Nihonium (Nh)

RIKEN's element 113 campaign produced one atom in July 2004, a second in April 2005, and then nothing at all for more than seven years. The accelerator kept running. The third atom arrived on 12 August 2012. Kosuke Morita's group had accumulated something on the order of 553 days of beam time to obtain three events, and it was the third one that won the argument.

Why the third atom mattered

The first two atoms decayed by a chain of alpha emissions that terminated in spontaneous fission of dubnium-262. That is suggestive, but it leaves the atomic number resting on the assumed identity of every link. Fission ends the chain without connecting it to anything independently known.

The 2012 event went further. It decayed through six successive alpha emissions — roentgenium, meitnerium, bohrium, dubnium, lawrencium — and finished at mendelevium-254, a nuclide whose properties had been established for decades. A chain that lands on known ground is not an inference; it is an arithmetic identification of the nucleus at the top, since each alpha decay subtracts exactly two protons.

The joint IUPAC/IUPAP reviewers weighed the competing claims in December 2015. Dubna and Livermore had also seen element 113, from 2003 onwards, but as an intermediate step in the decay chains of element 115 rather than as a directly produced nucleus, and the working party found those chains internally inconsistent in their individual decay energies. RIKEN was awarded the priority and invited to propose a name.

Asia's first element

No element had ever been named in Asia. Every square on the periodic table had been named in Europe or North America, and the omission was conspicuous enough that Japanese chemists had been conscious of it for a century.

The group had a precedent to reckon with. In 1908 the Japanese chemist Masataka Ogawa had announced an element he called nipponium, and the claim had not survived. Reusing the name was awkward on those grounds, and impossible on another: the natural symbol Np had long since been taken by neptunium.

They chose nihonium, from Nihon, one of the two Japanese readings of the characters meaning "land of the rising sun", with the symbol Nh. The name was approved in November 2016. Morita has said the intention was partly to give Japanese schoolchildren something of their own on a chart that had never before mentioned their country.

Why cold fusion still worked here

Element 113 was made with a bismuth target and a zinc-70 beam — a lead-and-bismuth-style combination of the kind that had built the previous six elements at Darmstadt. By 2004 that approach was running out of road, because the probability of a successful fusion falls steeply with atomic number.

The scale of the difficulty is easier to grasp as a rate than as a cross-section. RIKEN's apparatus produced roughly one atom of nihonium every three years of running. The experiment was justified not by the yield but by the certainty: the products of this route are cleanly separated and their chains can be traced link by link, which is exactly what the third atom demonstrated and exactly what the competing hot-fusion claim could not.

What element 113 might be like

Nihonium sits in group 13, below boron, aluminum, gallium, indium and thallium. No chemistry has been performed on it, and the isotopes available — nihonium-286 is the most durable, at a few seconds — do not permit any.

The predictions, and they are predictions, follow from thallium's behaviour taken one step further. Thallium already prefers the +1 state over the +3 that the rest of group 13 favours, because its 6s electron pair is relativistically stabilised and reluctant to bond. In nihonium the same effect on the 7s pair is expected to be stronger still, making +1 the dominant state and possibly making the element behave in some respects like a heavy alkali metal or even a noble metal. Calculations put its melting point around 700 K and its density near 16 grams per cubic centimetre.

Attempts have been made to study nihonium's volatility experimentally, using atoms produced in the decay chains of element 115, but the results so far have been inconclusive. For the moment, what is known about element 113 is its atomic number, a handful of decay chains, and the fact that it took nine years to prove.