In the lower right corner of the periodic table of elements, at the very bottom of group 17, a well-known element is hiding. Its name is iodine, and some will immediately remember the characteristic purple colour of its crystals.

This element, along with fluorine, chlorine and bromine, belongs to the group of halogen elements that were named after their ability to form salts in which they are included as anions (e.g. sodium chloride, potassium bromide or cesium iodide). It is one of the elements necessary for living, given that it makes a part of the thyroid hormone.

In nature, it is almost exclusively found in the form of chemical compounds, most often salt, which is a consequence of its great reactivity. Nevertheless, elemental iodine is often used in laboratory experiments, as well as in petrochemistry, and we are going to say a few words about it today.

At first glance, the crystals of elemental iodine are almost black, while under a strong light source they are deep purple. You may remember Lugol’s solution (a solution of iodine and potassium iodide) which is dark purple and iodine vapour is also purple. In its elemental state, two atoms of iodine together form a molecule, which, together with other similar molecules, forms a crystal, so iodine is a solid crystalline substance at room temperature.

If we were to heat those crystals, we would notice something interesting! Namely, contrary to expectations, iodine will not dissolve into a liquid by heating, as, for example, the ice dissolves into liquid water by heating, but purple vapour will already start to come out of the crystals. We say that by heating, iodine crystals will sublime, that is, iodine will directly change from a solid state to a gaseous state, without previous changing to a liquid state.

The same process takes place at room temperature, but at a much slower rate. In fact, it happens so slowly that the crystals you can see take almost 30 years to grow to a size of 1-2 cm.

As it is often the case when it comes to discoveries, these crystals grew quite by accident, too. How, you ask? Well, a chemist put a bottle of iodine in a desiccator, which at one point fell over due to moving, and its contents spilled over the desiccator. Distracted and in a hurry, the chemist put the desiccator aside, intending to clean it later, but it seems he had forgotten about it.

It was only many years later, when clearing out a closet, in which the desiccator was once put aside, that someone noticed the truly beautiful iodine crystals that had been growing all those years while the desiccator was closed, and which are still growing today.

Author of the text and photos: Alen Bjelopetrović, Dr.Sc.