How to find the Oxidation Number for Cr in CrO3 (Chromium trioxide)

To find the correct oxidation state of Cr in CrO3 (Chromium trioxide), and each element in the molecule, we use a few rules and some simple math. First, since the CrO3 molecule doesn’t have an overall charge (like NO3- or H3O+) we could say that the total of the oxidation numbers for CrO3 will be zero since it is a neutral molecule. We write the oxidation number (O.N.) for elements that we know and use these to figure out oxidation number for Cr. ---------- GENERAL RULES Free elements have an oxidation state of zero (e.g. Na, Fe, H2, O2, S8). In an ion the all Oxidation numbers must add up to the charge on the ion. In a neutral compound all Oxidation Numbers must add up to zero. Group 1 = +1 Group 2 = +2 Hydrogen with Non-Metals = +1 Hydrogen with Metals (or Boron) = -1 Fluorine = -1 Oxygen = -2 (except in H2O2 or with Fluorine) Group 17(7A) = -1 except with Oxygen and other halogens lower in the group ---------- We know that Oxygen usually is -2 with a few exceptions. When Oxygen is in a peroxide, like H2O2 (Hydrogen peroxide), it has a charge of -1. When it is bonded to Fluorine (F) it has an oxidation number of +2. Here it is bonded to Cr so the oxidation number on Oxygen is -2. Using this information we can figure out the oxidation number for the element Cr in CrO3.