1. CHAPTER OUTLINE Electronegativity Polarity & Electronegativity
Lewis Structures
Molecular Shapes

2. Least electronegative
ELECTRONEGATIVITY
Linus Pauling derived a relative Electronegativity Scale based on Bond Energies.
Electronegativity (E.N.) is the ability of an atom involved in a covalent bond to attract the bonding electrons to itself.
F 4.0
Cs 0.7
Most electronegative
Least electronegative

3. Electronegativity increases

4. BOND POLARITY & ELECTRONEGATIVITY
Polarity is a measure of the inequality in the sharing of bonding electrons
The more different the electronegativity of the elements forming the bond
The larger the electronegativity difference (EN)
The more polar the bond formed

5. POLARITY & ELECTRONEGATIVITY
As difference in electronegativity increases
Bond polarity increases
Most polar
Least polar

6. POLARITY & ELECTRONEGATIVITY
difference
Bond Type
EN = 0
Non-polar covalent
0 < EN <1.7
Polar covalent
1.7 < EN
Ionic

7. The molecule is nonpolar covalent
POLARITY & ELECTRONEGATIVITY
The molecule is nonpolar covalent H
Hydrogen Molecule
Electronegativity
2.20
EN = 0

8. The molecule is polar covalent
POLARITY & ELECTRONEGATIVITY
The molecule is polar covalent H Cl
Hydrogen Chloride Molecule + -
Electronegativity
2.20
3.16
EN = 0.96

9. POLARITY & ELECTRONEGATIVITY
No molecule exists
The bond is ionic
Sodium Chloride
Na+
Cl-
Electronegativity
0.93
3.16
EN = 2.23

10. (similar electronegativities) (small to moderate EN)
SUMMARY OF BONDING
Ionic Bond
(large EN)
EN > 1.7
Non-polar
(similar electronegativities)
EN = 0
Polar
(moderate EN)
Covalent Bond
(small to moderate EN)
0 < EN < 1.7

11. COMPARING PROPERTIES OF IONIC & COVALENT COMPOUNDS
Structural Unit
Ions
Molecules
Melting Point
High
Low
Boiling Point
Solubility in H2O
Low or None
Electrical Cond.
None
Examples
NaCl, AgBr
H2, H2O

12. LEWIS STRUCTURES
In Lewis symbols, valence electrons for each element are shown as a dot.
Lewis structures use Lewis symbols to show valence electrons in molecules and ions of compounds.
Lewis symbols for the first 3 periods of representative elements are shown below:

13. LEWIS STRUCTURES
In a Lewis structure, a shared electron pair is indicated by two dots between the atoms, or by a dash connecting them.
Unshared pairs of valence electrons (called lone pairs) are shown as belonging to individual atoms or ions.

14. LEWIS STRUCTURES
Writing correct Lewis structures for covalent compounds requires an understanding of the number of bonds normally formed by common nonmetals.

15. LEWIS STRUCTURES
Structures must satisfy octet rule (8 electrons around each atom).
Hydrogen is one of the few exceptions and forms a doublet (2 electrons).
Covalent molecules are best represented with electron-dot or Lewis structures.

16. LEWIS STRUCTURES Non-bonding electrons must be displayed as dots.
Bonding electrons can be displayed by a dashed line.

17. LEWIS STRUCTURES
More complex Lewis structures can be drawn by following a stepwise method:
1. Count the number of electrons in the structure.
2. Draw a skeleton structure.
3. Connect atoms by bonds (dashes or dots).
4. Distribute electrons to achieve Octet rule.
5. Form multiple bonds if necessary.

18. Example 1: H O H Write Lewis structure for H2O Step 1: H2O
= 8 electrons
2 (1) + 6 = 8
Step 2:
 
H O H
Step 3:
Skeleton structure should be symmetrical
Hydrogen has doublet
4 electrons used 4 electrons remaining
Octet rule is satisfied
Step 4:

19. Example 2: O C O Write Lewis structure for CO2 Step 1: CO2
= 16 electrons
4 + 2(6) = 16
Step 2:
 
 
O C O  
Step 3:  
 
 
Skeleton structure should be symmetrical
Step 4:
Octet rule is NOT satisfied
10 electrons used 6 electrons remaining
4 electrons used 12 electrons remaining
Octet rule is satisfied
Step 5:

20. Octet rule is satisfied Octet rule is NOT satisfied
Example 3:
Write Lewis structure for CO32-
Step 1:
CO32-
= 24 electrons
4+3(6)+2 = 24
Step 2:
6 electrons remaining
12 electrons remaining
0 electrons remaining
18 electrons remaining
 
 
O C O O
Step 3:
 
 
Step 4:
Step 5:
Octet rule is satisfied
Octet rule is NOT satisfied

21. Octet rule is satisfied
Example 4:
Write Lewis structure for NH3
Step 1:
NH3
= 8 electrons
5 + 3(1) = 8
Step 2:
 
H N H H
Step 3:
Step 4:
Octet rule is satisfied

22. Octet rule is satisfied
Example 5:
Write Lewis structure for ClO3
Step 1:
ClO3
= 26 electrons
7+3(6)+1 = 26
Step 2:
0 electrons remaining
20 electrons remaining
8 electrons remaining
2 electrons remaining
14 electrons remaining
 
 
 
O Cl O O
Step 3:
 
 
Step 4:
Octet rule is satisfied

23. EVALUATING LEWIS STRUCTURES
When evaluating Lewis structures for correctness, two points must be considered:
Are the correct number of electrons present in the structure?
Is octet rule satisfied for all elements? (Hydrogen is an exception)

24. Structure is incorrect
Example 1:
Determine if each of the following Lewis structures are correct or incorrect. If incorrect, rewrite the correct structure.
2(1) = 12
Octet is incomplete
Octet is complete
Doublets are complete
Structure is incorrect

25. Structure has 14 electrons Structure is incorrect
Example 2:
Determine if each of the following Lewis structures are correct or incorrect. If incorrect, rewrite the correct structure.
Structure has 14 electrons
Only 12 electrons shown
2(5) + 4(1) = 14 2 4 2
Structure is incorrect 2 2
Octets are complete

26. MOLECULAR SHAPES
A very simple model , VSEPR (Valence Shell Electron Pair Repulsion) Theory, has been developed by chemists to predict the shape of large molecules based on their Lewis structures.
The three-dimensional shape of the molecules is an important feature in understanding their properties and interactions.
All binary molecules have a linear shape since they only contain two atoms.
More complex molecules can have various shapes (linear, bent, etc.) and need to be predicted based on their Lewis structures.

27. MOLECULAR SHAPES
Based on VSEPR, the electron pair groups in a molecule will repel one another and seek to minimize their repulsion by arranging themselves around the central atom as far apart as possible.
Electron pair groups can be defined as any one of the following:
bonding pairs
non-bonding pairs
multiple bonds

28. SUMMARY OF VSEPR SHAPES
Number of electron pair groups around central atom
Molecular
Shape
Bond
Angle
Examples
Bonding
Non-bonding 2
Linear
180
CO2 3
Trigonal planar
120
BF3 1
Bent
SO2 4
Tetrahedral
109.5
CH4
Pyramidal
NH3
H2O