Moscovium (Mc)

Element 115 had a public reputation fourteen years before anyone made an atom of it, and the reputation had nothing to do with chemistry.

In 1989 a man named Robert Lazar appeared on Las Vegas television claiming to have worked on recovered extraterrestrial spacecraft at a site south of Area 51. The craft, he said, were powered by a stable superheavy substance he called element 115, which he described as available in quantity and as generating a gravitational field. At the time, element 115 was an unoccupied square that nobody had synthesised, which made the claim unfalsifiable in exactly the way such claims benefit from.

Element 115 was made in 2003. Its longest-lived isotope survives for less than a second. It cannot be accumulated, does not exist outside an accelerator, and has never been produced in a quantity greater than a handful of atoms — a total, across all experiments ever performed, of well under a hundred. The properties described in 1989 are not merely unobserved; they are incompatible with what element 115 turned out to be.

The story is worth telling on a reference page precisely because a great many people arrive at this square looking for it, and the honest answer is short: the substance described does not exist, and the element that occupies the square is one of the most ephemeral things ever made.

What was actually made

In August 2003, a collaboration between Dubna and Lawrence Livermore fired a calcium-48 beam at a target of americium-243 and observed decay chains attributed to two isotopes, moscovium-287 and moscovium-288. The results were published in early 2004.

Confirming an element from decay chains alone has a weakness: the chain gives you the differences between successive nuclei with confidence, but the absolute atomic number at the top depends on correctly identifying where the chain lands. For element 115, the chain ends at dubnium-268, whose long life allowed it to be extracted and chemically identified — which pinned the number directly.

An X-ray fingerprint from Darmstadt

The most satisfying confirmation came from a different laboratory and a different kind of evidence. In 2013 a team led by Dirk Rudolph of Lund University, working at the heavy-ion laboratory in Darmstadt, repeated the synthesis and measured not just the alpha particles but the X-rays emitted alongside them.

Characteristic X-ray energies are determined by the nuclear charge of the atom emitting them, so they identify an element the way a fingerprint identifies a person, independent of any assumption about the decay chain. The Lund measurement was the first time X-ray evidence had been brought to bear on a superheavy element claim, and it is the reason element 115's atomic number is regarded as directly established rather than inferred.

Recognition followed in December 2015, when the IUPAC and IUPAP reviewers credited the Dubna, Livermore and Oak Ridge collaboration. Oak Ridge's contribution was the americium target material.

Moscow Oblast

The name honours the region in which Dubna lies, roughly 120 kilometres north of Moscow. It had been proposed once before and used elsewhere: the Dubna team originally wanted moscovium for element 116, and only after that square was named for the Livermore laboratory did the name become available for 115. It was approved in November 2016 alongside nihonium, tennessine and oganesson.

Predicted chemistry, and the state of the isotopes

Element 115 has never been through a chemical experiment, and the isotopes in hand do not permit one: moscovium-290 lasts around two-thirds of a second and moscovium-288 rather less.

What theory expects, as prediction rather than observation: moscovium sits in group 15 beneath bismuth, and the trend that already makes bismuth prefer +3 over the +5 that nitrogen and phosphorus favour should intensify. The 7s electron pair is calculated to be relativistically stabilised to the point of being chemically inert, so the accessible states should be +1 and +3, with +1 unusually prominent for a group 15 element. Calculations describe it as a dense, relatively low-melting metal, with figures around 13.5 grams per cubic centimetre and a melting point near 670 K appearing in the literature. All of those numbers come from relativistic computations, and none has been measured.