ELECTRONEGATIVITY (9.5)
STUDY OBJECTIVES
- Describe the trends in electronegativity within the periodic table.
- Explain what is meant by a polar bond and predict the relative polarity
of bonds.
- Classify bonds in given substances as ionic, polar covalent, or covalent..
Electronegativity. Chemical bonds are rarely
purely covalent or completely ionic. Rather, most bonds exhibit some characteristics
of both. In the previous chapter we saw that atoms of the elements exhibit varying
tendencies in their ability to attract and hold free electrons in the gas phase.
In other words, electron affinity values show periodic variations. The term
electronegativity is used to describe
the ability of an atom within a molecule to attract a shared electron pair toward
itself.
Linus Pauling developed a method for determining the relative electronegativities
of the elements. These values are given in Figure 9.5 (text). Pauling assigned
the value 1.0 to Li and 4.0 to F. The values for second and third row elements
are given in Table 9.2. Electronegativity values exhibit periodic behavior.
In general, electronegativities increase from left to right across a period,
and decrease within a group from top to bottom as shown in Figure 9.3.
Table 9.2 Electronegativities of Second- and Third-Row Representative
Elements.
|
Second Row
|
Li
|
Be
|
B
|
C
|
N
|
O
|
F
|
| |
1.0
|
1.5
|
2.0
|
2.5
|
3.0
|
3.5
|
4.0
|
|
|
|
|
|
|
|
|
|
|
Third Row
|
Na
|
Mg
|
Al
|
Si
|
P
|
S
|
Cl
|
| |
0.9
|
1.2
|
1.5
|
1.8
|
2.1
|
2.5
|
3.0
|
Figure 9.3 Periodic trends in electronegativity.
As a consequence of the differing abilities of atoms in a bond to attract
the shared electron pair, most electron pairs are not shared equally. This imbalance
causes an electron pair to shift slightly toward the more electronegative atom,
giving rise to a polar covalent bond. In the HCl molecule, for instance,
the electronegativity c of Cl is 3.0, and
for H it is 2.1. The electronegativity difference 
c
is:
| c |
= cCl – cH |
| = 3.0 – 2.1 = 0.9 |
The chlorine atom with its higher electronegativity attracts the electron pair
more strongly. This makes the Cl atom slightly negative and H slightly positive.
Here d denotes a partial charge, that is,
a charge less than 1.0, as it would be in an ion.
Pure covalent bonding, which is the equal sharing of electron pairs, occurs
only in homonuclear diatomic molecules. Examples are H2, N2,
and Cl2. In a diatomic molecule with both atoms the same, c
must be zero, and the bonding electron pair is shared equally. Bonds of this
type are described as nonpolar covalent bonds, or pure covalent bonds.
In bonds involving different atoms, the electronegativity difference will
depend on the relative electronegativities of the two atoms. When Dc
2.0 a bond is mostly
ionic, for in this case one atom so outdoes
the other at attracting electron pairs that electrons can be considered to be
completely transferred to the more electronegative atom. Bonds between atoms
such that Dc < 2.0 are classified
as polar covalent bonds, or simply polar bonds. The "2.0 rule"
is an approximation and does not apply to all ionic compounds.
Nonpolar covalent bonds and completely ionic bonds represent extreme situations
in bonding. To refer to a bond as being "ionic" or "covalent" is an oversimplification.
Sometimes the term percent ionic character is used to describe the polar
nature of a bond. A pure covalent bond has zero percent ionic character. Bond
with Dc
2.0 will have at least 50%
ionic character and will be called ionic. There are no 100% ionic bonds. Compounds
of Group 1A or 2A metals with a nonmetal usually have ionic bonds, whereas compounds
of two nonmetals usually have polar covalent bonds. See Table 9.3 for examples.
Table 9.3 Bond Character of Some Common Bonds
|
| Ionic Compounds |
Dc |
|
| NaCl |
2.1 |
| NaBr |
1.9 |
| NaI |
1.6 |
| KCl |
2.2 |
| KBr |
2.0 |
| MgCl2 |
1.8 |
|
| Polar Covalent Bonds |
Dc |
|
| C—Cl |
0.5 |
| N—H |
0.9 |
| O—H |
1.4 |
| S—H |
0.4 |
| S—O |
1.0 |
| N—O |
0.5 |
|
EXAMPLE Bond Polarity
Arranging the following bonds in order of increasing ionic character: C—O,
C—H, and O—H.
C—O < O—H < C—H
C—O < C—H < O—H
C—H < C—O < O—H
C—H < O—H < C—O
O—H < C—O < C—H
O—H < C—H < C—O
Correct!
Click a Hint button for help.
As the electronegativity difference increases the bond becomes more polar and
its ionic character increases. Using 
Figure 9.5
(textbook), we can determine the differences.
for C—O Dc = 3.5 – 2.5 = 1.0
for C—H Dc = 2.5 – 2.1 = 0.4
for O—H Dc = 3.5 – 2.1 = 1.4
The ionic character increases in the order
C—H < C—O < O—H
•Comment
The electronegativity of hydrogen is unlike that of the other elements of Group
1A. In terms of electronegativity, hydrogen is similar to the nonmetal elements
boron and carbon. Bonds of H to nonmetal atoms are polar covalent bonds, rather
than ionic bonds, such as the bonds in LiCl and NaCl.
EXAMPLE Electronegativity Trends
Using the trends within the periodic table, determine which of the following
is the most electronegative element: As, Se, or S.
As
Se
S
Correct!
Click a Hint button for help.
Se and As are in the same period, and so the one further to the right has the
higher electronegativity. That one is Se. Now compare S and Se. They are in
the same group. The one nearer the top of the group has the greater electronegativity
which is sulfur.
EXAMPLE Types of Bonds
For the following pairs of elements label all bonds between them as ionic,
polar covalent, or pure covalent.
- Rb and Br
Correct!
Click a Hint button for help.
- S and S
Correct!
Click a Hint button for help.
- C and N
Correct!
Click a Hint button for help.
- Bond type depends on electronegativity differences. The value of Dc
for a Rb—Br bond is 2.0. Answer: RbBr is ionic.
- For S—S, Dc = 0.0, and so the bond
is a pure covalent bond.
- In the periodic table carbon and nitrogen are adjacent to each other in
period 2. The one on the right is N; it is more electronegative and so we expect
a polar covalent bond. Electronegativity values give Dc
= 0.5.
OBJECTIVE CHECK
Complete the following questions to check your understanding of the material.
Select the check button to see if you answered correctly.
- Which atom is the most electronegative?
Li
Cs
P
As
Ge
- List the following bonds in order of increasing ionic character:
N—O, Na—O, O—O, S—O
- Classify the O—H bond in CH3OH as ionic, polar covalent, or
nonpolar covalent.
ionic
polar covalent
nonpolar covalent
- Which of the following is a nonpolar covalent bond (pure covalent)?
H—Cl
Li—Br
Se—Br
Br—Br
- Classify the following bonds as ionic, polar covalent, or nonpolar covalent.
| Se—Cl |
|
| Al—Cl |
|
| K—F |
|
| Cl—Cl |
|
