GROUP VII: THE HALOGENS 315
Electron affinity and hydration energy decrease with increasing
atomic number of the halogen and in spite of the slight fall in bond
dissociation enthalpy from chlorine to iodine the enthalpy changes
in the reactions
|X 2 (g) + M+(g) + e~ -> M + X~(s) (11.1)|X 2 (g) + <r-^X-(aq) (11.2)both decrease and the reaction becomes less exothermic. Hence the
reactivity and the electrode potential (which is closely related to
reaction (11.2) and indeed defined by it under standard conditions)
decrease from fluorine to iodine. Table 11.2 gives the enthalpy
change (kJ mol~ *) for each halogen in reaction (1 1.2).
ELECTR ONEGATIYITY
The large value for fluorine, and the marked decrease from fluorine
to iodine, are points to be noted. The high value for fluorine means
that the bond between an element M and fluorine is likely to be
more ionic (more polar) than a bond formed by M with any other
elements. The low value for iodine indicates the possibility that
iodine may be electropositive in some of its compounds.
OXIDISING PROPERTIES
For fluorine, the reaction
is energetically highly favourable for the formation both of X~ and
of X~(aq). Hence gaseous fluorine is highly reactive towards metals,
giving essentially ionic fluorides; and in solution (as its high
electrode potential indicates) it is one of the most powerful oxidising
agents, oxidising water very readily (p. 100). Hence the fluoride ion
cannot be converted into fluorine in aqueous solution ; electrolysis
of a found fluoride must be used. In contrast, iodide ions in solution
are readily oxidised even by air (Table 4.3).
HALOGENS AS LIGANDS
The small fluoride ion shows a great tendency to act as a ligand and
form complex ions, for example [A1F 6 ]^3 ~, [PF 6 ]~, [FeF 6 ]^3 ~ in