Structure of Chromate and Dichromate Ions

Both the chromate ion (CrO₄²⁻) and the dichromate ion (Cr₂O₇²⁻) are oxoanions of chromium, and both act as moderately strong oxidising agents; in both ions, without exception, the chromium is present in the +6 oxidation state -- the difference between the two ions is purely structural (how many chromium centres and oxygen atoms are joined together), not a difference in the oxidation state of the chromium itself.

In aqueous solution, the chromate and dichromate ions exist in a genuine chemical equilibrium and are readily interconvertible into one another, with the position of that equilibrium controlled directly by the solution's pH: in an alkaline (basic) solution, the chromate ion (CrO₄²⁻) is the predominant species present, whereas in an acidic solution, the dichromate ion (Cr₂O₇²⁻) becomes the predominant species -- essentially, two chromate ions combine (with loss of a water molecule) to form one dichromate ion under acidic conditions, and this condensation reverses under basic conditions.

The precise structures of the two ions, as revealed by X-ray crystallographic determination, are as follows. The chromate ion, CrO₄²⁻ (Figure 4.8a), adopts a regular TETRAHEDRAL geometry: the central chromium atom is bonded to four oxygen atoms arranged symmetrically at the corners of a tetrahedron, and all four Cr-O bonds are equal in length, measured at 166 pm -- this bond-length equality across all four positions is consistent with resonance delocalisation of the ion's overall -2 charge symmetrically across all four oxygen atoms, rather than the charge being localised on any particular pair of oxygens. …