The Foundations of Chemistry

This reasoning is similar to that used

to explain the low IE 1 values for

Group VIA elements.

6-4 Electron Affinity 247

Elements with very negative electron affinities gain electrons easily to form nega-

tive ions (anions).

“Electron affinity” is a precise and quantitative term, like “ionization energy,” but it is
difficult to measure. Table 6-2 shows electron affinities for several elements.
For many reasons, the variations in electron affinities are not regular across a period.
The general trend is: the electron affinities of the elements become more negative from
left to right in each period. Noteworthy exceptions are the elements of Groups IIA and
VA, which have less negative values than the trends suggest (see Figure 6-3). It is very
difficult to add an electron to a IIA metal atom because its outer ssubshell is filled. The
values for the VA elements are slightly less negative than expected because they apply to
the addition of an electron to a half-filled set of nporbitals (ns^2 np^3 nns^2 np^4 ), which
requires pairing. The resulting repulsion overcomes the increased attractive force of the
nucleus.
Energy is always required to bring a negative charge (electron) closer to another nega-
tive charge (anion). So the addition of a second electron to a 1
anion to form an ion
with a 2
charge is always endothermic. Thus, electron affinities of anionsare always posi-
tive.

EXAMPLE 6-3 Trends in EAs

Arrange the following elements in order of increasing values of electron affinity, that is, from
most negative to least negative.

K, Br, Cs, Cl

Plan

Table 6-2 shows that electron affinity values generally become more negative from left to right
across a period, with major exceptions at Groups IIA (Be) and VA (N). They generally become
more negative from bottom to top.

Cu

–118

Ag

–125

Au

–282

H

–73

Li

–60

Na

–53

K

• 48
  Rb


• 47
  Cs


• 45

Be

(~0)

Mg

(~0)

Ca

Sr

Ba

B

–29

Al

• 43
  Ga
  –29
  In
  –29
  Tl
  –19

C

–122

Si

–134

Ge

–119

Sn

–107

Pb

–35

N

0

P

–72

As

–78

Sb

–101

Bi

–91

O

–141

S

–200

Se

–195

Te

–190

F

–328

Cl

–349

Br

–324

I

–295

He

0

Ne

0

Ar

0

Kr

0

Xe

0

(~0)

(~0)

(~0)

1 2 3 4 5 6

TABLE 6-2 Electron Affinity Values (kJ/mol ) of Some Elements*

*Estimated values are in parentheses.