Chemistry - A Molecular Science

Chapter 5 The Covalent Bond

C-H

H-I

Pb-Cl

H-ClC-O

Ag-Cl

O-H

Li-I

Si-O

Na-Cl

Rb-F

H-F

100 80 60 40 20 0

12 3

0

Tl-Cl

Dc

Percent Ionic Character

Mostly Covalent

Mostly Ionic

KBr

Figure 5.4 Percent ionic character in a bond versus the electronegativity difference between the bound atoms Bond types change continuously from covalent (blue) to ionic (yellow). Bonds that are over 50% i

onic are called ionic. Note that

bonds labled in white are considered polar covalent, but they are between a metal and a nonmetal. * The fact that these

atoms have such high electronegativities and form

covalent bonds with nonmetals is

an important feature that will be

used in Chapters 10 and 12 to explain some of their chemistry.

50% ionic, and that a bond is ionic if it is over 50% ionic. Thus, the C-H bond (

Δχ

= 0.4)

is less than 5% ionic and is considered

to be a covalent bond, while the HCl bond (

Δχ

=

1.0) is slightly less than 20% ionic, so it is considered to be a polar covalent bond. The KBr bond (

Δχ

= 2.3) is ~75% ionic, so it is considered to be ionic. No bond is 100% ionic

because every atom exerts some pull on the bonding electrons;

i.e

., no atom has zero

electronegativity. Hence, the bonding electron density near a metal in an ionic bond may be very low, but it is not zero.

The electronegativities of the nonmetals ar

e not that dissimilar, so bonds between

nonmetals are usually covalent or polar covalent. However, this is a broad generalization and care should be made in its use. For example,

Δχ

= 1.8 for an H-F bond, which makes

the bond between these two nonmetals

~55% ionic. Large values of

Δχ

occur most

frequently in bonds be

tween metals (low

) and nonmetals (high χ

), so these bonds are χ

usually

considered to be ionic, but care must also

be taken with this rule as well. Metals,

such as Ag, Hg, and Pb, that lie on the ri

ght side of the periodic table, have high

electronegativities (

= 1.9 for Ag and 2.3 for Pb), so their bonds to nonmetals are not χ

very ionic. In fact, the Ag-Cl and Pb-Cl bonds

(Δχ

= 1.1 and 0.7) are only ~30% and

~15% ionic, respectively!* Although there are exceptions, we will frequently make use of the generalization that

nonmetal-nonmetal bonds are covalent, while metal-nonmetal

bonds are ionic.

Example 5.1

Determine the more polar bond in each pair

and the direction of the bond dipole.

C-F or Si-F bond Fluorine is more electronegative than either C or Si; thus, the bond dipole points toward the fluorine atom in both bonds. The electronegativity difference in each bond is

Δχ

= χ

(F)

• χ

(Si or C). Because carbon is more electronegative than silicon,

Δχ

is greater for the Si-F

bond, which makes it the more polar bond. C-H or O-H bond χ(H) = 2.1, which makes it less electronegativ

e than most of the ot

her nonmetals; thus,

the bond dipole will point away from the

hydrogen atom. Oxygen is more electronegative

than carbon, which means that

the electronegativity difference,

Δχ

=

χ(O or C) -

χ(H), is

greater for the O-H bond

. The O-H bond is the more polar bond. It should be noted

that

Δχ

= 0.4 for a C-H bond. Such a low value for

Δχ

indicates that the C-H bond is

almost completely covalent (it has less than 5% ionic character).

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