1. CHEMICAL BONDING Molecular Orbital Theory Valance Bond Theory
Theory of Resonance
Metallic Bond

2. Chemical Bond Ionic bond Covalent bond Valance Bond Theory
Molecular Orbital Theory

3. Wave Nature of Electrons
Heitler and London in 1927 and updated by Pauling and Slater in 1931
directional properties of covalent bond
VALANCE BOND METHOD
Pairing of electrons (Heitler)
Direction and shapes of orbitals (pauling)

4. Molecular Orbital Theory

5. Molecular Orbital Theory
In the molecular orbital model, orbitals on individual atoms interact to produce new orbitals, called molecular orbitals, which are now identified with the whole molecule.
Molecular orbital (MO) theory is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule.

6. Figure: Energy match and molecular orbital formation

8. Three simple kinds of molecular orbitals
1. Sigma (bonding) orbitals (s).
2. Non-bonding orbitals (n)
3. Sigma star (anti-bonding) orbitals (s*)
Electrons delocalised along the axis between two nuclei. These may be represented as shared electrons, e.g. H:H or H:F; H-H or H-F
Orbitals that are essentially unchanged from atomic orbitals, and remain localised on a single atom (unshared).
These may be represented as a pair of electrons on one atom.
Orbitals with a node or nodes along the axis between two nuclei. These do not contribute to bonding, they “undo” bonding.

10. HF?

11. Valence MOs
Bonding in HF 2p
This diagram shows the outer, valence energy levels of H, F and HF.
The electronic configuration of the 8 valence electrons of HF is shown in blue.
There are four orbitals, each containing a pair of electrons.
How do we represent these? 1s ­¯ ­¯ ­¯ ­¯ ­¯ ­¯ ­¯ 2s H HF F

12. VBT H2O O 1s22s23p4 H-O-H angle should be 900, but it is 1050
Hybrid bond
BeCl2: SP-P bond
BF3: SP2-P bond
CH4: SP3-S bond
What about CCl4?

15. MOT Formation of H2 molecule Why He2 not formed? Atomic orbital σ
Molecular orbital σ and σ*: bonding and antibonding orbital
Non bonding orbital
Direct or head on overlap is σ bond
Sideways overlap of orbitals are π bond

18. N2

19. O2