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1
CHAPTER OUTLINE���
Electronegativity Polarity & Electronegativity Lewis Structures Molecular Shapes
2
ELECTRONEGATIVITY���
Electronegativity (E.N.) is the ability of an atom involved in a covalent bond to attract the bonding electrons to itself.
Linus Pauling derived a relative Electronegativity Scale based on Bond Energies.
Cs 0.7
F 4.0
Least electronegative
Most electronegative
3
ELECTRONEGATIVITY���
Electronegativity increases
4
BOND POLARITY &���ELECTRONEGATIVITY���
The more polar the
bond formed
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 (ΔEN)
5
POLARITY &���ELECTRONEGATIVITY���
As difference in electronegativity
increases
Bond polarity increases
Most polar
Least polar
6
POLARITY &���ELECTRONEGATIVITY���
Electronegativity difference
Bond Type
ΔEN = 0 Non-polar covalent
0 < ΔEN <1.7 Polar covalent
1.7 < ΔEN Ionic
7
H H
Hydrogen Molecule
The molecule is nonpolar covalent
Electronegativity 2.20
Electronegativity 2.20
POLARITY &���ELECTRONEGATIVITY���
ΔEN = 0
8
H Cl
Hydrogen Chloride Molecule
Electronegativity 2.20
Electronegativity 3.16
The molecule is polar covalent
δ+ δ-
ΔEN = 0.96
POLARITY &���ELECTRONEGATIVITY���
9 Sodium Chloride
Na+ Cl-
Electronegativity 0.93
Electronegativity 3.16
The bond is ionic No molecule exists
ΔEN = 2.23
POLARITY &���ELECTRONEGATIVITY���
10
SUMMARY���OF BONDING���
Ionic Bond (large ΔEN)
Covalent Bond (small to moderate ΔEN)
Non-polar (similar electronegativities)
Polar (moderate ΔEN)
EN > 1.7
EN = 0
0 < EN < 1.7
11
COMPARING PROPERTIES���OF IONIC & COVALENT���
COMPOUNDS���
Ionic Covalent
Structural Unit Ions Molecules
Melting Point High Low
Boiling Point High Low
Solubility in H2O High Low or None
Electrical Cond. High None Examples NaCl, AgBr H2, H2O
12
LEWIS ���STRUCTURES���
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:
In Lewis symbols, valence electrons for each element are shown as a dot.
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���
Covalent molecules are best represented with electron-dot or 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).
16
LEWIS���STRUCTURES���
Bonding electrons can be displayed by a dashed line.
Non-bonding electrons must be displayed as dots.
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:���
Write Lewis structure for H2O
H2O = 8 electrons 2 (1) + 6 = 8 Step 1:
Step 2: H O H
Skeleton structure should be
symmetrical
Step 3:
4 electrons used 4 electrons remaining Step 4:
• •
• •
Octet rule is satisfied
Hydrogen has doublet
19
Example 2:���
Write Lewis structure for CO2
CO2 = 16 electrons
4 + 2(6) = 16 Step 1:
Step 2:
O C O Skeleton structure should be
symmetrical
Step 3:
4 electrons used 12 electrons remaining
Step 4:
• •
• •
Octet rule is satisfied
• •
10 electrons used 6 electrons remaining
• •
• •
• •
Octet rule is NOT satisfied
Step 5:
20
Example 3:���
Write Lewis structure for CO32-
CO32- = 24
electrons 4+3(6)+2 = 24 Step 1:
Step 2:
O C O
O Step 3:
Step 4:
• •
• •
• •
• •
• •
• •
• •
• •
• •
18 electrons remaining
12 electrons remaining 6 electrons remaining 0 electrons remaining
Octet rule is satisfied Octet rule is NOT satisfied
Step 5:
21
Example 4:���
Write Lewis structure for NH3
NH3 = 8 electrons 5 + 3(1) = 8 Step 1:
Step 2:
H N H
H Step 3:
Step 4:
• •
Octet rule is satisfied
22
Example 5:���
Write Lewis structure for ClO3-
ClO3- = 26
electrons 7+3(6)+1 = 26 Step 1:
Step 2:
O Cl O
O Step 3:
Step 4:
• •
• •
• •
• •
• •
• •
• •
• •
• •
20 electrons remaining
14 electrons remaining 8 electrons remaining 2 electrons remaining
Octet rule is satisfied
• • 0 electrons remaining
23
EVALUATING LEWIS���STRUCTURES���
When evaluating Lewis structures for correctness, two points must be considered:
1. Are the correct number of electrons present in the structure?
2. Is octet rule satisfied for all elements? (Hydrogen is an exception)
24
Example 1:���
Determine if each of the following Lewis structures are correct or incorrect. If incorrect, rewrite the correct structure.
2(1) + 4 + 6 = 12
Octet is complete Doublets are complete
Octet is incomplete
Structure is incorrect
25
Example 2:���
Determine if each of the following Lewis structures are correct or incorrect. If incorrect, rewrite the correct structure.
2(5) + 4(1) = 14
Structure is incorrect
Only 12 electrons shown
2 2
2 2 4
Structure has 14 electrons
Octets are complete
26
MOLECULAR���SHAPES���
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.
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.
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 0 Linear 180 CO2
3 0 Trigonal planar 120 BF3
2 1 Bent 120 SO2
4 0 Tetrahedral 109.5 CH4
3 1 Pyramidal 109.5 NH3
2 2 Bent 109.5 H2O