Oxidation State of Chromium in CrO5 (The Peroxide Trap)

If you treat all 5 oxygens as normal -2, you get x + 5(-2) = 0, so x = +10. But +10 is impossible because chromium only has 6 valence electrons (3d5 4s1), so its maximum oxidation state is +6. CrO5 actually contains peroxide oxygens (-1), which lowers the count to +6.

CrO5 has two peroxide linkages (two O-O bonds). That means 4 of its 5 oxygen atoms are peroxo oxygens with oxidation state -1. Only 1 oxygen is doubly bonded to Cr as a normal oxide oxygen (-2). Its formula is often written CrO(O2)2 to show this butterfly-shaped structure.

Let Cr = x. There are 4 peroxo oxygens at -1 and 1 oxide oxygen at -2. The molecule is neutral, so: x + 4(-1) + 1(-2) = 0, which gives x - 4 - 2 = 0, so x = +6. Chromium is +6.

Whenever a compound contains an O-O single bond (peroxide or superoxide), the linked oxygens are NOT -2. In a peroxide, each such oxygen is -1. CrO5 is a peroxo compound (blue in ether), so you must assign -1 to the 4 oxygens in the two O-O bonds. This is the exact point NEET tests.

Chromium has electronic configuration [Ar] 3d5 4s1, giving 6 electrons available for bonding. So its maximum oxidation state is +6 (as in CrO3, K2Cr2O7, K2CrO4). Any answer above +6 for chromium must be wrong - this alone tells you +10 is a trap.

Yes. CrO5 is chromium peroxide, formed when acidified dichromate reacts with hydrogen peroxide (H2O2). It is deep blue in ether and contains two peroxide (O-O) linkages, which is why chromium stays +6 and not +10.

+6. This was directly asked in NEET 2019 (Odisha). The answer is +6 because of the two peroxide bonds.

Chromium's electron configuration is [Ar] 3d5 4s1, so only 6 electrons can be lost or shared. Its maximum possible oxidation state is +6, making +10 chemically impossible.

CrO5 has a butterfly (or pentagonal) structure: one oxygen doubly bonded to Cr (as Cr=O) and two side-on peroxide (O-O) units bonded to chromium, written as CrO(O2)2.