Question

I3- Oxidation number

Answer 1

-1

Answer 2

I don't agree with this answer but as I look on the web I find that most answers do agree with your answer.
By rule, and after all oxidation numbers are just a set of made up rules, the oxidation number (ON) of a mono-atomic ion is the charge on an ion. Therefore, the ON of I^- is -1. By rules the SUM of the oxidation states of a polyatomic ion is equals the charge on the ion. Therefrore, the charge on the [I^3]^- is -1/3 + -1/3 + -1/3 = -1 so EACH ion has an ON of -1/3. Yes, I know that [I3]^- is made up of I2 (zero ON) + I^- to make I3^- and many of the answers I see on the web wax eloquently about the bonding electrons and they give zero ON to the I2 and -1 ON to the lone I^- and follow then with [I3]^- being -1. That's baloney in my opinion. BY RULE, the ON of each I is -1/3. The CHARGE in the [I3]^- is -1 but the ON is -1/3 for each. Likewise, the charge on the I in KI is -1 but the ON of IODINE (NOT IODIDE ION) IS -1