The limits of the periodic table
In the quest for new chemical elements, scientists have pushed physics to its breaking point. Their efforts may take us beyond the bounds of the periodic table. Vanessa Seifert
Discovering a new element is every chemist’s dream. It comes with a lot of prestige and recognition: your name is given to the element, and a Nobel prize is almost certainly in the bag. But nowadays this dream has become quite difficult to achieve. The elements that Dmitri Mendeleev predicted through the gaps in his periodic table have now been discovered, and the table’s borders have grown to include heavier elements that chemists had not thought existed in the past. Today our technological means have been pushed to their limits, and it is not entirely clear whether we should expect the discovery of any new elements. There is a physical limit to the periodic table – but have we reached it yet?
Searching in nature
In the distant past, discovering an element was relatively straightforward. One would take matter and isolate an element through constant chemical processes. Chemists would take a chunk of matter and refine it until they ended up with something pure. It is believed that copper was the first element to have been discovered around 9000 BC in the Middle East. Lead and gold were discovered around 7000 and 6000 BC respectively. In ancient times and up until the 18th century, alchemists contributed greatly to the discovery of elements by developing experimental techniques that are still used in chemical practice, such as distillation – the process of boiling and condensing. They also discovered several chemical elements even if they did not recognise them as such at the time, including arsenic, antimony, and phosphorus.
In 1789, French chemist Antoine-Laurent Lavoisier offered the first definition of chemical elements. This was a major turning point. Lavoisier defined elements as substances which cannot be further decomposed by means of chemical analysis. By doing so, he established three parameters that would act as the empirical basis for an element’s discovery. Firstly, the substance had to be pure; there should not be any other chemical entities (elements, molecules, etc.) contained in it. Secondly, the new element had to stay relatively stable (under some set of conditions). Thirdly, the sample had to be of a sufficient amount so as to be examinable. Fulfilling this last parameter was essential for studying the chemical and physical properties of the discovered element.
For the last couple of centuries, and prior to the search for superheavy elements, the question of whether (and which) elements were awaiting discovery was largely set by the periodic table. In the 19th century, chemists started to propose specific orderings of the chemical elements that would help them systematise and organise the knowledge they had acquired about them. While competing classifications were proposed, Mendeleev’s table was the one that eventually won over the scientific community. In his proposed classification, chemical elements were ordered by atomic weight. With the determination of atomic structure in the early 20th century, this was amended so that elements were ordered in terms of their atomic number (i.e. number of protons). This resolved some discrepancies in the ordering of elements.
The periodic table contained gaps, suggesting that there were elements which had not been discovered. Moreover, patterns in the properties of sets of elements became much more apparent, and properties could be inferred by the position of the predicted element in the table. This gave chemists a good idea of which properties they should expect in elements that had not yet been discovered. Mendeleev’s classification and the use of his suggested table rendered the discovery of new chemical elements a more precise activity. The periodic table, with its empty gaps and predicted properties of elements, gave a clear direction as to which elements to look for and as to when chemists could confidently say (through the measurement of specific properties) to have discovered them.
Synthesising in the laboratory
All this started to change in the 20th century. In 1939 Marguerite Perey, at the Curie Institute in Paris, was trying to remove radioactive impurities from actium. Unable to purify the actium completely, she inferred that there was another element present – francium. This was the last naturally occurring element to be found.
Thereafter, chemists turned to synthesising elements that are not found in nature. The first element discovered via synthesis was technetium – taking its name from the Greek word technetos, or artificial – which was named in 1937 by Carlo Perrier and Emilio Segré. The element was created using a particle accelerator at the University of California, which scientists used to fire nuclei of deuterium – sometimes known as heavy hydrogen – at molybdenum (though it should be noted that others claimed to have identified the element earlier).
The turn to synthesis happened for several reasons. Firstly, new elements were very difficult to naturally isolate and keep stable in sufficient amounts to study. When radium was discovered by Marie and Pierre Curie in 1898, they extracted only one thousandth of a gram from ten tonnes of pitchblende ore. Secondly, some elements did not exist on Earth as they were highly unstable, having a very short time before their atoms decayed (commonly expressed in terms of their half-life). Thirdly, new technology, such as particle accelerators, allowed for the synthesis of new elements.
Synthesis opened a new chapter in the history of element discovery, largely motivated by the advancement of nuclear physics and its enormous applications in the military and for the development of nuclear energy. But synthetic elements tested the practicality of Lavoisier’s requirements for element discovery. Discoveries were no longer achieved by isolating a substantial amount of a relatively stable pure chemical substance which naturally occurs on Earth. While extracting a new chemical element without impurities was still a desirable objective, isolating a good amount of a substance for a good amount of time became extremely difficult.
Reinventing discovery
Synthesis meant that the elements that scientists set out to discover became heavier and heavier, containing ever more protons in their nucleus. This made it difficult to keep them stable, as protons electromagnetically repel each other. Eventually, this situation became so grave that it created tensions within the scientific community, leading, in the 1980s, to the so-called Transfermium Wars. These wars concerned priority disputes – battles for credit – over who had actually discovered the elements with atomic numbers between 101 and 109. These disputes were largely based on extra-scientific grounds; it was, after all, the Cold War and among the competing research groups were those based in the United States and the Soviet Union. However, the practical difficulties in establishing an element discovery should not be overlooked. None of the initial parameters set by Lavoisier could be maintained. It was not possible to isolate from nature elements with so many protons in their nuclei. Even by means of synthesis, the newly discovered elements were rarely (if ever) stable. Moreover, chemists could not synthesise enough to allow them to study the new element’s properties. In some instances, chemists would produce very few atoms that would very quickly decay into lighter elements.
