Francium is second only to astatine on the periodic table for the title of rarest naturally occurring element in the earth’s crust. The point is somewhat debatable as a number of transuranic elements are theorized to exist in nature in infinitesimally small quantities; perhaps as high up as curium but at least as far as measurable elements goes francium holds the #2 spot behind astatine. How rare is that? Well, it’s been calculated that if you were to put all of Earth’s francium atoms together you’d only have about an ounce of it(!)

Mendeleyev’s table showed a promising hole at the bottom of the alkali column. It proved to be a popular target for element hunters of the early 20th century. Many scientists predictably went looking for it in ores rich in lithium and other alkalis. It was a logical but ultimately fruitless trail. It wasn’t until 1939 when Marguerite Perey, protege of Marie Curie, correctly identified the merest trace of a signature in preparations of the recently discovered element actinium; itself an extremely rare occurrence in nature.

In any case, with a half life barely 22 minutes or so, francium’s exceedingly unstable atomic configuration means that putting any appreciable amount together will create so much heat from all those colliding particles that the grouping invariably ignite themselves into incandescence. So, in a very literal sense, it’s impossible to view the element in its metallic state even if we could somehow isolate enough of it in one place to make it visible. Indeed, the only photo taken, a cluster of just 3,000 atoms, appeared under an electron scope as a little star-like figure that had vaporized itself into plasma.

Here we present an - ahem - generous 21,000 atoms. Spread out over the contents of what looks like a pinch of Himalayan salt they’re sufficiently shielded from each other to avoid heating up. Thanks to the work done by Perey, and a number of researchers who came later, we now know that each decaying atom of isotope actinium-227 has a roughly 1% chance of becoming francium-223 for a few minutes before turning into radon or even the fabled astatine itself.

The sample is initially prepared from uranium-235 (yes, U235, the atomic-bomb making type!) which was extracted from uraninite. From this ore various chemical steps were used to isolate thorium-231. This in turn breaks down to protactinium-231 before becoming the donor atom actinium-227. Given its comparatively long half life of 22 years, Ac-227 therefore becomes, in human terms at least, a never-ending generator of Fr-223 atoms. By knowing the concentration of actinium present in the sample, confirmed through spectroscopic analysis, as well as the half lives of both isotopes, we can determine when the sample reaches a “steady state” in which as many decaying atoms to elements farther down the chain are at the same time being replenished from the decay of the actinium.