JEE Main · 2020 · 09 Jan 2020 · Shift 2 · d and f Block Elements

In the chromate ion $\mathrm{Cr}{\mathrm{O}}_4^{2-}$ the chromium sits at the centre of a tetrahedron bonded to four oxygens, giving $4$ chromium-oxygen bonds. The dichromate ion $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}$ is two such tetrahedra joined through one shared bridging oxygen, so each chromium is bonded to four oxygens; counting the three terminal bonds on each chromium plus the two bonds to the bridge gives $3+3+2=8$ chromium-oxygen bonds. The total over both ions is $4+8=12$.