1. VSEPR – a model for predicting 3-D Molecular Shapes
Coursebook Notes p HW 10-1, p. 87
Need Molecular Shapes Prelab with Lewis Dot Structures
Valence Shell Electron Pair Repulsion Theory
VSEPR – a model for predicting 3-D Molecular Shapes
DNA in 2D
DNA in 3D

2. BONDING ELECTRON PAIRS (shared valence e- pairs between O and H)
Lewis Dot Structures show the two- dimensional (2-D) arrangement of VALENCE ELECTRONS in a molecule.
NONBONDING Electron Pairs (sometimes called Lone Pairs) – belong to just one atom, in this picture Oxygen.)
THIS 2-D MODEL OF WATER DOES NOT EXPLAIN IMPORTANT OBSERVED PROPERTIES OF WATER.
EXAMPLE: LIFE APPARENTLY REQUIRES LIQUID WATER TO EXIST. 2-D MODEL OF WATER PREDICTS WATER SHOULD EXIST ONLY AS A GAS ON EARTH.
2-D
BONDING ELECTRON PAIRS (shared valence e- pairs between O and H)

3. NEWS FLASH – THE WORLD IS NOT FLAT (2-D) – ITS ROUND (3-D)

4. 3-D 3-D HELPS US BETTER UNDERSTAND PHYSICAL REALITY
TRUE PROPERTIES OF MOLECULES CAN ONLY BE FULLY UNDERSTAND BY MODELING REALITY IN THREE-DIMENSIONS!
3-D
3-D HELPS US BETTER UNDERSTAND PHYSICAL REALITY
Link to ship smacked by 100 foot water wave

5. Importance of Molecular Shapes
Our universe is three-dimensional – true properties of molecules can only be understood by looking at actual arrangements of molecules in space.
Example application: Chiral molecules – most drugs contain a carbon bonded to 4 different other atoms or groups of atoms.

6. Thalidomide – 1 form of drug treats morning sickness, the other causes birth defects
Effect of thalidomide mutations
DIFFERENCES IN BINDING OF CHIRAL FORMS

7. HW 10-1, p. 87
73) Why is the geometric structure of a molecule important, especially for biological molecules?

8. Key ideas
Molecular geometry is determined by the arrangement of bonding and non-bonding electron pairs around the central atom.
NON-Bonding (NB) electron pairs
Bonding electron pairs
4 BONDING PAIRS
3 BONDING PAIRS
2 BONDING PAIRS
0 NB PAIRS
1 NB PAIR
2 NB PAIRS

9. COUNTING BONDING AND NONBONDING ELECTRON PAIRS AROUND THE CENTRAL ATOM
Important Notes on Nonbonding Pairs (sometimes called LONE PAIRS:
Only Lone pairs (recall Pair = 2 e-) around CENTRAL Atom impact geometry.
Nonbonding Pairs on Central Atom are typically enclosed in a bubble .
Important Notes on Double and Triple Bonds:
- Double and Triple Bonds count as a single bonding region for the purposes of determining geometry.

10. How many Bonding Pairs (BP) and Nonbonding Pairs (NP) are present around the CENTRAL ATOM in each of the following molecules?
TAKE OUT MOLECULAR MODELS PRELAB; GRAB WHITE BOARD AND MARKER FOR EACH TEAM
BP: 2
BP: 2 5
BP:
NP: 1
NP:
NP:

11. MOLECULAR MODELS LAB 2

12. MOLECULAR MODELS LAB 3

13. MOLECULAR MODELS LAB 4

14. MOLECULAR MODELS LAB 5

15. MOLECULAR MODELS LAB 2 2

16. MOLECULAR MODELS LAB 3 1

17. MOLECULAR MODELS LAB 2

18. MOLECULAR MODELS LAB 6

19. MOLECULAR MODELS LAB 2 1

20. Key ideas
2) The bonding and nonbonding valence electron pairs arrange themselves as far APART in space as possible in order to minimize ELECTROSTATIC REPULSION between negative charges.
Link to Phet
LIKE CHARGES REPEL
NEGATIVE ELECTRON CLOUDS REPEL EACH OTHER

21. Key ideas
2) The bonding and nonbonding valence electron pairs arrange themselves as far APART in space as possible in order to minimize ELECTROSTATIC REPULSION between negative charges.
Link to Phet
Electron Pair Repulsion
LIKE CHARGES REPEL
NEGATIVE ELECTRON CLOUDS REPEL EACH OTHER

22. HW 10-1, p. 87
74) What general principles determine the molecular structure (shape of a molecule)?
75) How is the structure around a given atom related to repulsion between valence electron pairs on the atom?
76) Why are all diatomic molecules linear regardless of the number of valence pairs on each atom?
Geometry of molecule is determined by arrangement of bonding and nonbonding electron pairs around the central atom.
Bonding and Nonbonding electrons pairs around the central atom are arranged as FAR APART IN SPACE as possible to MINIMIZE REPULSION.
Only way electrons can be shared between two atoms is a straight line.

