1. Valence Bond Theory

2. Valence bond (VB) theory and molecular orbital (MO) theory are two important theories of covalent bonding
According to VB theory, a bond between two atoms is formed when a pair of electrons with their spins paired is shared by two overlapping atomic orbitals, one orbital from each of the atoms joined by the bond

3. The VB view of the formation of the covalent bond in H2
The amount the potential energy is lowered when the bond forms depends, in part, on the extent of orbital overlap
The VB view of the formation of the covalent bond in H2

4. Therefore, atoms tend to position themselves so the maximum amount of orbital overlap occurs
This yields the minimum potential energy and the strongest bonds
The fluorine atom has a singly occupied 2p orbital
It can overlap with the hydrogen 1s orbital to form the single bond in hydrogen fluoride

5. The H 1s orbitals position themselves for maximum overlap with the S 3p orbitals. The predicted 90º bond angle is very close to the experimental value of 92º.
There are many molecules with shapes and bond angles that fail to fit the VB model as developed

6. The bonds in the ethane molecule. (a) Overlap of orbitals
The bonds in the ethane molecule. (a) Overlap of orbitals. (b) The degree of overlap of the sp3 orbitals in the carbon-carbon bond is not appreciably affected by the rotation of the two CH3- groups relative to each other around the bond.

7. The nitrogen atom in ammonia has one lone pair of electrons
The nitrogen atom in ammonia has one lone pair of electrons. The experimental H-N-H bond angle is 107º.
The oxygen atom in water has two lone pairs of electrons and an experimental H-O-H bond angle of 104.5º.

8. Multiple bonds almost always consist of a single sigma bond and one or more pi bonds
The pi bonds are formed from the p orbitals that are not involved in the hybrid orbitals
Hydrocarbons frequently involve pi bonds
The double bond in alkenes is comprised of one sigma and one pi bond; the triple bond in alkynes consists of one sigma and two pi bonds

9. The carbon-carbon double bond in ethene
The carbon-carbon double bond in ethene. The carbon-carbon double bond consists of one sigma and one pi bond.

10. (a) the sp hybrid orbitals on each carbon form the sigma bond in acetylene (ethyne). (b) sideways overlap of the 2px and 2py orbitals form two pi bonds. (c) the pi-bonds after they form.

11. If the CH2 group in front was to rotate relative to the one in the rear, the unhybridized p orbitals would no longer be aligned, meaning the pi-bond would be broken. Bond breaking requires more energy than is available through normal bending and stretching at room temperature.
Groups connected by single bonds can freely rotate
Rotation is restricted around double bonds

12. VB theory can be summarized:
The basic framework of the molecule is determined by the arrangement of the sigma-bonds
Hybrid orbitals are used to form the sigma-bonds and the lone pairs of electrons
The number of hybrid orbitals needed by an atom in a structure equals the number of atoms to which it is bonded plus the number of lone pairs of electrons in its valence shell

13. 4) When there is a double bond in a molecule, it consists of one sigma and one pi bond 5) When there is a triple bond, it consists of one sigma and two pi bonds

14. Molecular Orbital Theory

15. Bonding MOs are lower in energy than antibonding MOs This can be represented using energy-level diagrams
MO energy-level diagrams for H2 and He2. The bond order is one (1) for H2 and zero (0) for He2.

16. MOs follow the same filling rules as atomic orbitals:
Electrons fill the lowest-energy orbitals that are available
No more than two electrons, with spin paired, can occupy any orbital
Electrons spread out as much as possible, with spins unpaired, over orbitals that have the same energy

17. The approximate relative energies of molecular orbitals in second period diatomic molecules. (a) Li2 through N2, (b) O2 through Ne2.

18. MO theory correctly predicts the unpaired electrons in O2 while VB theory does not
MO theory handles easily the things that VB theory has trouble with
MO theory is a bit difficult because even simple molecules require extensive calculations
MO theory describes “resonance” very efficiently

19. The sigma (a) and pi (b) framework of benzene
The sigma (a) and pi (b) framework of benzene. The double donut – shaped electron cloud (c) formed by the pi-electrons. In MO terms, the electrons are delocalized and the extra stability is the delocalization energy. Functionally, the resonance and delocalization energy are the “same”.