1. Chapter 10 Chemical bonding: Molecular shapes, valence bond theory, and molecular orbital theory

2. Valence Shell Electron Pair Repulsion Theory
VSEPR theory:
Electrons repel each other
Electrons groups in a molecule arrange themselves so as to be as far apart as possible
Minimize repulsion
Determines molecular geometry
Electron groups are atoms and lone pairs

8. Defining Molecular Shape
Electron pair geometry: the geometrical arrangement of electron groups around a central atom
Atoms and lone pairs count as electron groups
Molecular Geometry: the geometrical arrangement of atoms around a central atom
Ignore lone pair electrons

9. 2 e- groups surrounding the central atom
e- pair geometry: linear
MG: linear
AXE designation: AX2E0
A: Central Atom
X: Bonding pairs
E: Non-bonding pairs
Example: BeCl2, CO2

10. 3 e- groups 3 Bonds, 0 Lone Pairs 2 Bonds, 1 Lone Pair
e- PG: Trigonal Planar (Triangular planar)
MG: Trigonal Planar
AX3E0
BF3
2 Bonds, 1 Lone Pair
e- PG: Trigonal Planar (Triangular planar)
MG: Bent/angular
AX2E1
GeCl2

11. 4 e- groups 4 bonds, 0 Lone Pairs 3 bonds, 1 Lone Pair
e- PG: Tetrahedral
MG: Tetrahedral
AX4E0
CH4
3 bonds, 1 Lone Pair
e- PG: Tetrahedral
MG: Triangular Pyramidal
AX3E1
NH3

12. 2 bonds, 2 Lone Pairs
e- PG: Tetrahedral
MG: Bent/Angular
AX2E2
H2O

15. Molecular Polarity

17. Determining molecular polarity
Draw Lewis dot structure taking into account 3-D molecular geometry
Indicate the dipole moment for each bond using EN values for each atom
Sum up the dipole moments as vectors and determine if there is a net dipole moment

18. Valence bond theory
Electrons are more appropriately represented in a quantum mechanical manner
Electrons exist in atomic orbitals that can interact with other atomic orbitals
Interactions between two atoms can be analyzed through their potential energy

19. Valence bond theory summary
When bringing atoms close together, when orbitals with unpaired electrons interact this results in a stabilizing (more negative) energy, promoting a chemical bond
Geometry and shape of orbitals interacting will affect the overall molecular geometry

20. Valence bond theory: Hybridization
Orbitals in a molecule are not the same as in an atom
Orbitals can mix together to form hybrid orbitals
This occurs mostly with atoms that tendto make more bonds, like carbon
Hybrid orbitals will have altered energy and shape, depending on the orbitals that are mixing
Reasoning for hybridization….ENERGY!!
Results in overall lower potential energy for the molecule

21. sp3 hybridization
Mixing one s orbital and three p orbitals will give you four sp3 orbitals
Can explain 4 EPG geometries

24. sp2 hybridization and double bonds
Mixing one s orbital and two p orbitals makes three sp2 orbitals
Explains 3 EPG geometries with double bonds

27. Sigma (σ) and pi (π) bonds

28. Bond rotation

29. sp hybridization
Mixing one s orbital and one p orbital makes 2 sp orbitals
Explains triple bonds

32. sp3d hybridization
Mixing one s, three p, and one d orbital produce five sp3d orbitals
This can explain 5 EPG geometries

34. sp3d2 hybridization
Mixing one s, three p, and two d orbitals produce five sp3d2 orbitals
This can explain 6 EPG geometries

37. Molecular orbital (MO) theory!!!
Solving the Schrodinger equation allowed for the calculation of the atomic orbitals (1s, 2p, 3d, etc.)
A similar calculation can be applied for molecular orbitals, this results in a process similar to hybridization
Linear combination of atomic orbitals (LCAOs) – a weighted linear sum of the valence atomic orbitals
a mixing of the atomic orbitals to produce molecular orbitals

38. Making molecular orbitals from atomic orbitals

39. Molecular orbital energy diagram
Similar to orbital diagrams for atoms, but now for molecules
Molecular orbitals are labeled based on
the interaction between the orbitals (σ in the case below)
The type of bonding orbital (bonding or antibonding)
The orbitals involved (1s orbitals)

40. Bond order
Bond order – relates to the strength of a bond and is dependant on the number of electrons in bonding and antibonding orbitals

41. Summary of LCAO-MO Theory
We can represent molecular orbitals (MOs) as a linear combination of atomic orbitals (AOs), where the total number of atomic orbitals will equal the total number of produced MOs
When two AOs mix, a lower energy bonding MO is produced, and a higher energy antibonding MO is produced
Filling in the molecular orbital energy diagram with the total number of electrons from the atoms, following general electron configuration rules
Bond order can be determined from the bond order formula

42. Period two homonuclear diatomic molecules
With period two diatomic molecules 2s and 2p orbitals must be considered (the valence orbitals in these atoms)

43. p orbital based MOs
For B2, C2, and so on, we need more orbitals to store electrons
Mixing of the positive and the negative to produce the bonding and antibonding MOs

46. Explanation of change in orbital ordering
Orbitals with similar phases and similar energies can mix
The more 2s and 2px mixing lowers the σ2s energy and raises the σ2p energy

47. Summary

48. Power of the MO theory
Lewis dot structure shows oxygen with no unpaired electrons
From the MO diagram you can see that it does have 2 unpaired electrons
Real life liquid O2 displays paramagnetism, as expected from a compound with unpaired electrons

49. Second period heteronuclear diatomic molecules
Different atoms have different energies for their atomic orbitals
Mixing is not as strong in heteronuclear diatomic molecules

50. Polyatomic molecules

51. No MO Chapter 10