Modern inorganic chemistry

STRUCTURE AND BONDING 33

ELECTRON AFFINITIES

Typical elements in Groups V, VI and VII would be expected to

achieve a noble gas configuration more easily by gaining electrons

rather than losing them. Electron affinity is a measure of the energy

change when an atom accepts an extra electron. It is difficult to

measure directly and this has only been achieved in a few cases; more

often it is obtained from enthalpy cycle calculations (p. 74).

Group trends

Table 2.4 gives the energy values for the reaction

1 mole

together with atomic and ionic radii.

Table 2.4

Atomic

number

9

17

35

53

85

Element

¥

a

Br

I

At

Atomic

radius* (g)

(nm)

0.064

0.099

0.111

0.130

—

Radius

ofX~ ion

(nm)

0.133

0.181

0.196

0.219

—

Electron

affinity

(kJmol"^1 )

-333

• 364


• 342
  -295


• 256

See footnote to Table 2.1.

Energy is evolved in each case. The table clearly indicates that
the electron affinity falls with the increasing size of the atom. The
anomalous value for fluorine is explained on the grounds that since
the fluorine atom is small, the incoming electron encounters strong
repulsion by the nine electrons already closely shielding the nucleus.
In each case, the ion produced by electron addition is larger than
the atom from which it was formed. After the addition of the first
electron, subsequent electron addition must take place against the
repulsion of a negatively-charged ion. Two-electron affinities are
known in only a few cases. The values for oxygen and sulphur are
given in Table 2.5.
Energy is released on formation of the singly-charged ion but a
greater amount of energy is required to add a second electron and