1. Chemistry 1011 Y8Y,U Paul G. Mezey
Chapter 10: Chemical Bonding II: Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory Artificial Sweeteners: Fooled by Molecular Shape (Suggested Reading) VSEPR Theory: The Five Basic Shapes [10.1] VSEPR Theory: The Effect of Lone Pairs [10.1] VSEPR Theory: Predicting Molecular Geometries [10.1] Molecular Shape and Polarity [10.2] Valence Bond Theory: Orbital Overlap as a Chemical Bond [10.3 & 10.4] Valence Bond Theory: Hybridization of Atomic Orbitals [10.3 & 10.4]
Chemistry Y8Y,U Paul G. Mezey 1

2. Chapter 10 Chemical Bonding II
Lewis dot structures .
Lewis dot structures only give an idea of the electron distribution in the species. There is NO INFORMATION about the molecular geometry, which depends on the relative position of nuclei around the central atom. ?
Molecular Geometry and Chemical Bonding, Paul G. Mezey

3. Chapter 10 Chemical Bonding II
VSEPR Model .
One may connect the information of electron distribution in a Lewis dot structure to molecular geometry by using the
Valence-Shell Electron-Pair Repulsion (VSEPR) theory.
The essence of the VSEPR theory:
GROUPS of electrons repel each other, ending up as far from each other as possible.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

4. Chapter 10 Chemical Bonding II
Some textbooks talk about
repulsion of ELECTRON PAIRS.
The term
repulsion of ELECTRON GROUPS
is perhaps better, because multiple bonds are treated the same way as ONE PAIR of electrons in VSEPR theory even though in a multiple bond there are more than one pair of electrons present. .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

5. Chapter 10 Chemical Bonding II.
Considering directions around a central atom,
A lone pair is ONE GROUP of electrons
A single bond is ONE GROUP of electrons
A double bond is ONE GROUP of electrons
A triple bond is ONE GROUP of electrons
Molecular Geometry and Chemical Bonding, Paul G. Mezey

6. Chapter 10 Chemical Bonding II
Electron distribution vs. geometry .
Electron distribution
Molecular geometry
The “shape” of electron group distribution
The “shape” of nuclear positions around the central atom
The “shape” of electron distribution INCLUDES
all lone pairs
Lone pairs influence molecular geometry, but they are not part of this “shape”, since there are no terminal nuclei on lone pairs
Molecular Geometry and Chemical Bonding, Paul G. Mezey

7. Chapter 10 Chemical Bonding II
Electron distribution vs. geometry .
For simple molecules with a central atom:
If the central atom has NO lone pairs on it, then
the electron group distribution and the molecular geometry
ARE THE SAME!
Molecular Geometry and Chemical Bonding, Paul G. Mezey

8. Chapter 10 Chemical Bonding II
Figure of shapes (GROUPS) .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

9. Chapter 10 Chemical Bonding II
AXn notation .
The central atom A is bonded to n atoms or functional groups, denoted as X.
This notation ignores lone pairs, so it is suited for categorizing molecular geometries, which also ignore lone pairs.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

10. Chapter 10 Chemical Bonding II
Figure of shapes (2 to 4 GROUPS) .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

11. Chapter 10 Chemical Bonding II
Figure of shapes (5 GROUPS) .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

12. Chapter 10 Chemical Bonding II
Figure of shapes (6 GROUPS) .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

13. Chapter 10 Chemical Bonding II
Comment .
With experience, we tend to start drawing Lewis dot structures
with molecular geometry information included…
instead of
instead of
Molecular Geometry and Chemical Bonding, Paul G. Mezey

14. Chapter 10 Chemical Bonding II
Getting geometry information .
1. Draw the Lewis dot structure
2. Determine the number of electron groups on the central atom to get electron group arrangement
Use the number of lone pairs and the arrangement to determine the molecular
geometry
Molecular Geometry and Chemical Bonding, Paul G. Mezey

15. Chapter 10 Chemical Bonding II
Advanced geometry considerations .
Lone pairs are the “biggest” electron groups
(best at repelling other electron groups).
Triple bonds are the next “biggest” groups.
Double bonds are “smaller”.
Single bonds are the “smallest” electron groups.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

16. Chapter 10 Chemical Bonding II
Tetrahedral arrangement (advanced) .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

17. Chapter 10 Chemical Bonding II
Trigonal planar arrangement (advanced) .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

18. Chapter 10 Chemical Bonding II
Problem .
What is the arrangement of electron groups, and geometry around the central atom for the following molecules?
SF2 XeO4
H3O+ AsF5
Molecular Geometry and Chemical Bonding, Paul G. Mezey

19. Chapter 10 Chemical Bonding II
Molecular dipole moments .
If there are polar covalent bonds (partial charge separations) in a molecule, the molecule MAY OR MAY NOT have a permanent dipole moment.
A permanent dipole moment means there are regions of the entire molecule that are permanently partially negative and permanently partially positive.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

20. Chapter 10 Chemical Bonding II
Permanent dipole moments .
To determine if a molecule has a permanent dipole moment, we add together the vectors
that describe the charge separation of polar covalent bonds.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

21. Chapter 10 Chemical Bonding II
Permanent dipole moments .
To add vectors, we chain vectors by putting the tail of the next vector on the head of the previous vector.
The resultant vector is then drawn from the tail of the first vector to the head of the last vector in the chain.
This resultant vector is the permanent dipole moment.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

