Chapter 9 chemical bonds 503 it may be more

Chapter 9 Chemical Bonds
503

it may be more appropriate to draw the covalent bond as
with the electrons in the bond being nearer to the F atom than the H atom. Because
the electrons in the bond are nearer to the F atom, this side of the molecule takes
on a partial negative charge, which is represented by δ − ( δ is the lowercase Greek
letter delta). The other side of the molecule, the H atom, adopts a partial positive
charge, which is represented by δ +:
A covalent bond between different atoms that attract the shared electrons by
different amounts and cause an imbalance of electron distribution is called a polar
covalent bond
14
.
Technically, any covalent bond between two different elements is polar. However,
the degree of polarity is important. A covalent bond between two different
elements may be so slightly imbalanced that the bond is, essentially, nonpolar. A
bond may be so polar that an electron actually transfers from one atom to another,
forming a true ionic bond. How do we judge the degree of polarity?
Scientists have devised a scale called electronegativity
15
, a scale for judging how
much atoms of any element attract electrons. Electronegativity is a unitless
number; the higher the number, the more an atom attracts electrons. A common
scale for electronegativity is shown in Figure 9.2 "Electronegativities of the
Elements" .
14. The unequal sharing of
electrons in a covalent bond.
15. A qualitative scale for judging
how much atoms of any
element attract electrons.
Chapter 9 Chemical Bonds
9.4 Other Aspects of Covalent Bonds 504

Figure 9.2 Electronegativities of the Elements
Electronegativities are used to determine the polarity of covalent bonds.
The polarity of a covalent bond can be judged by determining the difference of the
electronegativities of the two atoms involved in the covalent bond, as summarized
in the following table:
Electronegativity Difference Bond Type
0 nonpolar covalent
0–0.4 slightly polar covalent
0.4–1.9 definitely polar covalent
>1.9 likely ionic
Chapter 9 Chemical Bonds
9.4 Other Aspects of Covalent Bonds 505

EXAMPLE 7
What is the polarity of each bond?
1. C–H
2. O–H
Solution
Using Figure 9.2 "Electronegativities of the Elements" , we can calculate the
difference of the electronegativities of the atoms involved in the bond.
1. For the C–H bond, the difference in the electronegativities is 2.5 − 2.1 =
0.4. Thus we predict that this bond will be slightly polar covalent.
2. For the O–H bond, the difference in electronegativities is 3.5 − 2.1 = 1.4,
so we predict that this bond will be definitely polar covalent.
Test Yourself
What is the polarity of each bond?
1. Rb–F
2. P–Cl
Answers
1. likely ionic
2. polar covalent
The polarity of a covalent bond can have significant influence on the properties of
the substance. If the overall molecule is polar, the substance may have a higher
melting point and boiling point than expected; also, it may or may not be soluble in
various other substances, such as water or hexane.