+

18. Valence Bond Theory in a Nut Shell…
All covalent bonds are formed by the sharing of two or more electrons. All bonds will have at least one of these bond pair concentrated in the space between the two interacting nuclei forming a sigma bond.
The appropriate set of hybrid orbitals used to form these sigma bonds is determined by the geometry of the molecule.
Atoms that share more than one pair of electrons will do so in pi bonds, which lie above and below the nuclei.
Molecules with two or more resonance structures can have pi bonds that extend beyond the two bonded atoms, allowing electrons to become delocalized.

19. Molecular Orbital (MO) Theory
Though valence bond theory effectively conveys most observed properties of ions and molecules, there are some concepts better represented by molecular orbitals.

20. Molecular Orbital (MO) Theory
In MO theory, we invoke the wave nature of electrons.
If waves interact constructively, the resulting orbital is lower in energy: a bonding molecular orbital.

21. Molecular Orbital (MO) Theory
If waves interact destructively, the resulting orbital is higher in energy: an antibonding molecular orbital.

22. MO Theory
MO Theory begins with the construction of molecular orbitals.
Whenever two atomic orbitals overlap, two molecular orbitals form.
Let’s consider H2. Notice, two orbitals will overlap, and thus, two molecular orbitals formed.

23. Let’s turn this into a diagram…

24. MO Theory
In H2 the two electrons go into the bonding molecular orbital.
The bond order is one half the difference between the number of bonding and antibonding electrons.
Recall, bond order indicates the stability of the bond formed.

25. MO Theory
For hydrogen, with two electrons in the bonding MO and none in the antibonding MO, the bond order is 1 2
(2 - 0) = 1

26. MO Theory In the case of He2, the bond order would be 1 2 (2 - 2) = 0
Therefore, He2 does not exist.

27. Example
Write the electron configuration in terms of MOs and determine the bond order of H2- and H2+.

28. Example
According to molecular orbital theory, would either Be2 or Be2+ be expected to exist? Explain.

29. MO Theory
For atoms with both s and p orbitals, there are two types of interactions:
The s and the p orbitals that face each other overlap in  fashion.
The other two sets of p orbitals overlap in  fashion.

30. MO Theory The resulting MO diagram looks like this.
There are both s and p bonding molecular orbitals and s* and * antibonding molecular orbitals.

31. MO Theory
The smaller p-block elements in the second period have a sizeable interaction between the s and p orbitals.
This flips the order of the s and p molecular orbitals in these elements.

32. Second-Row MO Diagrams

33. Paramagnetism vs. Diamagnetism
Molecular orbital theory allows one to predict the behavior of a substance in the presence of a magnetic field.
Molecules with one or more unpaired electrons are attracted into a magnetic field. This behavior is called paramagnetism.
Substances with no unpaired electrons are weakly repelled from a magnetic field…they exhibit diamagnetism.

34. Example
Predict the magnetic properties and bond order of the peroxide ion, O22- and the acetylide ion, C22-.

35. Molecular Orbital Theory
This model represents electrons in allowed energy states called molecular orbitals which can spread out across all the atoms of a molecule.
MO diagrams represent the combination and relative energies of molecular orbitals.
The diagrams allow for easy calculation of bond order, indicating the stability of the bond formed.
Bonding and antibonding MOs formed by the combination of s orbitals are called sigma molecular orbitals while the combination of p orbitals are called pi molecular orbitals.
The MO model correctly predicts properties like paramagnetism and diamagnetism.