23. Each team grab a bag containing 5 Styrofoam balls and 4 toothpicks
Using Concept of Minimizing Repulsion to predict geometry and bond angles for molecules with 2, 3 and 4 bonds
Each team grab a bag containing 5 Styrofoam balls and 4 toothpicks
Link to tutorial

24. Conventions for Drawing in 3-D
Use to represent bonds in plane of paper
Use to represent bonds receding into plane of paper
Use to represent bonds coming out of plane of paper
Use to represent nonbonding (lone) electron pairs on central atom

25. Practice Drawing; Indicate bond angle
Linear:
120o
Triangular or Trigonal Planar:

26. Practice Drawing; Indicate bond angle
Tetrahedral:
(View 1)
Tetrahedral:
(View 2)
Tetrahedral:
(View 3)

28. Effect of Nonbonding Electrons Pairs on Bond Angles
Nonbonding electrons pairs are more spreadout and therefore exert a greater repulsion than bonding pairs.
Example: Bond angles for CH4, NH3, H2O
VSEPR animations

29. Practice Drawing; Indicate bond angle
Trigonal Pyramid
(View 1)
Trigonal Pyramid
(View 2)
Trigonal Pyramid
(View 2)

30. HW 10-1, p. 87
77) Although the valence electron pairs in ammonia have a tetrahedral arrangement, the overall geometric structure is described as trigonal pyramid not tetrahedral. Explain

31. HW 10-1, p. 87
78) Although both BF3 and NF3 molecules contain the same number of atoms, the BF3 molecules are flat whereas the NF3 molecule is a trigonal pyramid. Explain.

32. MOLECULAR MODELS LAB
Rotate through 9 stations around room (duplicates on each side of room). Draw each molecule in 3-D

33. Steps for Applying VSEPR:
Link to VSEPR video use first 4:00 min
Link to VSEPR 2
Draw Lewis Dot structure (be sure to include all nonbonding (lone) pairs on central atom).
Count the number of bonding and nonbonding electron pairs on the central atom.
Double and triple bonds count as only ONE bonding pair (one bonding “region”).
Consult chart from your notes below that describes arrangement of electron pairs in space that minimizes electrostatic repulsions.

34. 2
180 o 3
120 o 2 1
<120 o 4
109.5 o

35. 3 1
107 o 2 2
104.5 o
180 o 5
120 o
90 o 6
180 o
90 o

36. Total Prs
2 charge clouds, linear

37. Linear 2 bonding electrons pairs (2 atoms attached to center atom)
0 Non Bonding e- pairs
(lone pairs)
Bond angle = 180o
Type: AB2
Ex. : BeF2, CO2
Link to Phet Simulation

38. 2
180 o

39. Three Clouds
With three electron clouds, the farthest the electron clouds can get away from each other is 120o, the corners of an equilateral triangle. This shape is known as trigonal planer.
Link to Phet Simulation

40. Trigonal Planar
3 bonding electron pairs (3 atoms attached to center atom)
0 nonbonding pairs
Bond angle = 120o
Type: AB3
Ex. : BCl3

41. 2
180 o 3
120 o

42. Bent (3 total e- pairs) 2 Bonding e- pairs 1 nonbonding e- pairs
Bond angle < 120 o
Type: AB2E2
Ex. : O3
Link to Phet Simulation

43. 2
180 o 3
120 o 2 1
<120 o

44. Four clouds
When four clouds are attached to a central atom, the farthest they can get away from each other is the four corners of a tetrahedron. The tetrahedral angle is 109.5o.
Link to Phet Simulation

45. Tetrahedral 4 Bonding electron pairs 0 Non-Bonding e- pairs
Bond angle = 109.5o
Type: AB4
Ex. : CH4

46. 2
180 o 3
120 o 2 1
<120 o 4
109.5 o

47. Trigonal Pyramidal 3 Bonding e- pairs 1 Non-bonding e- pair
Bond angle = 107o
Type: AB3E
Ex. : PH3
Link to Phet Simulation

48. 3 1
107 o

49. Bent (4 total e- pairs) 2 Bonding e- pairs 2 Non Bonding e- pairs
Bond angle 104.5o
Type: AB2E2
Ex. : H2S
Link to Phet Simulation

50. 3 1
107 o 2 2
104.5 o

51. Trigonal Bipyramidal 5 Bonding e- pairs 0 Non Bonding e- pairs
Bond angle =
equatorial -> 120o
axial -> 90o, 180o
Type: AB5
Ex. : PBr5
Link to Phet Simulation

52. 3 1
107 o 2 2
104.5 o
180 o 5
120 o
90 o

53. Octahedral 6 Bonding e- pairs 0 Non Bonding e- pairs Bond angle = 90o
Type: AB6
Ex. : SF6
Link to Phet Simulation

54. 3 1
107 o 2 2
104.5 o
180 o 5
120 o
90 o 6
180 o
90 o

55. Molecules with multiple centers
A central atom is any atom with more than one atom bonded to it
Perform exercise individually for each atom
Molecular shape will refer only to the atoms/lone pairs immediately attached to that atom