Element 61 in our International Year of the Periodic Table series is promethium. Only trace amounts of promethium form naturally, which limit its applications, though it has previously been trialled in atomic batteries.

All isotopes of promethium are radioactive, and though it may once have existed in Earth’s crust, it would have decayed away to other elements relatively rapidly. Its most stable isotope only has a half-life of a little under 18 years. Promethium does still form in trace amounts in uranium ores, but we’re really talking minuscule quantities – in fact, promethium was only even discovered when produced synthetically.

Recently, Nature Chemistry published an article debating which element in the periodic table was the most boring. Without wanting to be too mean to promethium, I think it can consider itself a little lucky not to have made it into their final shortlist for the title. Perhaps the one fact about promethium that does make it at least a little interesting is that astronomers have detected that it’s somehow being produced by a distant star.

Due to its scarcity and radioactivity, it doesn’t have a lot of applications. It was trialled for use in pacemakers, but the arrival of lithium-ion batteries rendered this use obsolete. Some sources claim that these promethium atomic batteries are used in military missiles and radios, though there seems to be little in the way of authoritative sources for this claim. Admittedly, I suspect missile manufacturers aren’t in the habit of publishing publicly accessible lists of missile components, so its use in them has to remain a possibility.

One other previous use of promethium was as a replacement for radioactive radium to make glow-in-the-dark watch dials. Though promethium is also radioactive, it was considered safer than radium as it only emits beta radiation. However, the short half-life of promethium meant this application wasn’t particularly long-lived.

Remember, you can keep track of all of the previous entries in this series on the site here, or on the Royal Society of Chemistry’s dedicated page.