376 THE TRANSITION ELEMENTS
one electron required (and also giving the vanadium a formal
oxidation state of — 1).TESTS FOR VANADIUMThe colour sequence already described, for the reduction of van-
adium(V) to vanadium(II) by zinc and acid, gives a very character-
istic test for vanadium. Addition of a few drops of hydrogen peroxide
to a vanadate(V) gives a red colour (formation of a peroxo-complex)
(cf. titanium, which gives an orange-yellow colour).CHROMIUMTHE ELEMENTChromium occurs quite extensively, mainly as the ore chromite or
chrome ironstone, a mixed oxide of iron(II) and chromium(III).
Presence of chromium in the mineral beryl produces the green
colour of emeralds and the red colour of ruby is due to the substitu-
tion of Cr(III) for Al(III) in the mineral alurmnium(III) oxide; hence
the name ^chromium' derived from the Greek for colour. Direct
reduction of chromate by heating with carbon and calcium oxide
gives an alloy of iron and chromium, ferrochrome, which can be
added to steel, to make stainless steel (12-15 % chromium). The pure
metal can be prepared by reduction of the + 3 oxide, Cr 2 O 3 , using
powdered aluminium, or by electrolytic reduction of the + 6 oxide
CrO 3. The metal is extensively used in chromium plating because
it is relatively inert to chemical attack. However, the extent of inert-
ness is dependent on its purity. It is inert to the oxidising oxo-acids
(phosphoric, nitric, aqua-regia, concentrated sulphuric); these
render it passive, probably by formation of a surface layer of oxide.
It remains bright in air, despite formation of a surface layer of oxide.
When pure (no oxide layer) it is readily soluble in dilute hydro-
chloric acid (to give a chromium(II) cation, see below) and displaces
copper, tin and metal from solutions of their salts.
In the older form of the periodic table, chromium was placed in
Group VI, and there are some similarities to the chemistry of this
group (Chapter 10). The outer electron configuration, 3d^5 4s^1.
indicates the stability of the half-filled d level, 3d^5 4sl being more
stable than the expected 3d^4 4s^2 for the free atom. Like vanadium
and titanium, chromium can lose all its outer electrons, giving
chromium(VI); however, the latter is strongly oxidising and is