Tennessine (Ts)

The hardest part of making element 117 was not the physics. It was getting a small quantity of berkelium from Tennessee to Moscow before it decayed.

Five crossings of the Atlantic

Oak Ridge produced 22 milligrams of berkelium-249, using its high-flux reactor and the shielded separations building beside it — a campaign that tied up the reactor for months and yielded a substance with a half-life of about 330 days. From the moment it was separated, the material was on a clock, losing a fifth of a per cent of itself every day, and everything downstream had to be completed before too much of it was gone.

In the summer of 2009 the berkelium was packed into five lead containers and put on a commercial flight from New York to Moscow. Russian customs officers were presented with several large containers covered in hazardous-material labelling and paperwork that did not adequately describe the contents. They put the whole consignment back on the same aircraft to New York.

It was turned back a second time. The berkelium finally entered Russia on its fifth transatlantic crossing, having spent weeks in the air and in customs halls while the isotope decayed. From Moscow it went east to the reactor institute at Dimitrovgrad on the Volga, where it was deposited onto a thin titanium foil to make the target, and then to Dubna. Six months of continuous bombardment with a calcium-48 beam followed, and produced six atoms of element 117.

Why berkelium and nothing else

The choice of target was not a preference. Element 117 needs 117 protons, and a calcium-48 projectile supplies twenty of them, so the target has to have ninety-seven — which is berkelium and only berkelium.

That makes element 117 uniquely dependent on a single supply chain, and a thin one. Californium is the heaviest actinide that reactors turn out in the amounts a target needs, and every other superheavy campaign leans on it; berkelium, one square below, comes out of the same reactors as a minor product and has no such margin. Only a couple of facilities worldwide can produce even the tens of milligrams a target consumes, and what they produce decays away within a year of leaving the reactor. Every attempt at element 117, before or since, has had to solve the same logistical problem.

An -ine ending, and a symbol chemists already use

The 2010 discovery was recognised by the IUPAC and IUPAP joint working party in December 2015, credited to the Dubna, Oak Ridge and Livermore collaboration along with Vanderbilt University and the University of Tennessee. The name honours the state of Tennessee, in recognition of the reactor and the separations facility without which the experiment could not have happened. It is the second element named after a US state.

The ending is prescribed rather than chosen. IUPAC requires that a new element in group 17 take the halogen suffix -ine, so element 117 had to be tennessine rather than tennessium — a grammatical assertion of group membership that, as it happens, the element itself may not honour.

The symbol has caused some low-level irritation. Ts is the standard abbreviation for the tosyl group in organic chemistry, where it appears in a great many structures and reaction schemes. Nomenclature committees considered the clash acceptable on the grounds that no organic chemist is likely to encounter element 117 in a flask.

A halogen that is probably not one

The name says halogen. The calculations say otherwise, and this is one of the sharper cases in the periodic table where a group assignment and a predicted behaviour part company.

A halogen's defining chemistry is its appetite for one more electron, filling the p subshell and forming a −1 ion. Relativistic calculations predict that in element 117 the 7p subshell is split so widely by spin–orbit coupling that the outermost electron sits in a high-lying, loosely held orbital, while the lower pair are tucked away. The consequences expected are a much smaller electron affinity than iodine's, a −1 state that is weak or unavailable, and easier access to +1 and +3 states instead. Some calculations suggest tennessine would be metallic in bulk rather than the molecular substance its lighter relatives form.

Every one of those statements is a computation, and no experiment on the horizon will change that, because tennessine falls below the threshold of the only technique that could test it. Superheavy chemistry is done one atom at a time in the gas phase: the atom is swept along a channel of detectors held at a temperature gradient and the question asked is where it sticks. That takes seconds. Tennessine-294 lasts about fifty milliseconds and tennessine-293 around twenty, which is an order of magnitude short. Fewer than two dozen atoms of the element have ever existed, and none of them survived long enough to be offered a surface.