What is the most expensive thing you can buy?

Before you go too far down the rabbit hole and suggest anything from space rockets to, hey just maybe France would consider selling the Mona Lisa….. no, the answer is astatine. As a commodity at least - something priced by weight or volume - this obscure element wins the contest by a very long shot. Pose the question to your average know-it-all nerd at work or school and the automatic responses will likely be something trite like platinum or diamonds (Google says antimatter which is true enough but that’s not something you can buy).

The fact is that the cost of almost any earthly substance you can think of is dirt cheap if you price it by the milligram, nanogram or picogram; that last being a decimal point with twelve zeroes and a one. But astatine, as you will now learn, is priced by the atom. One. Frikkin’. Atom. If you’re good with chemistry and know your moles and Avogadro’s, and are pedantic enough to do the math, you’ll find the corresponding weight of that atom to be a number so filled with zeroes that it makes more sense to use scientific notation rather than risk your eyes going all googly.

Oh, and it gets worse. You see, despite the considerable price tag, this atom doesn’t even have the decency to become your permanent collectible for it falls apart rather quickly. At the atomic level, astatine’s nucleus is one of the most unstable arrangements imaginable. Its 200+ grouping of protons and neutrons are unable to hold their unholy marriage together longer than a few hours before divorcing into smaller atoms with less quarrelsome internal bits.

Luckily - if that’s the right adverb here - the loss of one astatine atom is probabilistically being continuously replaced with a new one thanks to a little bit of actinium. A starting amount of just 1 μCi of Ac-227 disintegrates at the rate of 37,000 atoms per second. This yields a 1.38% chance of it decaying to Fr-223 and a 0.006% chance of that decaying to At-219. So you'll get, on average, around 1.38 atoms of At-219 every 55 seconds. While the amount of actinium would seem to those of us without degrees in physics to be disappearing alarmingly fast, nah, no worries. It’s got decades’ worth of atom pops before exhausting itself. The depletion rate of the mass of actinium is, in any case, offset somewhat by the fact that the intermediate francium atom decays faster than that of astatine meaning that, for the next few years at least, the net rate of At-219 will actually increase.

Who knows, you might be lucky enough to at some point have two simultaneously viable astatine atoms in that sample. What. A. Bargain!