Darmstadtium (Ds)

Element 110 was made on 9 November 1994 by a method whose name causes endless trouble. In nuclear physics, cold fusion means fusing two nuclei in a way that leaves the combined nucleus barely excited, so that it can settle down by shedding a single neutron instead of boiling several off. It has no connection whatever to the 1989 claim about producing energy in an electrochemical cell, which was a different thing entirely and did not work. The nuclear kind works, and it built six elements.

Fusing without heating

The problem with making a superheavy nucleus is that fusing two nuclei requires enough energy to overcome their mutual electrical repulsion, and that energy has to go somewhere afterwards. It goes into the new nucleus as heat, and a hot superheavy nucleus almost always fissions before it can cool. Every extra neutron it has to evaporate is another chance to fall apart.

Yuri Oganessian's insight at Dubna in the 1970s was that some target–projectile combinations produce far less excitation than others. Fire a medium-mass ion such as nickel at a lead or bismuth target — both close to strongly bound shell configurations — and the fused nucleus is formed almost cold. One neutron out, and it survives.

Darmstadt turned this into an industrial process. The Universal Linear Accelerator delivered the beam, and the products were separated by SHIP, a velocity filter that uses crossed electric and magnetic fields to let through only particles moving at the speed a fused nucleus would have, sweeping away the enormous background of scattered beam. The separation happens in about a microsecond, which is why nuclei that live for milliseconds can be studied at all.

Sigurd Hofmann's group used the technique to produce elements 107, 108, 109, 110, 111 and 112 over thirteen years, from one laboratory. For element 110 the combination was a lead target and a nickel-62 beam, and the first identification rested on a small number of decay chains.

Cold fusion has a ceiling, though. Above element 112 the cross-sections collapse to the point where the method stops paying for the beam time it consumes. One further square was wrung out of it at RIKEN, at a cost in years that nobody has been willing to repeat, and everything past element 113 came from a different reaction entirely.

Earlier claims

Element 110 had been reported before 1994. Dubna announced it in 1987 and a Berkeley group in 1991, and neither result was accepted — in both cases the evidence was too thin to establish the atomic number. The IUPAC and IUPAP joint working party assessed the file and credited Darmstadt in 2001, with the naming following in 2003.

Wixhausium, and a joke about the emergency services

The naming discussion produced some genuine oddities before it settled.

• Wixhausium was seriously considered by the discoverers, after Wixhausen, the suburb of Darmstadt in which the laboratory actually stands. It would have been the most precisely geographic element name ever coined.
• Policium was floated as a joke, because 110 is the telephone number for the police in Germany. It got further into public discussion than a joke normally does, and had to be disclaimed.

The group settled on the city, and IUPAC approved darmstadtium in 2003. Few element names point that precisely at the ground the discoverers stood on; most settle for a country, a region or a person.

A density that depends on which calculation you read

There is no chemistry of element 110, and the obstacle is arithmetic rather than difficulty. Darmstadtium-281, the most durable isotope, survives on the order of ten seconds, and the number of atoms ever made across every isotope is in the low hundreds. Even a single gas-phase adsorption measurement needs more material than that.

What exists instead is a body of calculation, and it should be read as calculation. Element 110 is expected to be a group 10 metal, chemically a heavier platinum, with the +6, +4 and +2 states likely accessible, and to condense as a body-centred cubic solid.

Its density is the number worth being careful with, because the literature does not offer one. The early relativistic extrapolations that filled in the whole of row 7 put darmstadtium near 34 or 35 grams per cubic centimetre; later solid-state calculations bring it down to the high twenties. Both sit well above osmium, which at a shade over 22 is the densest substance anyone has actually weighed — but neither makes darmstadtium the leader. On both families of prediction the row peaks one and two squares earlier, at meitnerium and hassium, for the same reason the row above peaks at iridium and osmium: that is where the d shell is around half filled, cohesion is strongest and the atoms sit tightest.

None of it has been observed. These are statements about what a solid would do if there were ever enough of the element to make one, and there is no realistic prospect of that. A density printed against darmstadtium in a reference table is the output of a computer, and the gap between that and a weighing is the whole difference between the top of the periodic table and the rest of it.