What is the oxidation state of chromium in the oxide CrO3?

The oxidation state is a direct measure of the number of valence electrons in the outer shell of an atom. Every atom wants its outer shell to be a complete 8. If it has fewer electrons, it tends to give them away and acquires a positive oxidation state.

If it has more electrons it tends to gain a few more electrons to complete its outer shell and acquires a negative oxidation state. Chromium has an oxidation state of +6. This is because oxygen, with an oxidation state of -2, can attach to the atom three times in the chromium oxide compound.

The oxidation state of chromium in CrO3 is +6. For those that don’t know, CrO3 is chromium trioxide. It’s a bright red powder, but turns orange when it’s wet. That’s really cool, isn’t it? Anyway, the oxidation state of a particular metal is the amount of electrons lost.

So, there are six more protons than electrons in the chromium trioxide. It follows that it has a positive charge, and there is a good reason for that. When there are more protons than there are electrons, the item has a positive charge. This is the same reason you get a static charge in the winter when you rub socks on a carpeted floor: you gather extra electrons, and discharge them at the door knob.