1. Chapter 9 – Molecular Geometry and Bonding Theories
Homework:
11, 13, 15, 19, 20, 21, 25, 26, 31, 34, 35, 36, 39, 41, 42, 43, 44, 47, 49, 51, 54, 56, 96, 100

2. 9.2 – The VSEPR Model Two balloons Three balloons Four balloons
Linear arrangement
Three balloons
Trigonal-planar arrangement
Four balloons
Tetrahedral arrangement

3. Electrons in molecules behave like balloons
A single covalent bond forms between atoms when a pair of electrons is between the atoms
A bonding pair of electrons defines a region in which the electrons are most likely to be found between two atoms
This area we find electrons is called an electron domain
A nonbinding pair (or lone pair) defines an electron domain located around one atom

4. Example Four electron domains here
In general, each nonbinding pair, single bond or multiple bond produces an electron domain around the central atom

5. Because electron domains are negatively charged, they repel each other.
The best arrangement of a given number of electron domains is the one that minimizes the repulsions between them.
This is the basic idea behind the VSEPR model.

6. Similar to Balloons? You bet! Two domains makes linear arrangement
Three domains makes trigonal-planar arrangement
Four domains makes tetrahedral arrangement

7. pg. 349

8. The arrangement of electron domains about the central atom is called its electron–domain geometry.
In contrast, the molecular geometry is the arrangement of only the atoms in a molecule or ion
So any non-bonding pairs are not a part of the molecular geometry

9. The VSEPR model predicts electron-domain geometry
From this and knowing how many domains are due to nonbinding pairs, we can predict the molecular geometry
When all the electron domains in a molecule come from bonds, the molecular geometry is the same as the electron-domain geometry
But if one or more domains comes from lone pairs, we must ignore those domains for molecular shape

10. pg. 351

11. Example NH3 Already done this. 4 electron domains around central atom
So electron-domain geometry is tetrahedral
We know 1 of those domains comes from lone pairs
So the molecular geometry of NH3 is trigonal pyramidal
Tetrahedral with one less end, see pg. 347

12. Steps using VSEPR model to predict shape of molecules
Draw Lewis structure
Count number of electron domains around central atom
Determine electron-domain geometry
Use table 9.1, 9.2 or 9.3
Use the arrangement of the bonded atoms to determine the molecular geometry
Use table 9.2 or 9.3

13. Example CO2 Draw Lewis Structure
How many electron domains around the central atom are there?

14. What is the electron-domain geometry for this?
Linear
What molecular geometry is possible?

15. Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles
We refine the VSEPR model to predict and explain slight variances from the ideal bond angles
Methane (CH4), ammonia (NH3) and water (H2O) all have tetrahedral electron-domain geometries
But their bond angles are a little different
CH4 = 109.5º, NH3= 107º and H2O = 104.5º
Differences based around which type of electron pairs make up the electron domains

16. Bond angles decrease as the # of nonbonding electron pairs increase.
Bonding pair of electrons attracted by both nuclei of the bonded atoms
Lone pair of electrons attracted primarily by one nucleus

17. Since H2O had the most lone pairs, it gets the shortest bond angles
Because lone pair has less nuclear attraction, it’s domain becomes more spread out
So electron domain for lone pairs exert more repulsive force on adjacent electron domains
This compresses (lessens) the bond angles
Since H2O had the most lone pairs, it gets the shortest bond angles

18. Multiple Bonds an Bond Angles
Multiple bonds have a higher electron-charge density than single bonds
Also creates larger electron domains
So electron domains for multiple bonds exert a greater repulsive force on adjacent electron domains than single bonds do
So multiple bonds (double or triple) will decrease the bond angles too

19. Phosgene (Cl2CO) Central atom has three electron domains
3 single bonds
Trigonal planar geometry
Double bond acts like a lone pair, reducing the Cl-C-Cl bond angle

20. How Do These all Compare?
In terms of volume occupied by electron pairs
In other words, who compresses the most?
Lone pair > triple bonds > double bonds > single bonds

21. Molecules with Expanded Valence Shells
So far we have assumed the molecules have no more than an octet of electrons
But the most common exception to the octet rule is a central atom having greater than 8 valence electrons
So we need to deal with molecules with 5 or 6 electron domains

22. pg. 354

23. Example
Use the VSEPR model to predict the electron and molecular geometry of ClF3
Step 1: Lewis structure
How many electron domains around central atom? 5

24. How many bonding domains? How many non-binding domains?
5 electron domains
Gives us an electron geometry of trigonal bipyramidal
How many bonding domains? 3
How many non-binding domains? 2
So its molecular geometry is
T-shaped

25. Shapes of Larger Molecules
The VSEPR model can be extended to more complex molecules than we’ve been dealing with.
Consider acetic acid
CH3COOH

26. Acetic acid has 3 interior atoms
Carbon, and each oxygen
We can use VSEPR to look at each central atom individually

27. 9.3 – Molecular Shape and Molecular Polarity
Remember that bond polarity measures how equally the electrons in a bond are shared between the two atoms
Higher bond polarity = less equal sharing
Higher electronegativity difference = higher bond polarity

28. For every bond in the molecule, we can look at the bond dipole
The dipole moment depends on both the polarities of the bonds and the geometry of the molecule
Last chapter we focused just on the polarity effect on the dipole moment
For every bond in the molecule, we can look at the bond dipole
The dipole moment that is due ONLY to the two atoms in the bond

29. Example
CO2
O=C=O
Each C=O bond is polar (O is more electronegative than C)
Since we have two O=C bonds, the bonds are identical
We end up with high electron density around the O, and low electron density in the middle

30. Bond dipoles and dipole moments are vectors
The overall dipole moment is the sum of the bond dipoles that make it up
But, must consider both the amount of the dipole, and the direction of the dipole
We have two identical C=O bonds, so the amount of the dipoles are the same
But the DIRECTION of the dipoles are opposite
This causes the individual bond dipoles to cancel each other out
So the geometry of CO2 indicates that it is a NONPOLAR molecule, even though it contains polar bonds.

31. Bond Dipole Activity
Bond Dipole Activity

32. Steps to Determine Molecular Polarity
Draw Lewis structure
Determine molecular geometry
Look at effects of electronegativity differences

33. 9.4 – Covalent Bonding and Orbital Overlap
The VSEPR gives as a method to predict the shape of molecules
Does not explain WHY the bonds exist between atoms
A mixture of Lewis’ notion of electron-pair bonds and atomic orbitals leads to a model of chemical bonding
This mixture of views is called the valence-bond theory

34. In Lewis theory, covalent bonding occurs when atoms share electrons
The sharing concentrates electron density between the two nuclei involved
In valence-bond theory, the build-up of electron density between the nuclei is thought of as occurring when
a valence atomic orbital of one atom merges with a valence atomic orbital of another atom

35. This merger of orbitals
Means that they share a region of space
Called overlap
The overlap of orbitals allows two electrons of opposite spin to share the common space between the nuclei
Forming an atomic bond
See figure 9.14 on pg. 360

37. Distance
There is always an optimum distance between the two bonded nuclei in a covalent bond
Too close = too much repulsion between the nuclei
Too far = not much overlap, not a strong bond

38. 9.5 – Hybrid Orbitals