Modern inorganic chemistry

GROUP VII: THE HALOGENS 313
promotion to 3s requires too much energy. Thus fluorine is normally
confined to a valency of 1 although in some solid fluorides bridge
structures M—F—M are known in which fluorine acquires a
covalency of 2.
All the remaining halogens have unfilled d orbitals available and
the covalency of the element can be expanded. Compounds and
complex ions are formed both with other halogens and with oxygen
in which the halogen can achieve a formal oxidation state as high
as + 7, for example chlorine has formal oxidation states of +1 in
the chlorate(I) anion CIO" ; -f 5 in the chlorate(V) anion CIO 3, and

• 7 in the chlorate(VII) anion C1OJ.

ELECTRODE POTENTIALS AND REACTIVITY OF THE

HALOGENS

One surprising physical property of fluorine is its electron affinity

which, at — 333kJmol~l, is lower than that of chlorine, —364

kJmol"^1 , indicating that the reaction X(g) + e" -» X~(g) is more

exothermic for chlorine atoms. In view of the greater reactivity of

fluorine a much higher electron affinity might reasonably have been

expected. The explanation of this anomaly is found when the steps

involved in a complete reaction are considered. For example, with

a Group I metal ion M+(g) the steps to form a crystalline solid are,

(1) iX 2 (g) -* X(g) Bond dissociation enthalpy

(2) X(g) + e~ -* X"(g) Electron affinity

(3) X~(g) 4- M + (g) -> M+X~(s) Lattice enthalpy

the overall reaction being

e~ +±X 2 (g) + M + (g)-M+X-(s)

The enthalpies for the reactions of chlorine and fluorine are shown

graphically in Figure 112 as the relevant parts of a Born-Haber

cycle. Also included on the graph are the hydration energies of the

two halogen ions and hence the enthalpy changes involved in the

reactions

iX 2 (g) + <?-^-

The very low bond dissociation enthalpy of fluorine is an important

factor contributing to the greater reactivity of fluorine. (This low

energy may be due to repulsion between non-bonding electrons on

the two adjacent fluorine atoms.) The higher hydration and lattice

enthalpies of the fluoride ion are due to the smaller size of this ion.