1. Molecular Geometry and Valence Bond / Molecular Orbital Theory
Chemical Bonding II
Molecular Geometry and Valence Bond / Molecular Orbital Theory
Chapter 10

2. Molecular Shapes
Sugars and sweeteners taste sweet because they fit into spots on the tongue that trigger that response
Artificial or ‘no calorie’ sweeteners fit in the same place but do not get metabolized as quickly or at all… meaning they pass through fast and do not absorb

3. VSEPR Theory The Valence Shell Electron Pair Repulsion (VSEPR) Theory
States that electron pairs (defined as lone pairs or single, double, triple bonds, and even single e-’s) repel one another and want to push as far away as possible
Affects the shape
(molecular geometry)

4. Shapes
The preferred shape of a molecule is the one that allows maximum distance between electrons (bonding and lone pairs)
Depends on
How many come off the central atom(s)
How many lone pairs / bonding pairs

5. TWO electron Groups LINEAR: Maximizes separations by 180⁰
Draw out BeCl2
CO2

6. THREE electron Groups
Trigonal planar geometry: maximum separation is ~120⁰
Draw BF3
Draw CH2O
Notice the
bond angles (ok
to round to 120⁰

7. FOUR electron Groups
Tetrahedron geometry – maximum bond angle is 109.5⁰
Draw CH4

8. FIVE electron Groups Trigonal Bipyramidal Geometry Draw out PCl5
2 bond angles for maximum separation: 90 ⁰ & 120⁰
Draw out PCl5
“Axial chlorines” (top and bottom) vs the three “equatorial Chlorines”

9. SIX electron Groups
Six electron groups around a central atom assume an octahedral geometry
Draw (think about) SF6
Maximum bond angle
is ALL 90⁰

10. Practice Determine the molecular geometry of NO3- Draw it out first
Resonance ?
How many bonding sites??
Three… what does this tell about its shape?

11. Bonded vs Un-bonded electrons
The previous shapes were all “bonded” electron groups
Think about what happens with un-bonded electron pairs (lone electron pairs)
VSEPR theory says these are also greatly (-) and will push away and repel from one another in a similar fashion
Only they create different shapes because there is not an atom to “see” on the lone pairs

12. Nonbonding electrons still push (very, very negative)
They still have repulsions and according to the VSEPR theory they will migrate as far away as possible to alleviate this stress

13. Most Repulsive Least Repulsive
REPULSIONS
Lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair
Most Repulsive Least Repulsive
Think about how this can affect bond angles
The more repulsive (the greater they push away) the greater the bond angle

14. FOUR electron groups with ONE LONE PAIR
Think about ammonia
There are 4 electron groups (3 bonded and 1 lone)
So, push them apart just like a tetrahedron… but there is nobody bonded on one side
This ‘push’ creates a pyramidal shape (trigonal pyramidal)

15. Trigonal Pyramidal

16. FOUR electron groups with TWO LONE PAIRS
Think about Water
There are 4 electron groups (2 bonded and 2 lone)
So, push them apart just like a tetrahedron… but there is nobody bonded on two sides
This ‘push’ creates a BENT shape

17. Bent Molecular Geometry

19. FIVE electron groups with ONE LONE PAIR
Draw out / Consider SF4
In order to get these as far away as possible:
SEESAW
Geometry

20. FIVE electron groups with TWO LONE PAIRS
Draw out / Consider BrF3
In order to get these as far away as possible:
T-SHAPED
Geometry

21. FIVE electron groups with THREE LONE PAIRS
Draw out / Consider XeF2
In order to get these
as far away as possible:
LINEAR Geometry

22. SIX electron groups with ONE LONE PAIR
Think about BrF5
There are 6 electron groups (5 bonded and 1 lone)
So, push them apart just like a octahedron… but there is nobody bonded to one of the sides
This ‘push’ creates a SQUARE PYRAMIDAL MOLECULAR GEOMETRY

