1. Unit 7 Molecular Geometry

2. Essential Question
What is a Lewis structure and what does it tell us?

3. Lewis Structures Properties are determined by molecular shape
Lewis structures can be used to determine shape
Shows the arrangement of electrons in a compound

4. Writing Lewis Structures
Step 1: Add up valence electrons
Ex: H2O
H = 1 valence electron X 2 atoms
O = 6 valence electrons
8 valence electrons

5. Writing Lewis Structures
Step 2: Determine the central atom
If Carbon is in the compound, it is the central atom
If Carbon is NOT present, the LEAST electronegative element is central
Hydrogen is NEVER the central atom

6. Writing Lewis Structures
Step 3: Draw a pair of electrons between the central atom and each bonded atom
A pair of bonding electrons can be represented by a dash or two dots between the bonding atoms
H : O : H or H – O – H

7. Writing Lewis Structures
Step 4: Fill in the rest of the electrons to satisfy the octet rule for all elements in the compound
Hydrogen must satisfy the duet rule
H : O : H or H – O – H

8. Writing Lewis Structures
Step 5: Count up all the electrons. Make sure everything has an octet (duet for hydrogen).
If you always check your work, you can’t make a mistake!!
HINT: For these instructions and more helpful tips see page 22 in your packet 

9. Writing Lewis Structures
If necessary, use two pairs of electrons to form a double bond between two atoms
Ex: CO2

10. Writing Lewis Structures
If necessary, use three pairs of electrons to form a triple bond between two atoms
Ex: N2

11. Essential Question
What is polarity?

12. Polarity
Polar bonds are an intermediate between covalent bonds (equal sharing) and ionic bonds (complete transfer of electrons)

13. Polarity and Electronegativity
Recall from previous units…
Electronegativity is the ability of an element to attract elements in a compound
High = very attracted to electrons
Low = not so attracted to electrons
Some values can be located on page pg 21 in your packet.

14. Polarity and Electronegativity
You can determine if a bond is nonpolar covalent, polar, covalent or ionic using electronegativity values (see pg 21 in your packet)
How big is the difference?
3.3
1.7
0.3
0.0
Ionic
Polar
Covalent
Nonpolar
Covalent

15. Now we can determine Lewis structure for a molecule and polarity for a bond
Weren’t we looking for shape??

16. What is molecular geometry?
Essential Questions
What is molecular geometry?
What determines basic molecular geometry?

17. VSEPR
Valence
Shell
Electron
Pair
Repulsion

18. VSEPR VSEPR theory determines the molecular geometry of the molecule
Molecular Geometry – 3D arrangement of a molecule’s atoms in space
Number and type of electron pairs determines shape

19. VSEPR We can classify as bonding pairs or lone pairs
Bonding Pair – shared between two atoms
Lone Pair – unshared electrons
Number of each type of electron pair determines shape

20. VSEPR Molecular geometry determines the bond angle
Electron pairs avoid each other, so they will stay as far apart as possible
See page 21 in your packet

21. VSEPR (reference pg 17 or 21 in your packet)

22. VSEPR To use determine the shape: Draw Lewis Structure Either
A - Count the number of lone and bonding pairs, and compare to the chart on the previous slide (this would require you to MEMORIZE the chart) or
B - Compare to the chart on page 21 in your packet (that’s what you get on the test)

23. Molecular Polarity Some molecules that have polar bonds are nonpolar
Molecular shapes can cause polarity to cancel out

24. Molecular Polarity Polar molecules are said to have dipoles
The dipoles tell you which parts of the molecule are slightly negative and slightly positive

25. Molecular Polarity For BINARY Compounds Nonpolar Shapes Polar Shapes
Tetrahedral
Linear
Trigonal Planar
Polar Shapes
Pyramidal
Bent

26. Molecular Polarity Ternary Compounds will generally be polar
Different elements have different electronegativities, so when there are more than two types of atoms the molecule will not be symetrical

27. What Does All This Mean?!?
Polarity determines several properties

28. “Like Dissolves Like”
Polar substances can be dissolved by other polar substances
Nonpolar substances can be dissolved by other nonpolar substances
Polar substances will NOT dissolve in nonpolar substances

29. “Like Dissolves Like”
When two liquids are capable of mixing, they are miscible
Immiscible means that they will not mix

30. “Like Dissolves Like” Water (H2O) is a POLAR substance
So is glass (SiO2)
When water is in a glass container, why does the meniscus curve up?

31. “Like Dissolves Like” We know glass is POLAR Hexane is NONPOLAR
Explain the behavior of this meniscus

32. “Like Dissolves Like”
Is density the only reason why your salad dressing separates??

33. More Properties

34. More Properties

35. What type of bear dissolves in water??