1. Bond
Theories

2. Lewis Structures Tell us about bonds in a molecule
Do not tell us about the shape of the molecule

3. Valence Bond Theory Based on the quantum model
Says that covalent bonds form when orbitals of different atoms overlap

4. Bonds: When Orbitals Overlap Types
sigma (σ) – the ends of the orbitals overlap
pi (π) – the sides of the orbitals overlap

5. Single Bond
s sublevel
sigma bond
Chemistry textbook, p. 162

6. Single Bond
p sublevel
sigma bond
Chemistry textbook, p. 162

7. Question
Which type of bond forms first between covalently bonded atoms?
Sigma Pi
Depends on the atom

8. Double Bond
sigma and pi bond
Chemistry textbook, p. 162

9. Triple Bond
sigma and 2 pi bonds
Chemistry textbook, p. 162

10. Bonds pi bond double bond triple bond forms only after a sigma bond
weaker than a sigma bond
double bond
stronger than a single bond
triple bond
stronger than a double bond

11. Resonance S O S O SO2 molecule
The molecule does not oscillate between the two shapes.
An average (1.5) of the 2 bond possibilities results.

12. Resonance
O3 molecule O O

13. Resonance
NO3 molecule − N O O N − O N −

14. Octet Exceptions
Molecules with an odd number of valence electrons to share
O − N O
Example: NO2

15. Octet Exceptions F F − B − F
Elements (such as Boron) that settle for less than 8 valence electrons
F − B − F F
Example: BF3

16. Extra electrons go to the d sublevel.
Octet Exceptions
Elements with more than 8 valence electrons
Extra electrons go to the d sublevel.
Example: SF6

17. The Need for Another Theory
Valence bond theory predicts liquid oxygen (O2) to be colorless and nonmagnetic.
However, it is actually pale blue and magnetic.

18. Molecular Orbital (MO) Theory
Each atom’s orbitals are replaced by totally new orbitals when a molecule forms.
Molecular orbitals are ranked in order of increasing energy.

19. Molecular Orbital (MO) Theory
Electrons fill molecular orbitals from lowest to highest energy.
Apply the same concept to molecules that has been applied to atoms.

20. Molecular Orbital (MO) Theory
Unpaired electrons can cause paramagnetism.
The lower energy molecular orbitals are bonding orbitals; the higher ones are antibonding orbitals.