The rarest natural element on earth will evaporate itself if you collect too much of it

At last count there are 118 confirmed elements in the periodic table. Yet the chance that you will come into contact with just five elements is more than nine times greater than with the other 113.

That’s because, of all the dozens of elements found in the Earth’s crust, it is these five – oxygen, silicon, aluminum, iron and calcium – that total more than 92 percent. In fact, almost half of the atoms we encounter every day are made up of oxygen, making it by far the most abundant element on Earth.

At the other end of the scale, however, we have astatine: the rarest naturally occurring element on Earth. It’s thought that less than 25 grams – that’s less than an ounce – of the stuff exists at any given time in the entire world. In fact, it is so rare that scientists still don’t know basic information about the element, such as what it looks like.

Astatine is named after the Greek word for “unstable,” and it’s an appropriate name: it’s incredibly radioactive, with a half-life of just over eight hours, even in its most stable form, astatine-210. That means that even if you were able to get some, there would only be an eighth of them left after 24 hours; the rest would have decayed into bismuth-206 or polonium-210.

And that’s the most stable isotope – most forms of astatine have half-lives of one second or less. In its elemental form, things get even more volatile: It’s so radioactive that if you had enough to see with the naked eye, it would literally vaporize itself under its own heat.

Although scientists can deal with astatine more or less directly, they can only do so by artificially creating it via nuclear reactions – usually by bombarding bismuth-209 with alpha particles.

Most of what we know about the element therefore comes from theoretical work rather than practical experiments. Us for example think astatine probably looks like a black solid because it lies in the halogen column of the periodic table. Halogens darken as they gain weight: fluorine is essentially colorless, chlorine is yellow-green, bromine is red-brown, and iodine is dark gray-violet. Therefore, as the next halogen in the list, astatine should be used. be even darker.

Of course, we’re assuming that the element is actually no more like a metal than a halogen—a question that divides chemists, since astatine also lies along a diagonal line in the periodic table, which includes metalloids like boron and silicon. In chemical reactions, it sometimes behaves like a halogen and sometimes like a metal, making it not easy to classify even by experts.

Despite being so rare as to be practically non-existent – ​​and so confusing that we don’t actually know anything about it for sure – astatine may turn out to have some pretty important practical uses. As the element decays, it emits α particles: radioactive particles formed from a combination of two protons and two neutrons, which for various reasons Real good at targeting cancer cells.

“Astatine is the Goldilocks of α-rays,” Mehran Makvandi, a radiologist at the University of Pennsylvania Medical School, told Chemical and Engineering News, a journal published by the American Chemical Society, in 2020. It emits fewer α particles than other isotopes. Like actinium-225, Makvandi explained – that makes it less powerful, but more focused on its emissions. It has an extremely short half-life, meaning it doesn’t stick around for long. What’s important is that only emits α particles – the least harmful of the different types of radiation.

If scientists could link astatine isotopes to cancer-targeting molecules, they might be able to create an anti-cancer treatment that could cut through a cancer cell’s DNA and leave surrounding tissue relatively unscathed. When it comes to potential cancer treatments, Makvandi said, “nothing comes close to that same targeted potential.”

Of course, there is a significant caveat: researchers must first get their hands on the stuff. Because it is so rare and unstable, this is not an easy task. That may be why the element is still officially listed as one whose biological role or use outside of research is unknown.

But maybe that’s for the best. If we knew it was really important, we’d probably need a lot more than 25 grams of the stuff, right?

An earlier version of this article appeared in March 2023.

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