Iodine is at the bottom of the halogens and is the least reactive of the halogens. Iodine is an oxidizing agent, though not as strong as bromine or chlorine. It can become the colorless iodide ion if another atom or molecule gives an iodine atom one of its electrons. This is called reduction. Iodine reacts with hydrogen sulfide or hydrazine to make hydriodic acid. Iodine dissolves in oleum to make a bright blue solution of I and sulfur dioxide. This cation is also made when iodine reacts with antimony pentafluoride. 