24. SIX electron groups with TWO LONE PAIRS
Think about XeF4
There are 6 electron groups (4 bonded and 2 lone)
So, push them apart just like a octahedron (AGAIN)… but there is nobody bonded to TWO of the sides
This ‘push’ creates a SQUARE PLANAR MOLECULAR GEOMETRY

26. Summary of VSEPR Theory
The geometry of a molecule is determined by the number of electron groups on the central atom(s)
The number of e- groups can be determined from the Lewis structure of the molecule. If the molecule has resonance structures, use one of them – doesn’t matter which one
Each of the following counts: lone pair, single bond, double bond, triple bond, or a single electron

27. Summary (cont’d)
The geometry of the e- groups is determined by minimizing their repulsions:
Lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair
Bond angles will vary, the presence of lone pairs will usually make bond angles smaller than the ideal angle of the particular geometry

28. Page 414-415 Copy this table On a piece of Computer paper (provided)
Review sheet /
Study guide /
Summary for
Molecular
geometry

29. Quick Practice
Which of the following statements is always true according to VSEPR theory?
The shape of a molecule is determined by repulsions among bonding electron groups.
The shape of a molecule is determined by repulsions among nonbonding electron groups
The shape of a molecule is determined by the polarity of its bonds
The shape of a molecule is determined by repulsions among all electron groups on the central atom

30. VSEPR Theory: Predicting Shapes
Draw the Lewis structure
Determine the total number of electron groups around the central atom
Determine the number of bonding groups and the number of lone pairs around the central atom
Use Table 10.1 to determine the electron geometry and molecular geometry

31. Polarity
As we discussed in chapter 9, bonds can be polar (unequal sharing of electrons, EN)
Entire molecules can be POLAR!!

32. WE STILL NEED TO USE OUT TABLE
Polarity
Bonds are easy… just look at the differences in electronegativity
For molecules, there must be a ‘pull’ in different directions, but not equal and opposite
WE STILL NEED TO USE OUT TABLE
FROM CHAPTER 9!

34. Nonpolar Molecules Think about two kids playing tug-o-war…
If they are equal in strength, there will be no net dipole moment
They cancel out
Go nowhere

35. Polar Molecules
Consider the same two kids pulling… if now instead, one kid pulls up and one kid pulls to the left… there WILL be a net movement (they will not cancel out

36. In summary, to determine whether a molecule is polar:
Draw a Lewis structure, determine the molecular geometry
Determine whether the molecule contains polar bonds
Determine whether the polar bonds add together to form a net dipole moment (add up vectors) – what is a vector???

37. Vector Addition Refer to page 420 for VECTOR ADDITION
One dimension = two kids pulling left and right (left / left OR right/ right)
Two dimensions = pulling up and to the right (in different directions, off of the same plane)

38. Page 421:

39. Practice Determine whether NH3 is polar
Draw the Lewis Structure
Determine where the molecule has polar bonds
Determine whether the bonds add together to form a net dipole moment
The three dipole moments sum to a net dipole moment. The molecule is POLAR!

40. PRACTICE #2
Determine whether CF4 is polar.

41. Interactions Nonpolar and Polar molecules DO NOT MIX
Think about water (polar) and oil (nonpolar)
They will separate and keep their distance
Polar molecules interact STRONGLY with other polar molecules
So when you mix P and NP together, the P will group and bunch up while the NP stay away and will not penetrate the group

44. Page 422 Chemistry in your day
How Soap Works
Read through this excerpt and answer the question in your notes
Consider the detergent molecule below (in your book). Which end do you think is polar? Which end is nonpolar?

45. Hybridization
Hybridization is a mathematical preocedure in which the standard atomic orbitals are combined to form new atomic orbitals called Hybrid Orbitals
It is essentially adding two orbitals together
SP3 hybridization  ___ ___ ___ ___

46. Pi and Sigma bonds
A pi (π) bond is the overlap between the half-filled p orbitals overlap side-by-side
A sigma (σ) bond is the overlap end-to-end

47. A pi (π) bond is formed after the sigma bond
A sigma (σ) bond is always the FIRST BOND formed

49. Short unit…
Molecular shapes through the pi / sigma bonds