Chapter 23 - The Transition Elements - 23-4 Concept Assessment - Page 1052: 23-4

The backbone $\mathrm{Cr}-\mathrm{O}-\mathrm{Cr}-\mathrm{O}-\mathrm{Cr}$ is surrounded by three tetrahedral structures in the $\left[\mathrm{Cr}_{3} \mathrm{O}_{10}\right]^{2-}$ anion.The central $\mathrm{Cr}$ atom of the backbone is bonded to two additional $\mathrm{O}$ atoms and the $\mathrm{Cr}$ atoms at the ends to three other $\mathrm{O}$ atoms. The overall number of $\mathrm{O}$ atoms= 10. The oxidation state of Cr is +6 while that of $\mathrm{O}$ is -2, which is responsible for the -2 charge on the anion.