Chemistry · D And F Block Elements · NEET
You get +10 only if you wrongly treat all five oxygens as normal oxide ions (each -2). That gives x + 5(-2) = 0, so x = +10. But +10 is impossible for chromium, because its highest possible oxidation state is +6 (it only has 6 electrons in 3d and 4s to lose). So the -2 rule cannot apply to all oxygens here. This is a signal that CrO5 must contain peroxide oxygens.
CrO5 is written more correctly as CrO(O2)2. It has one oxo oxygen (a normal O double-bonded to Cr, charge -2) and two peroxo groups (two O-O linkages, each group is O2 with charge -2). Its shape is often called a 'butterfly' structure. So there are not five separate -2 oxide ions; there is 1 oxo oxygen plus 4 oxygens that are joined in pairs as peroxides.
In a normal oxide, oxygen is -2. But in a peroxide the two oxygens share a bond (O-O), so each oxygen is only -1. In CrO5 the four peroxo oxygens are -1 each (total -4), and the one oxo oxygen is -2. Adding gives -6 from oxygen. This is the key NEET fact that changes the answer from +10 to +6.
Let Cr = x. Method 1 (by peroxo groups): 1 oxo oxygen = -2, and 2 peroxide groups = 2 x (-2) = -4. So x + (-2) + (-4) = 0, giving x = +6. Method 2 (by each atom): 1 oxo oxygen = -2, four peroxo oxygens = 4 x (-1) = -4. So x + (-2) + (-4) = 0, giving x = +6. Both methods give Cr = +6.
Yes. When acidified potassium dichromate (K2Cr2O7) is treated with hydrogen peroxide (H2O2), it forms a deep blue CrO5 (chromium peroxide). This blue colour is a common lab test for chromate/dichromate. Even though a peroxide reagent (H2O2) is used, the chromium stays at +6 because the peroxide oxygens carry the extra reduced charge, not the metal.
The oxidation state of Cr in CrO5 is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is +6. CrO5 has the structure CrO(O2)2 with one oxo oxygen (-2) and two peroxide links (each -2 in total), so the oxygens add to -6 and Cr comes out as +6.
+10 comes from wrongly assuming all five oxygens are normal oxide ions (-2 each). CrO5 actually has peroxide oxygens (-1 each), so the total oxygen charge is only -6, not -10. Also, chromium can never reach +10; its maximum is +6.
There are two peroxide (O-O) linkages. Together with one double-bonded oxo oxygen, that accounts for all five oxygen atoms (1 oxo + 4 peroxo).
CrO5 (blue) is formed when acidified potassium dichromate reacts with hydrogen peroxide (H2O2). This blue colour is used as a test for chromate and dichromate ions.
NEET repeatedly tests peroxo species like CrO5 as a 'trick' oxidation-state question. If you remember that Cr cannot exceed +6 and that CrO5 has peroxide links, you avoid the common +10/+12 trap and answer in seconds.
