173

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

Electronegativity and Bond PolarityACTIVITY 09-2

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Foundations of Chemistry

KEY QUESTIONS1. 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?

EXERCISES1. Using position in the Periodic Table as the criterion (do not look at a table or chart of electronega-

tivity values), arrange the elements in each of the following groups in order of increasing electro-negativity. 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.

Activity 09-2 Electronegativity and Bond Polarity 175

b) Identify the atom that carries a slight positive charge in each bond.

c) Use an arrow ( ) to indicate the direction and extent of electron density shift in each 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 electronega-tivity values.

3. Identify whether or not metals and nonmetals always form mostly ionic bonds. Can you fi nd metal-nonmetal pairs that produce mostly covalent or polar covalent bonds?

INFORMATIONThe 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.

Chapter 9: Molecular Structures176

Foundations of Chemistry

EXERCISES4. 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 Structure Polar or nonpolar?

CO2

CO O

H2O OH

H

NH3N

H HH

NF3N

F FF

NO3- O N

O

O

CHCl3

Cl

CCl Cl

H

BF3

FB

F

F

CH2F2

F

CF H

H

Activity 09-2 Electronegativity and Bond Polarity 177

Molecule Structure Polar or nonpolar?

cis- PCl3F2 C PF

Cl

Cl

F

trans- PCl3F2 C PCl

Cl

F

F

CH3CH3 C

H

HH

C

H

HH

CH3CH2CH3

C H

HH

C

H

HH

C

HH

CH3NH2 N

HH

H

C

HH

CH3OH

H

C

HH

H

O

CH3CH2OH

O H

C

H

HH

C

HH