Oxidation number prob!!?

Question

In which one of the following does iodine have an oxidation number of +1?

(1) I3 -
(2) IO 3 -
(3) NaIO4
(4) IO -

answer for this is (4)...Please help me by showing me how the oxidation state/number changes for all the 4 options..i really need help on this..thanks

(4) IO-
I = +1; O = -2

From the above, i have 1 question to ask..
Why is I = + 1 instead of +2 because O = -2?

Answer 1

the first ans. shows change in oxidation state. yes the ans is 4. The overall charge of the ion is -1. O is -2, part of that list of conditions.(ie. group IA is +1 etc.) so I must be +1.

Answer 2

(1) I3- The oxidation number is -1/3 or -0.33. It seems silly, but that's how it works out. Oxidation nyumbers have no physical significance. They are an arbitrary system of bookkeeping to help balance redox equations. Try to get the hang of this, because you're going to have to balance some tough equations.

(2) I = +5; O = -2
5 + 3(-2) = -1
So IO3-

(3) Na+ IO4-
I = +7; O = -2
7 + 4(-2) = -1

(4) IO-
I = +1; O = -2