THE TRANSITION ELEMENTS 377therefore only found in combination with oxygen and fluorine. Of
the lower oxidation states, the + 3 is the most stable and common.
COMPOUNDS OF CHROMIUM
Oxidation state + 6
In this state, chromium compounds are usually coloured yellow or
red (but due to charge transfer (p. 60) and not to the presence of d
electrons on the chromium ion). The only halide known is the
unstable chromium(VI) fluoride CrF 6 , a yellow solid. However,
oxide halides are known, for example CrO 2 Cl 2 Cchromyl chloride'),
formed as a red vapour when concentrated sulphuric acid is added
to a chromate(VI) (or dichromate) mixed with a chloride:
Cr 2 O?- -h 4Cr + 6H 2 SO 4 -> 2CrO 2 Cl 2 + 6HSO4 + 3H 2 O(This reaction may be used to distinguish a chloride from a bromide,
since CrO 2 Br 2 is unstable under these conditions).
The most important compounds containing Cr(VI) are the oxide
CrO 3 and the oxoanions CrOj", chromate(VI) and C^O^".
dichromate(VI).
CHROMIUM(VI) OXIDE (CHROMIUM TRIOXIDE)
Chromium trioxide is obtained as bright red crystals when concen-
trated sulphuric acid is added cautiously to a concentrated aqueous
solution of a chromate or dichromate(VI). It can be filtered off
through sintered glass or asbestos, but is a very strong oxidising
agent and so oxidises paper and other organic matter (hence the use
of a solution of the oxide — "chromic acid' — as a cleansing agent for
glassware).
Chromium(VI) oxide is very soluble in water ; initially, "chromic
acid', H 2 CrO 4 , may be formed, but this has not been isolated. If it
dissociates thus :
H 2 CrO 4 ^H+ + HCrO;
then the HCrO^ ions probably form dichromate ions :
^ CrO~ + HO
Chromium(VI) oxide is acidic, and the corresponding salts are the
chromates and dichromates, containing the ions CrO| ~ and Cr 2 O7 ",
i.e. [CrO 4 4- CrO 3 ]^2 ~. The oxidation state of chromium is +6 in
each ion (cf. sulphur in SO^" and S 2 O7~).