The
chromates and dichromates are interconvertible in aqueous solution
depending upon pH of the solution The oxidation state of chromium
in chromate and dichromate is the same 2 CrO4
2– + 2H
+ ® Cr2O7
2– + H2O
Cr2O7
2– + 2 OH
- ® 2 CrO4
2– + H2O
The 
structures 
of
chromate ion, CrO4
2– and
the dichromate ion, Cr2O7
2–
are shown below The
chromate ion is tetrahedral
whereas the dichromate ion
consists of two tetrahedra
sharing one corner with
Cr–O–Cr bond angle of 126°