1. Experiments show O2 is paramagnetic
MOLECULAR STRUCTURE
CHAPTER 11

2. Objectives:
Extend atomic concepts to electronic structures of molecules
Utilize two quantum mechanical theories of molecular electronic structure
Valence bond theory
Bonding as a result of a shared electron pair between two atoms
Molecular orbital theory
Concept of atomic orbital extended to molecular orbital
Molecular orbital is a wavefunction spread over entire molecule

3. Fig 11.1 A molecular potential energy curve
Equilibrium bond length
Dissociation energy, Do
v = 0
Depth of well
Do = De - 1/2ћω

4. Change in electron density as two hydrogen atoms approach each other.

5. Fig 11.2 Representations of valence-bond
wavefunctions
Electron 1
Electron 2 A B

6. ∴ σ(1,2) = - σ(2,1) Molecular potential energy curve for H2
Ψ(1,2) = {A(1)B(2) + A(2)B(1)}
σ(1,2)
According to Pauli principle: Ψ(1,2) = -Ψ(2,1)
∴ σ(1,2) = - σ(2,1)
and σ(1,2) = √1/2{α(1)β(2) - α(2)β(1)}
a singlet state
with valence-bond model

7. Fig 11.3 Orbital overlap and spin pairing in two
collinear p orbitals to form a σ bond

8. Fig 11.4 A π bond resulting from orbital overlap
and spin pairing

9. :N≡N: Fig 11.5 The structure of bonds in the N2 molecule one σ bond
two π bonds
:N≡N:

10. Fig 11.6 Valence-bond description of bonding in
H2O molecule
O atom: 2s2 (2px)2 (2py)1 (2pz)1
H 1s1
H 1s1
However,
the 2py and 2pz are
at 90° to each other:
Observed bond angle = 104.5°

11. Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals.
Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals.
Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process.
Covalent bonds are formed by:
Overlap of hybrid orbitals with atomic orbitals or
Overlap of hybrid orbitals with other hybrid orbitals

12. Fig 11.7 An sp3 orbital formed from superposition
of s and three p orbitals on same atom
Four LCAOs give
four equivalent hybrid MOs:
h1 = s + px + py + pz
h2 = s - px - py + pz
h3 = s - px + py - pz
h4 = s + px - py - pz

13. e.g., wavefunction for overlap
Fig Each sp3 hybrid orbital forms a σ bond
by overlap with an H 1s orbital
109.5°
e.g., wavefunction for overlap
of hybrid h1 and H 1s:
Ψ = h1(1)A(2)+ h1(2)A(1)

14. Fig 11.9 More detailed representation of
formation of an sp3 hybrid orbital

15. Fig 11.10 An sp2 orbital formed from superposition
of s and two p orbitals on same atom
Three LCAOs give
three equivalent hybrid MOs:
h1 = s + √1/2 py
h2 = s + √3/2 px - √1/2 py
h3 = s - √3/2 px - √1/2 py
Remaining unhybridized p orbital

16. Fig 11.11 Representation of double bond in C2H4

17. Fig 11.12 Representation of triple bond in C2H2

18. How do I predict the hybridization of the central atom?
Count the number of lone pairs AND the number
of atoms bonded to the central atom
# of Lone Pairs +
# of Bonded Atoms
Hybridization
Examples 2 sp
BeCl2 3
sp2
BF3 4
sp3
CH4, NH3, H2O 5
sp3d
PCl5 6
sp3d2
SF6