It was while making sulfuric acid that Jöns Jacob Berzelius — ‘the father of Swedish chemistry’ — noticed a red residue which he first took for tellurium, as Russell Boyd from Dalhousie University notes in this month’s ‘in your element’ article (subscription required). A more meticulous investigation, however, revealed that the residue displayed different properties, resembling those of sulfur. The new element fell into place between tellurium and sulfur in the chalcogen family of the periodic table, and Berzelius named it selenium (after the Greek word for Moon) owing to its similarity with tellurium (named after the Latin word for Earth).
Although often eclipsed by sulfur in textbooks, selenium has a reactivity of its own. I particularly like the fact that its grey allotrope, the most stable form, conducts electricity better in the light than in the dark, and converts electric current from AC to DC — properties which have not gone unnoticed in the fabrication of photovoltaic cells and rectifiers, respectively. Its red tint also went on to account for a worldwide application: selenium dioxide (which adopts a one-dimensional chain structure) imparts vibrant reds and pinks to glass.
It was only much, much later (140 years after its discovery) that the role of selenium in biological systems was identified. It replaces sulfur in some proteins, which in recent years have been shown to help the prevention of cancer, by hindering radical attacks on cells or possibly also by slowing tumour growth. It is introduced in the body by ingestion — dietary recommendations however follow a fine line between too little and too much, both with potentially very serious consequences. While ingesting too little can lead to a weakened immune system or heart problems, selenium poisoning comes with unpleasant skin or breath odour side effects, can affect mental awareness, and can even be life-threatening at high doses. Around 55 micrograms per day sounds just right — I was very surprised to read that it is contained in a single dried Brazil nut!
Have a look at the article to find out more, but I wouldn’t recommend daily Brazil nut fests.
Anne Pichon (Associate Editor, Nature Chemistry)