1. Chemical Bond Theories
Atomic Orbitals
Molecules ?
Valentim Nunes, Engineering Unit of IPT February, 2018

2. TOM MOLECULAR ORBITAL THEORY — Robert Mulliken)
Valence electrons are delocalized
Valence electrons occupy molecular orbitals that extends to all the molecule

3. TLV VALENCE BOND THEORY Linus Pauling
Valence electrons are localized between the atoms (or are isolated pairs).
Semi filled atomic orbitals overlap to form the chemical bond.

4. Valence Bond Theory
The Valence Shell Electron Pair Repulsion Model, based on the Lewis structures do not explain the chemical bonds!
Does not explain why the F-F chemical bond is weaker than the H-H bond, although both chemical bonds result from the sharing of one electron pair.
Accordingly to the TLV, the formation of the H-H covalent bond results from the spatial overlap of two atomic orbital 1s of both atoms.

5. Valence Bond Theory Sharing of two electrons between two atoms
Dissociation energy of bonds
Bond lenght H2 F2
436,4 kJ/mol
150,6 kJ/mol
74 pm
142 pm
Valence Bond Theory — the electrons in a molecule occupy atomic orbitals of individual atoms.
Overlap
2 1s
2 2p
Sharing of two electrons between two atoms

6. Formation of a chemical bond  (Sigma)
Two s orbitals overlap

7. Formation of a  chemical bond
Two s orbitals overlap
Two p orbitals overlap

8. Valence Bond Theory Chemical bonds in BF3 Triangular plane
angle = 120o

9. Chemical bond in BF3 Hybridization of atomic orbitals
How to explain the three bonds with an angle with 120o using one orbital s and p orbitals that forms angles of 90o?
Pauling introduced the concept of:
Hybridization of atomic orbitals
The hybridization consists in the mixture of atomic orbitals producing a new set of orbitals — Hybrid orbitals

10. Hybridization — coalescence of two or more atomic orbitals to produce a new set of hybrid orbitals.
Coalescence of, at least, 2 atomic orbitals non-equivalent atomic orbitals (ex., s and p). The hybrid orbitals have a different shape of the original atomic orbitals.
The number of hybrid orbitals is equal to the number of atomic orbitals that participate in the hybridization process.
The chemical bonds are formed by:
Overlap of hybrid orbitals with atomic orbitals.
Overlap of hybrid orbitals with other hybrid orbitals.

11. Chemical bond in BF3 redistribution of electrons hybridization 2p 2s
Non occupied orbital p
Three hybrid orbitals sp2

13. BF3
The three hybrid orbitals results from the mixture of one s orbital and two p orbitals  3 sp2
Then we have 3 hybrid orbitals semi filled that are used to form the B-F sigma chemical bonds.

14. BF3
One orbital of each F overlaps with the sp2 hybrid orbital to form para forma the sigma B-F chemical bond.

15. Chemical bond in CH4
How to justify 4 C—H sigma chemical bonds with an angle of 109o?
We must use 4 atomic orbitals — s, px, py, e pz — to form 4 hybrid orbitals.

17. Chemical bonds in CH4

18. Hybridization of Orbitals
Count the number of isolated pairs and the number of atoms bonded to the central atom.
Isolated pairs + Atoms bonded
Hybridization
Examples 2 3 4 5 6 sp
sp2
sp3
sp3d
sp3d2
BeCl2
BF3
CH4, NH3, H2O
PCl5
SF6

20. Bonds in Glicine

21. Bonds in Glicine

22. bonds in Glicine

23. Bonds in Glicine

24. Bonds in Glicine

25. Multiple Bonds
Consider the ethene, C2H4

26. sp2 hybridization of a carbon atom
Fundamental state
Electron promotion
sp2 hybridization
state

27. The 2pz orbital is perpendicular to the hybrid orbitals plane.

28. Bonds in C2H4
Pi bond (p) – electronic density above and below the nuclei of atoms.
Sigma bond (s) – electric density between the 2 atoms.

29. π bond in C2H4
The p orbital non utilized in each C contains an electron and this p orbital overlaps with the p orbital of the neighbor atom to form the π bond.

30. s and π bonds in C2H4

31. s and π bonds in CH2O

32. s and π bonds in C2H2

33. sigma (s) and pi (p) C H O Simple bond 1 sigma bond Double bond
1 sigma bond + 1 pi bond
Triple bond
1 sigma bond + 2 pi bonds
s ligações = 6
+ 1 = 7
p ligações = 1

34. Consequences of multiple bonds
Rotation is restricted along one C=C chemical bond