316 GROUPVIhTHE HALOGENS
which the central atom exhibits a high co-ordination number. The
other larger halide ions show this tendency to a greatly diminished
extent and the complexes formed are usually less stable, although
certain metals (e.g. mercury) form iodo-complexes, for example
[HgI 4 ]^2 " which are more stable than fluoro- or chloro-complexes.
In certain cases there is insufficient space around the atom for as
many iodine atoms as for other halogens, for example phosphorus
forms pentahalides with fluorine, chlorine and bromine (and in the
case of fluorine the ion [PF 6 ] ~), but no pentaiodide. The large size
of iodine also accounts for the fact that there are few complexes
with more than four iodine ligands.
An important reason for low coordination of iodide ions is that
high coordination Implies a high oxidation state of the central atom,
which often (but not always) means high oxidising power—and this
means oxidation of the easily oxidised iodide ligands. Thus the non-
existence of, for example, phosphorus(V) pentaiodide is to be
explained by the oxidation of the iodide ligands and reduction of
phosphorus to the +3 state, giving only PI 3 , not PI 5.OCCURRENCE AND EXTRACTION
FLUORINE
Fluorine occurs widely in nature as insoluble fluorides. Calcium
fluoride occurs as fluospar or fluorite, for example in Derbyshire
where it is coloured blue and called 'bluejohn'. Other important
minerals are cryolite Na 3 AlF 6 (p. 141) and fluorapatite CaF 2 3Ca 3
(PO 4 ) 2. Bones and teeth contain fluorides and some natural water
contains traces.
Fluorine cannot be prepared directly by chemical methods. It is
prepared in the laboratory and on an industrial scale by electrolysis.
Two methods are employed: (a) using fused potassium hydrogen-
fluoride, KHF 2 , in a cell heated electrically to 520-570 K or (b)
using fused electrolyte, of composition KF :HF = 1:2, in a cell at
340-370 K which can be electrically or steam heated. Moissan, who
first isolated fluorine in 1886, used a method very similar to (b) and
it is this process which is commonly used in the laboratory and on
an industrial scale today. There have been many cell designs but the
cell is usually made from steel, or a copper-nickel alloy (^4 MoneF
metal). Steel or copper cathodes and specially made amorphous
carbon anodes (to minimise attack by fluorine) are used. Hydrogen
is formed at the cathode and fluorine at the anode, and the hydrogen
fluoride content of the fused electrolyte is maintained by passing in