What is it that limits or constrains the number of chemical elements we know? The International Year of the Periodic Table, 2019, commemorates the 150th anniversary of the discovery of the Periodic System by the Russian scientist, Dmitri Mendeleev in 1869.
Like many discoveries, there were many contemporaries who were devising schemes to organise and classify the elements. Four years before the announcement of Mendeleev, John Newlands, a British chemist, had attempted to publish his work which recognised that there were similarities between elements which he called the Law of Octaves. The work was dismissed by the Chemical Society with a rather harsh rebuttal suggesting the value of the Law of Octaves was of little more value than listing the elements in alphabetical order. A shortcoming of Newlands work was that there was not space for the future elemental discoveries, with a pattern that sometimes required two elements in a single box to create a sequence. Mendeleev, on the other hand, left gaps where things did not fit and was not exclusively guided by the atomic weights but chemical properties and ordered elements thus.
At the time of Newlands the noble gases, helium, neon, argon, krypton, were yet to be discovered. The first of them was not found on earth, but in the sun where characteristic absorption of light in the chromosphere of the Sun due to quantum jumps of the electrons in atomic orbits results in dips in intensity at particular wavelengths, the Fraunhofer lines, revealed a hitherto unknown atomic substance. This was later isolated on Earth where it is produced by the radioactive alpha-decay of heavy elements producing a gas which is so light it escapes the Earth’s gravitational pull and vanishes into space. The element was named after the location of its first observation - Helios.
The discovery of oxygen was made by Priestley in 1774 when beams of Helios sunlight were focussed on mercuric oxide, releasing a gas which made mice more active and with life-giving properties. In 1780 Priestley came to Birmingham and became a member of the Lunar Society along with Watt and Boulton, until the early 1790s when he was forced to flee the city in what were called the Priestley Riots – for his politically and theologically controversial ideas.
In more recent times the lengths discoverers will go to have been extreme. At the time of the Second World War, the heaviest known element was number 92 – uranium. Transuranic (beyond uranium) elements were in part discovered in the fallout from the atomic weapons testing programme. Einsteinium was recovered following the analysis of filter papers flown through the explosion clouds of the atomic bomb tests. The name is ironic given his role in the bomb programme. Here the rapid capture of neutrons in the explosive environment of the bomb detonation created exotic isotopes that radioactively decayed to new elements. Rather adventurous colleagues have proposed that a good way to synthesise even more exotic elements might be to have a double detonation, but that is unlikely to find favour unless in some unhappy accident.
Part of the attraction of searching for new elements is the opportunity to name them. The names tell their own story. Americium, Lawrencium, Californium, Berkelium, elements 95, 97, 98 and 103 are all tributes to the great work done at the Lawrence Berkeley Laboratory in California, names never to be lost in time. The newest discoveries have Russian and Japanese origins as the programme has shifted to countries prepared to make the investment required.
The discovery of the element 113 at the RIKEN laboratory in Japan has been named Nihonium, following from the Japanese name for Japan. The team was led by the charming and unassuming Morita-san. The equally impressive Yuri Oganessian has led the work at the Flerov Laboratory near Moscow to discover elements 115, 117 and 118; named Moscovium, Tennessine and Oganesson. The last of these a wonderful recognition for a man who had dedicated his life to the discovery and characterisation of new chemical elements. The collaboration was with the Oak Ridge National Laboratory in Tennessee where the target for making these new elements was the already synthetically synthesised Berkelium made in the Oak Ridge reactor; synthesis upon synthesis.
The process for making these elements pushes experiments to their limits where beams of exotic nuclei irradiate exotic targets for weeks on end, pushing the targets to their melting point to create a few atoms of the new element. The new elements are identified by their alpha (or helium) decay to known elements; just as helium was first itself characterised but now is a tool for element discovery.
The properties of these new elements are challenging our ideas of chemistry and the table of Mendeleev. The atomic charge is so large, the electrons in the atom move so quickly that their properties are modified and the new elements no longer follow the traditional pattern. A new era of elemental chemistry is being entered.
So, to the original question – is that it, what is the limit to the existence of the elements? The answer lies in nuclear physics. Just as there are shells in atomic physics, with the noble gases corresponding to shells filled with electrons and the periodic pattern, there are shells in nuclei. Nuclei with closed shells of protons and neutrons are more stable – they are noble nuclei. It is these nuclei which offer hope. An Island of Super Heavy nuclei close to element 126, a closed shell, is predicted. How this will be synthesised remains a mystery but indeed may need a double atomic device or perhaps may being synthesised as we speak in the cataclysmic collisions of neutron stars out there in the cosmos, observed through gravitational waves, which create many of the elements of our universe. Its name awaits.