1. Lewis Dot Structures, VSEPR & Polarity
Thursday, December 7th, 2017

2. Dot Diagrams vs. Structures
Lewis Dot Diagrams show the number of valence of electrons electrons (in the form of dots) for a single element
Lewis Structures are diagrams that show the bonding between atoms of a molecule and the lone pairs that may exist in the molecule

3. Drawing Lewis Structures Review
Sum the # of valence electrons from all atoms
Anions: add e– (CO32- : add 2 e– ) Cation: subtract e– (NH4+: minus 1 e– )
Predict the arrangement of the atoms
Usually the first element is in the center (often C, never H)
Make a single bond (2 e–) between each pair of atoms
Arrange remaining e– to satisfy octets (8 e– around each)
Place electrons in pairs (lone pairs)
Too few? Form multiple bonds between atoms: double bond (4 e-) and triple bond (6 e-)
Check your structure!
All electrons have been used
All atoms have 8e-
Exceptions: Remember that H only needs 2e– !

4. Lewis Structure “Useful Hints”
C group: Forms a combo of 4 bonds and no LP (Lone Pairs)
N group: Forms a combo of 3 bonds and 1 LP
O group: Forms a combo of 2 bonds and 2 LP
F group (halogens): Forms 1 bond and 3 LP
Note that these are NOT always true!

5. Lewis Structures Practice
Practice drawing the following compounds in your notebook.
CO2 NH3 HCl CH4

6. Lewis Structures Practice - ANSWERS
CO2 NH3 HCl CH4

7. How do you determine if a structure is polar or nonpolar?
A structure will be nonpolar if is symmetrical around the central atom.
A structure will be polar if it is asymmetrical around the central atom.

8. Identify the following structures as polar or nonpolar.

10. Identify the following structures as polar or nonpolar.

11. WHATTTTTT?! That is NOT symmetrical!
NONPOLAR

13. VSEPR REFERENCE CHART
You may use this handout on all quizzes/tests/the midterm in this class.
DO NOT LOSE IT.

14. Molecular Shape
VSEPR theory assumes that the shape of a molecule is determined by the repulsion of electron pairs.

15. VSEPR Theory Based on Electron Dot (Lewis structures)
VSEPR Theory predicts shapes of compounds
VSEPR (pronounced “vesper”) stands for Valence Shell Electron Pair Repulsion
VSEPR predicts shapes based on electron pairs repelling (in bonds or by themselves)
Electrons around central nucleus
repel each other. So, structures
have atoms maximally spread out

16. VSEPR overview tetrahedral trigonal pyramidal Bent Linear
Each shape has a name (you should know these 5)
Names of Shapes:
tetrahedral
trigonal pyramidal
Bent
Linear
trigonal planar

17. Triangular Planar
Bent
Tetrahedral
Linear
Trigonal pyramidal

18. Example 1: Methane (CH4) Example 2: Ammonia (NH3) Example 3: Water(H2O)

19. methane, CH4
Tetrahedral
109.5°
Bonds are all evenly spaced electrons

20. ..
ammonia
NH3 .. .. ..
Trigonal Pyramidal
Less repulsion between the bonding pairs of electrons

21. .. ..
water, H2O
109.5° (109.5°)
109.5° (107°)
109.5° (104.5°)

22. .. ..

23. Bent or V
2 unshared pairs of e’s at top of O repel bonds and force them to bend

24. Steps to Success
Draw the Lewis Structure for each atom
- Count up the valence electrons, determine central atom (put a star), bond terminal (outside) atoms, recount, add lone electron pairs, check & shift to share if necessary
Determine VSEPR Structure
-- # atoms bound on central atom
-- # unpaired electrons on central atom
Redraw as VSEPR Structure

25. Example 4: Carbon Dioxide (CO2)

26. Example 4: Carbon Dioxide (CO2)