When the Russian chemist Dmitri Mendeleev published his periodic table of elements in 1869, there were just fifty-nine entries on it. The table grouped those elements—hydrogen, oxygen, and carbon, along with less familiar substances like osmium, rhodium, yttrium—according to their shared chemical properties and the weight of their atoms.

The table also contained thirty-three empty spaces that implied that there were elements still to be discovered. He gave these still-hypothetical elements names like “ekasilicon,” “ekaaluminum” and “ekaboron,” based on their expected similarities to known substances; the spaces were filled by germanium, gallium and scandium, respectively. (“Eka-” is a Sanskrit prefix meaning “one,” so you can think of the names as silicon 1, aluminum 1, and so on.) By year 1939, all of Mendeleev’s boxes had been filled in; the last one was “ekacesium,” now called francium.

What Mendeleev couldn’t have imagined was that scientists would one day begin creating elements not found naturally. Just a few days ago, the fabrication of one of the heaviest elements yet was confirmed by Swedish scientists working at the G.S.I. Helmholtz Center for Heavy Ion Research, in Darmstadt, Germany. Its provisional name is ununpentium.

What makes an element distinct is the number of protons it has in its nucleus: hydrogen has one proton, helium has two, and on up the periodic table to uranium, which has ninety-two. Creating new elements began with physicists bombarding existing ones with other particles; as the nuclei careened around they would sometimes smash together and form atoms with more than ninety-two protons. First came neptunium, in 1940, with ninety-three protons, then plutonium, with ninety-four (which, it turns out, does exist in trace quantities in nature). In the years since, scientists continued creating heavier and generally more unstable atoms. Ununpentium element has a hundred and fifteen protons. (It’s name means “one-one-five.”)

A few artificial elements have important practical applications. Plutonium can be used in nuclear weapons; it’s also the fuel for some nuclear reactors and has been used to power space probes, including Voyager 1 and 2. Ununpentium has no practical uses yet. It’s so unstable that it doesn’t stay around long enough to make anything out of it. Almost immediately after the Swedish scientists created it by smashing twenty-proton calcium nuclei into ninety-five-proton americium nuclei at high speed, the ununpentium decayed into element 113—ununtrium—which itself decayed into lighter elements. Its half-life was found to be only a hundred and seventy-three milliseconds. But the decay chain, plus the X-rays and gamma rays the short-lived nuclei spat out in their death throes, convinced the physicists that the one that got away was indeed element 115.

In fact, this was the second sighting of the element: Russian scientists had claimed the discovery of element 115 back in 2003, but the International Union of Pure and Applied Chemistry—chemistry’s equivalent of the International Astronomical Union, which famously demoted Pluto from planet status in 2006—wouldn’t acknowledge it without a confirming experiment from another team. The Helmholtz Center’s work must still be reviewed by both the I.U.P.A.C. and the International Union of Pure and Applied Physics, but ununpentium is now a step closer to inclusion on the periodic table. If that happens, the International Union will assign it a permanent, official name.

Ununpentium’s brief life doesn’t mean it’s completely useless: nuclear physicists can try and deduce from properties more about how atomic nuclei are put together and how they fall apart. The latest experiment created about thirty atoms’ worth of element 115; physicists will continue trying to make larger batches in order to explore its properties more fully.

Ununpentium is not, however, the heaviest known atom. In general, it’s harder to make new elements the further up the periodic table you go. But owing to the complex structure of heavy nuclei, atoms more massive than ununpentium were created earlier. I.U.P.A.C. has already signed off on element 116 (livermorium), element 117 (ununseptium) and element 118 (ununoctium), although the latter two haven’t been assigned permanent names yet. Ununoctium’s half life is just 0.89 milliseconds.

While it’s generally true that heavier atoms are more unstable than lighter ones, there’s at least the possibility, first proposed by the physicist Glenn Seaborg back in the nineteen-sixties, that if an atom gets heavy enough it could arrive at an “island of stability,” far less prone to immediate decay than its neighbors on the periodic table. Unbinilium, for example—the still-theoretical element 120—might be one of them. Perhaps it could live for minutes, or even days. That’s still far too short a time for almost any practical use. But for scientists who study nuclei for a living, the prospect of keeping an element that heavy around for more than a handful of milliseconds is an almost unimaginable luxury.

Illustration by Greg Robson/Pumbaa.