Question

What are the oxidation numbers of the underlined elements in the following and how do you rationalise your results?

KI₃

Answer 1

KI₃

In KI₃, the oxidation number (O.N.) of K is +1. Hence, the average oxidation number of I is `-1/3`. However, O.N. cannot be fractional. Therefore, we will have to consider the structure of KI₃ to find the oxidation states. 

In a KI₃ molecule, an atom of iodine forms a coordinate covalent bond with an iodine molecule.

+1  0   0   -1
K⁺  [I - I ← I]^-

Hence, in a KI₃ molecule, the O.N. of the two I atoms forming the I₂ molecule is 0, whereas the O.N. of the I atom forming the coordinate bond is -1.