22. Chapter 10 Chemical Bonding II
Recall HCl .
We saw earlier that the diatomic molecule HCl has a polar covalent bond.
Since there is only one bond, this one vector of charge separation ALSO describes the permanent dipole moment of HCl.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

23. Chapter 10 Chemical Bonding II
Water .
The permanent dipole moment in water can be seen by adding together the charge separation vectors of the two polar covalent O-H bonds.
Permanent dipole moment
Lewis structure
Adding vectors
Molecular Geometry and Chemical Bonding, Paul G. Mezey

24. Chapter 10 Chemical Bonding II
Symmetry and dipole moments .
A molecule with more than one polar bond MIGHT NOT have a permanent dipole moment when the charge separations are symmetrically distributed so that the resultant vector sums up to to zero.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

25. Chapter 10 Chemical Bonding II
Geometry and dipole moments .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

26. Chapter 10 Chemical Bonding II
Permanent dipole moments and molecular properties .
Ionic bonds are generally strong because of the strong electrostatic attraction between
positive and negative charges.
Molecules with permanent dipole moments have regions with partial positive and negative charges that attract the opposite regions on other molecules of the same type.
Such intermolecular forces affect the bulk properties of collections of molecules.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

27. Chapter 10 Chemical Bonding II
Quantifying dipole moments .
Dipole moments measure the amount of charge separation (in Coulombs) that occurs over the bond length (in meters) in a derived unit called a debye (D)
1 D = 3.34 x Cm
Dipole moment for HCl is 1.08 D
Molecular Geometry and Chemical Bonding, Paul G. Mezey

28. Chapter 10 Chemical Bonding II
Problem .
The molecule BrF3 has a dipole moment of 1.19 D. Which of the following geometries are possible: trigonal planar, trigonal pyramidal, or T-shaped?
b) The molecule TeCl4 has a dipole moment of 2.54 D. Is the geometry tetrahedral, seesaw, or square planar?
Molecular Geometry and Chemical Bonding, Paul G. Mezey

29. Chapter 10 Chemical Bonding II
Valence bond theory .
Bonds form between atoms when:
Orbitals (the “allowed” electron distributions) in the atoms overlap to create molecular bonding orbitals.
2. Each molecular bonding orbital has NO MORE THAN 2 electrons in it.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

30. Chapter 10 Chemical Bonding II
Bond strength .
Covalent bonds are strongest when there is maximum orbital overlap between atomic orbitals. This maximum overlap occurs in the same direction as the atomic orbitals point.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

31. Chapter 10 Chemical Bonding II
Hybrids .
Hybrids occur when we mix two or more different types of things from the same class.
The resultant hybrid shows similarities to the original things, but is distinctly different from them.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

32. Chapter 10 Chemical Bonding II
Hybrid orbitals .
The atomic orbitals of atoms can be
mixed together (WHEN REQUIRED!)
to form hybrid atomic orbitals
that are different from the source orbitals.
Such hybrid orbitals are used to better explain molecular geometry and bonding.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

33. Chapter 10 Chemical Bonding II
Oxygen atom orbital diagram .
We would expect water to have a 90 angle between its bonds, based on the atomic orbitals on oxygen.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

34. Chapter 10 Chemical Bonding II
Oxygen hybrid orbitals .
plus
gives
Molecular Geometry and Chemical Bonding, Paul G. Mezey

35. Chapter 10 Chemical Bonding II
Oxygen hybrid orbitals .
plus
gives
One s and three p orbitals combine to give four sp3 hybrid orbitals
Molecular Geometry and Chemical Bonding, Paul G. Mezey

36. Chapter 10 Chemical Bonding II
In general: .
A total of n atomic orbitals combine to give n hybrid orbitals of a given kind.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

37. Chapter 10 Chemical Bonding II
Hybrid orbitals .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

38. Chapter 10 Chemical Bonding II
Oxygen hybrid orbital diagram .
We would expect water to have a ~109.5 angle based on the hybrid sp3 orbitals on oxygen.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

39. Chapter 10 Chemical Bonding II
Determining hybrid orbitals diagrams .
Draw the Lewis dot structure
Use VSEPR theory to predict electron group arrangement
Use Table 10.2 to determine what hybrid orbitals have the same arrangement
4. Create the hybrid orbital diagram based on changing the ground state diagram
Molecular Geometry and Chemical Bonding, Paul G. Mezey

40. Chapter 10 Chemical Bonding II
Problem .
Describe the bonding of I3-
in terms of valence bond theory.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

41. Chapter 10 Chemical Bonding II
Multiple bonding .
Multiple bonds (double or triple bonds) are possible when more than one set of orbitals can overlap between two atoms.
The first bond is the sigma (s) bond, which occurs from orbital overlap on the axis between the atoms.
The second and third bonds are pi (p) bonds that occur from orbital overlap both above and below the axis between the two atoms.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

42. Chapter 10 Chemical Bonding II
Ethene has a double bond .
No orbital overlap between these p orbitals
Notice we’ve chosen to create sp2 hybrid orbitals
Molecular Geometry and Chemical Bonding, Paul G. Mezey

43. Chapter 10 Chemical Bonding II
Ethyne has a triple bond .
Notice we’ve chosen to create sp hybrid orbitals and not sp3 or sp2
Molecular Geometry and Chemical Bonding, Paul G. Mezey

44. Chapter 10 Chemical Bonding II.
Molecular Geometry and Chemical Bonding, Paul G. Mezey