An Overview about the Chemical Element Iodine

Ernest Capraro's image for:
"An Overview about the Chemical Element Iodine"
Image by: 

Eons ago (or decades at least), iodine was commonly found in the medicine cabinet, used to sterilize cuts and scrapes. It has been replaced in this role by less harmful products, and as a result, people are less familiar with this fascinating element.

Iodine (I) is a dark blue-black color in its crystal form, with an almost metallic appearance. Don't let its looks fool you though, it doesn't behave anything like a metal. It belongs to the nonmetal, and is brittle. It won't conduct electricity or heat well either. In fact, heat iodine a little bit and it doesn't melt. Instead, it sublimes - it turns directly from a solid into a lovely purple gas.

Iodine belongs to the group of halogens (the same group as fluorine, chlorine, and bromine) and tends to react chemically in the same manner. That lovely purple gas just mentioned, for instance, will combine readily with water to form hydroiodic acid. Breathing iodine vapor is quite painful as a result, because the acid then forms in the dampness of your throat and lungs, causing painful burning in small amounts, and death in larger doses. Like all the halogens, iodine can react to form an ion with a charge of negative one (it gains an electron from another atom, usually a metal). In its elemental state, it exists as a diatomic molecule - two atoms of iodine bound together (by a single bond).

Iodine is a hazardous chemical, thanks to its reactivity (it is an oxidizer). The practice of sterilizing cuts with it was effective because it did destroy the bacteria that would infect a wound. On the down side, it would also damage human cells as well. It causes an intense burning sensation in cuts, and no wonder, since it actually causes chemical burns. Iodine is slightly soluble in water, which means it can be washed off without much trouble. (Just make sure you get plenty of water on the exposed surfaces, and spend some time at it.)

Iodine is a fairly large atom, and an even larger (sometimes referred to as "soft") ion. (The gain of an electron results in an expansion of the electron cloud, due to the increased repulsion of similar charges.) With an atomic mass of 126.9 amu (double that for the molecular weight), iodine is the largest stable halogen. (The larger ones are radioactive.) Its markedly larger size means that iodine has a lower electronegativity than the smaller halogens, and that in organic molecules it tends to form bonds that are only weakly polarized.

For chemistry teachers - iodine is an excellent chemical for demonstrating the phase changes of sublimation and deposition. Working in a fume hood (or other well ventilated area), a few iodine crystals are placed in the bottom of a small beaker, atop a hot plate (or small flame). A watch glass is placed over the top of the beaker, with a few ice cubes in it. The iodine is heated (low heat! too high and the vapors escape en masse) and students can easily visualize the purple gas as it forms and disperses to fill the beaker. The vapors then deposit as a solid again on the underside of the cold watch glass, giving it a shiny coating of tiny iodine crystals.

For safety information, visit:

More about this author: Ernest Capraro

From Around the Web