Element of the Month: Promethium!

February's Element of the Month:
Promethium!
Pm
61

Atomic Mass: 145ish amu
Melting Point: 1042 °C
Boiling Point: 3000 °C

Most of your fancy-schmancy online physics sites want to tell you two different things about Promethium. Up front, they’ll say that Promethium doesn’t occur in nature. Then, in the fine print, they’ll admit that, yeah, it does, a little bit. Trace amounts. What it boils down to is that there is a certain small mass of natural Promethium on the Earth at any given moment in time, but it’s in the form of extremely dispersed individual molecules, and not many of them at that.

Promethium is radioactive. You don’t really expect serious radioactivity until you get up to Element #84, Polonium, but Promethium (#61) and another oddball, Technetium (#43) are precocious that way. Promethium comes in several different isotopes, but even the most stable version has a half-life of less than 18 years. That means that, according to the relentlessly effective laws of chance, any (and every) atom of that isotope of Promethium will have a 50/50 chance, in a given 18 years, of shedding a subatomic particle and becoming an atom of something else, probably Neodymium or Samarium.

Now this explains why you won’t find significant quantities of Promethium in nature: it simply doesn’t stick around. You don’t even have to think about it on a geological timescale. If you had bought a million pounds of the long-lasting 18-year half-life Promethium fairly recently – to celebrate the first staging of the still-popular play “Hamlet,” for instance – only about two ounces of your heap would still be Promethium today. Assuming I did the math right.


The Centerfold!

Online images of Promethium clearly aren’t of Promethium, since they all show larger samples than have ever existed in one place at one time.  Apparently any significant Promethium pebble heats up quickly from the alpha decay, and its surface develops a coating of a pink oxide.  Cool!  But I've got no visual on that, so to avoid giving you false confidence about the appearance of Element #61, here’s an album cover from an obscure British metal band.

OK: we’ve explained why Promethium doesn’t occur in nature: it disappears through radioactive decay. But what about the other half of the question: why is it that Promethium really does appear in nature, except only in tiny undetectable quantities? The answer is that Element #63, Europium, has an isotope that is, well, sort of radioactive. Which is to say, it really is radioactive, but its half-life is so long that on any meaningful scale – including, for example, the age of the universe – the radioactivity just isn’t very relevant. (This is apparently true, incidentally, for pretty much all the elements over #40.) But still, if you had five pounds of Europium sitting on the table, every minute or so there would be an alpha decay – an alpha particle, two protons and two neutrons, would go calving off – and one atom of your Europium would have changed to Promethium for a while. So that’s where natural Promethium comes from. But since Europium is a pretty rare Element, and since it only occurs naturally as a trace element within mineral compounds, natural Promethium is definitely real but also, in practical terms, strictly theoretical: it must exist, but you could never actually find any.

Also, Promethium can result from the somewhat more complicated action that goes on in Uranium ores. In fact, the Uranium process is probably a more significant Promethium generator than is the Europium process. But still, we’re only talking about a grand total of around one pound of highly dispersed natural Promethium existing on the Earth’s crust at any one time.

Fakey Promethium has been around since, depending on who you ask, 1938, 1942, 1945, or 1963. If you look in an old chemistry textbook from the 1950s or earlier, you'll probably see a blank spot on the periodic table where they figured something must be.  Synthetic Promethium actually has a few uses, for instance in atomic batteries. At one time, they used it for glow-in-the-dark paint. They make a little bit of it every year out of nuclear waste.