Q.Give the oxidation state of halogen in the following.

Step 1. Fix the rule for fluorine.

Fluorine, being the most electronegative element, is always -1 in every compound, including its compounds with oxygen — this reverses the 'usual' assumption that oxygen is always -2, since with fluorine oxygen is instead forced to a positive state.

Step 2. Work out OF2.

Let oxidation state of O = x. Overall neutral molecule: x + 2(-1) = 0, so x = +2. Oxygen is +2, fluorine is -1.

Step 3. Work out O2F2.

Let oxidation state of each O = x. Overall neutral: 2x + 2(-1) = 0, so 2x = 2, x = +1. Each oxygen is +1 (this is the peroxide-type O-O linkage, analogous to peroxide oxygen normally being -1, but here reversed because of the more electronegative fluorine present), fluorine is -1.

Step 4. Work out Cl2O3.

Here oxygen reverts to its usual -2 (chlorine is less electronegative than oxygen). Let oxidation state of each Cl = x. Overall neutral: 2x + 3(-2) = 0, so 2x = 6, x = +3. Chlorine is +3.

Step 5. Work out I2O4. …

Apply the rule that fluorine is always -1 (it being more electronegative than oxygen), so in fluorine-oxygen compounds oxygen must be assigned a positive oxidation state to balance the neutral …

Mechanically assigning oxygen -2 in OF2 and O2F2 (as one would in a normal oxide) without recognising that fluorine's higher electronegati …