322 GROUP VII: THE HALOGENS
temperature, for example K, B, Si P. S, I. Other elements ignite
when gently warmed in the gas. for example Ag and Zn, and even
gold, platinum and xenon are attacked if heated strongly. Graphite
is attacked slowly—hence the use of special electrodes in the
extraction of fluorine—and diamond only above 950 K. Some
metals, for example copper and nickel alloys, become coated with
a superficial layer of fluoride. This prevents further reaction and
hence vessels of these materials are used for the preparation and
storage of fluorine. Oxygen and nitrogen do not combine directly
with fluorine.
Chlorine reacts with most elements, both metals and non-metals
except carbon, oxygen and nitrogen, forming chlorides. Sometimes
the reaction is catalysed by a trace of water (such as in the case of
copper and zinc). If the element attacked exhibits several oxidation
states, chlorine, like fluorine, forms compounds of high oxidation
state, for example iron forms iron(III) chloride and tin forms tin(IV)
chloride. Phosphorus, however, forms first the trichloride, PC1 3 ,
and (if excess chlorine is present) the pentachloride PC1 5.
Bromine has a lower electron affinity and electrode potential than
chlorine but is still a very reactive element. It combines violently
with alkali metals and reacts spontaneously with phosphorus,
arsenic and antimony. When heated it reacts with many other
elements, including gold, but it does not attack platinum, and
silver forms a protective film of silver bromide. Because of the
strong oxidising properties, bromine, like fluorine and chlorine,
tends to form compounds with the electropositive element in a
high oxidation state.
Iodine, though generally less reactive than bromine, combines
directly with many elements, for example silver, gold and aluminium,
forming iodides. Mercury is also attacked and mercury(I) iodide.
Hg 2 I 2 , is first formed but in the presence of excess iodine this is
oxidised to mercury(II) iodide, HgI 2. Iodine and phosphorus (red
and white) react in the presence of water to form first phosphorus(III)
iodide, PI 3 , which is then hydrolysed to yield hydrogen iodide (p.
333). Iodine reacts with the other halogens to form interhalogen
compounds (p. 345).
WITH COMPOUNDSThe reactions with water
The oxidising power of fluorine is seen in its reaction with water: in
the liquid phase, water reacts to give hydrogen peroxide and some