The Foundations of Chemistry

The value of EA for Cl can also be
represented as
5.79 10
^19 J/atom
or
3.61 eV/atom. The electron
volt (eV) is a unit of energy
(1 eV1.6022 10
^19 J).

246 CHAPTER 6: Chemical Periodicity

thus have an electron affinity (EA) equal to zero. We can represent the electron affinities

of helium and chlorine as

He(g)e
88n He
(g) EA
 00 0 kJ/mol

Cl(g)e
88nCl
(g)349 kJ EA
349 kJ/mol

The first equation tells us that helium will not add an electron. The second equation tells

us that when one mole of gaseous chlorine atoms gain one electron each to form gaseous

chloride ions, 349 kJ of energy is released (exothermic).Figure 6-3 shows a plot of electron

affinity versus atomic number for several elements.

Electron affinity involves the additionof an electron to a neutral gaseous atom. The

process by which a neutral atom X gains an electron (EA),

X(g)e
88nX
(g) (EA)

is notthe reverse of the ionization process,

X(g)e
88nX(g) (reverse of IE 1 )

The first process begins with a neutral atom, whereas the second begins with a positive

ion. Thus, IE 1 and EA are notsimply equal in value with the signs reversed. We see from

Figure 6-3 that electron affinities generally become more negative from left to right across

a row in the periodic table (excluding the noble gases). This means that most represen-

tative elements in Groups IA to VIIA show a greater attraction for an extra electron from

left to right. Halogen atoms, which have the outer electron configuration ns^2 np^5 , have the

most negative electron affinities. They form stable anions with noble gas configurations,

... ns^2 np^6 , by gaining one electron.

General trends in electron affinities
of A group elements with position in
the periodic table. There are many
exceptions.

Electron affinity

Increase

(Less negative)

Decrease

(More negative)

Figure 6-3 A plot of electron affinity versus atomic number for the first 20 elements. The

generalhorizontal trend is that electron affinities become more negative (more energy is

released as an extra electron is added) from Group IA through Group VIIA for a given

period. Exceptions occur at the IIA and VA elements.

Period

Atomic number

F

CO

Be N

B

HLi

He

K

Ar

Cl

S

P

Si

Al

Mg

Na

Ne

1 2 3 4

Ca

Electron affinity (kJ/mol)

0 5 10 15 20 25

 400

300

200

100

0

 100

 200

 300