‘Synthesis opened a new chapter in the history of element discovery, motivated by the advancement of nuclear physics and its applications in the military’
Chemists and physicists decided to resolve these disputes by explicitly setting criteria that could be used to evaluate a purported element discovery. The International Union for Pure and Applied Chemistry, together with the International Union for Pure and Applied Physics, established the Transfermium Working Group which was assigned to produce two reports. In 1991, the first report titled ‘Criteria that must be satisfied for the DISCOVERY OF A NEW CHEMICAL ELEMENT TO BE RECOGNIZED’, established new criteria for a purported element discovery.
This report represented the first revision of what we admit as an element discovery. Firstly, the range of accepted stability for a newly discovered element became significantly smaller, in the range of one hundred trillionth of a second. Secondly, it was no longer required that a significant amount of a sample was discovered. The purported discovery of a single nuclide sufficed! Thirdly, purity in the examined samples was no longer essential. The traditional ways of pursuing element discovery were officially over.
The 1991 report offered a widely accepted and institutionally approved set of criteria for element discovery. But it was also the first sign that we were entering unchartered waters. A revised version from 2018 shows that the scientific community has started to adopt an even more flexible and fluid concept of element discovery. It is not clear which criteria should be taken as decisive, nor how to interpret them in a way that could be applied to all instances of element discoveries. The report itself states that the recommended criteria should be weighed on a case-by-case basis.
The reasons for this flexibility are both practical. The practical difficulties of discovery become more pressing as we search for elements with more protons. Elements with over 104 protons are called superheavy. Overall, scientists have discovered 118 elements – 14 of them superheavy. In 2006, the discovery of oganesson was confirmed. This element has 118 protons in its nucleus and became the most recent superheavy element to be discovered via synthesis. It is only one of two elements to be named after a living scientist – Yuri Oganessian, leader of the Joint Institute for Nuclear Research in Russia which first identified the element in 2002. It is quite revealing that its discovery was admitted by the official scientific committees only in 2015. The reason it took so long is largely because of the difficulty to synthesise even just a couple of its atoms. Collecting sufficient empirical evidence that would convince the community that a discovery had actually been achieved was particularly challenging.
The end of the line?
By 2010 all the gaps in the table were filled, and the last (seventh) row was completed with the latest discoveries. Nowadays, scientists debate the physical possibility of heavier elements. Is there are limit to how heavy elements can be? And if so, does this mean that there is an end to this quest that humanity has pursued for thousands of years?
That there may not be (many) more elements to discover is not an unreasonable hypothesis. Based on nuclear physics, scientists regard the idea that the periodic table has a limit – that only a finite number of elements can persistently exist – as fairly uncontroversial. This hypothesis is based on Einstein’s theory of relativity, which suggests that there is a point beyond which relativist effects no longer allow for a nuclide to stick together. That point marks the final limit for elements; beyond that, one cannot reasonably regard these groups of protons and neutrons as a persisting entity.
But there is no certainty about where that limit exactly lies. At the moment, there is only conjecture. Some expect nuclides with atomic numbers up to 190. Others point out that there is no reliable evidence that elements with atomic numbers beyond 118 exist, even though there are some models which suggest that elements with atomic numbers up to 172 could be possible. Even the renowned physicist Richard Feynman made a guess, claiming that the final element would be element 137. Although this element remains a hypothesis, it has been pre-emptively named feynmanium in his honour.
Some believe that there is a realm where superheavy elements reside and await their discovery: that place is called the island of stability. It is based on the idea that nuclides have a specific shell structure (called the nuclear shell model) which allows for specific numbers of protons and neutrons – called magic numbers! – to render those nuclides relatively stable. If we accept this hypothesis, then the goal is to synthesise the precise configuration of protons and neutrons that could stay stable on the basis of this model. To achieve this, however, one must first track where the island of stability lies. Much effort is put into identifying the magic numbers so that scientists have a guide as to which nuclides they should attempt to synthesise.
It is far from clear what expectations we should have for the future of element discovery. What it means to discover elements has become fuzzy and can only be judged on a case-by-case basis. Moreover, the physical possibility of heavier and heavier elements is pushed to its limits, making it unclear whether there are even any new elements to discover. On the other hand, this is a very exciting time for science. The challenges associated with element discovery only make the endeavour more fulfilling. Even if very few elements are left to be found, what we learn in the process about the essence of the minutest things is extremely exciting.
From a more philosophical perspective, this also raises some interesting questions. How do scientific discoveries come about, and how can they shift our views of nature? In answering these questions the periodic table has played an important role. However, the aforementioned difficulties in element discovery may require us to change how we view the periodic table’s role in scientific practice. The project of discovering new elements no longer revolves around analysing chemistry’s periodic table, but rather is based on predictive models developed in nuclear physics. In any case, one thing is clear: the story is far from over.
Dr Vanessa Seifert is a Marie Curie postdoctoral researcher at the University of Athens, studying the nature of chemical reactions. She previously studied Chemical Engineering and Philosophy of Science. She regularly writes about the interesting connections of philosophy and chemistry.
For more on her work, visit www.vanessa-seifert.com.
For more about the history of elements, Vanessa recommends Philip Ball's The Elements: A Visual History of their Discovery (2021).