ACTIVITY 09-2
Electronegativity and Bond Polarity
WHY?
Electronegativity is a measure of the ability of an atom in a molecule to attract electrons. The
difference in the electronegativities of two atoms profoundly affects the properties of the chemical
bond between the two atoms and, consequently, has dramatic effects on the physical and chemical
properties of materials. You therefore need to be able to identify polar bonds and polar molecules
from atomic electronegativities and molecular geometries (i.e., shapes).
LEARNING OBJECTIVES
•
Master the use of electronegativity to predict bond characteristics
•
Identify how bond characteristics affect material properties
SUCCESS CRITERIA
•
Correctly identify bonds in order of increasing polarity
•
Correctly identify polar molecules
PREREQUISITES
•
Activity 07-5: Periodic Trends in Atomic Properties
•
Activity 09-1: Valence Shell Electron Pair Repulsion Model
MODEL: ELECTRONEGATIVITY (EN) AND BOND CHARACTERISTICS
∆EN
Ion/Covalent Character
> 1.7
Mostly ionic
0.4 – 1.7
Polar covalent
< 0.4
Mostly covalent
0
Nonpolar covalent
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Foundations of Chemistry
KEY QUESTIONS
1. What is the general trend in electronegativity of atoms across the rows of the Periodic Table?
2. What is the general trend in electronegativity of atoms down the columns in the Periodic Table?
3. What kind of a bond is formed from two atoms that have the same electronegativity?
4. What kind of a bond is formed from two atoms that have very different electronegativities?
5. What is the relationship between electronegativity and bond polarity?
EXERCISES
1. Using position in the Periodic Table as the criterion (do not look at a table or chart of electronegativity values), arrange the elements in each of the following groups in order of increasing electronegativity. Use the < symbol in your arrangement.
(F, C, N, O)
(Cl, F, I, Br)
(O, S, F)
(Cs, Ca, Cu, S, Se, Cl)
(As, F, Sb, Cl, S, Se)
2. Perform tasks a) through d) for each of the following groups of bonds:
(C-F, Si-F)
(C-F, C-Cl, C-Br)
(N-O, N-F, C-F)
(Cl-Cl, F-Cl, F-Br, Br-Cl)
a) Order the bonds in order of increasing bond polarity. Use the < symbol in your arrangement.
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Chapter 9: Molecular Structures
b) Identify the atom that carries a slight positive charge in each bond.
) to indicate the direction and extent of electron density shift in each
c) Use an arrow (
bond, based on electronegativity values, with no arrow representing a nonpolar covalent bond.
The arrowhead points to the atom that is more electronegative. These arrows are called bond
dipoles.
d) Classify each bond as mostly ionic, polar covalent, or nonpolar covalent based on electronegativity values.
3. Identify whether or not metals and nonmetals always form mostly ionic bonds. Can you find metalnonmetal pairs that produce mostly covalent or polar covalent bonds?
INFORMATION
The overall molecular dipole is determined by adding the individual bond dipoles. In this addition
process, both the magnitude of the bond dipole and the geometry of the molecule contribute
because, if two bond dipoles with the same magnitude are pointing in opposite directions, then
they offset each other and the molecule is nonpolar.
Activity 09-2 Electronegativity and Bond Polarity
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Foundations of Chemistry
EXERCISES
4. Determine which of the following molecules are polar and show the directions of the molecular
dipoles. Note that lone pairs are not shown in the structures. Use your textbook to learn the meaning
of cis and trans if you do not already know.
Molecule
CO2
Structure
O
C
Polar or nonpolar?
O
H
H 2O
O
H
NH3
NF3
N
H
H
H
N
F
F
F
O
NO3-
O
N
O
Cl
CHCl3
C
Cl
H
Cl
F
BF3
B
F
F
F
CH2F2
C
F
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H
H
Chapter 9: Molecular Structures
Molecule
Structure
Polar or nonpolar?
Cl
cis- PCl3F2
C
Cl
P
F
F
F
trans- PCl3F2
C
Cl
P
Cl
F
H
H
CH3CH3
C
H
C
H
H
H
H
H
C
H
H
CH3CH2CH3
C
H
C
H
H
H
H
CH3NH2
C
H
N
H
H
H
H
C
CH3OH
H
O
H
O
H
CH3CH2OH
C
H
Activity 09-2 Electronegativity and Bond Polarity
H
H
H
C
H
H
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