Ah, the periodic table of the elements: that ubiquitous chart in the realm of chemistry. In the heady days of the 19th century new elements were being discovered, filling in the table left and right, culminating with the last discovery of a naturally-occurring stable element, rhenium, in 1925. New radioactive elements heavier than uranium (element 92) were being synthesized by the mid 20th century, some of which had great practical applications. Plutonium (element 94) is commonly used in nuclear reactors, nuclear warheads, and radioisotope thermoelectric generators; americium (element 95) is commonly used in (of all things) smoke detectors. This series of practically-relevant elements ended with californium (element 98), which is used to start up nuclear reactors and as a neutron source. The next-heaviest element, einsteinium (element 99), is the heaviest element that has been produced in macroscopic quantities. Beginning with fermium (element 100), new elements have only been produced in microscopic quantities, their half-lives measured in milliseconds. Such elements have been synthesized up to oganesson (element 118), though it’s likely with continued work the periodic table will be extended.
A simple periodic table of all 118 known elements, courtesy of Offnfopt of Wikipedia. CC-BY-SA 4.0
An Island of Stability
So is this the end of the road, barring new elements whose half-lives are but an instant, their existences a cross between a technicality and a scientific curiosity? Perhaps not. A big driver behind the effort to synthesize elements up to 118 has been the theoretically-predicted but tantalizingly-yet-to-be-reached “island of stability”, a region of superheavy elements that might be relatively stable, with half-lives long enough for them to be produced and used in macroscopic quantities.
Island of stability derived from Zagrebaev, by Lasunncty of Wikipedia. CC-BY-SA 4.0
None of the isotopes predicted to lie on the “island of stability” have been synthesized yet, largely because our particle accelerators are insufficiently powerful to deliver the amount of neutrons required to create them, but early results from the latest elements to be synthesized, the “transactinides”, indicate they are indeed more stable than their predecessors on the periodic table, just as the island of stability theory predicts.
Where exactly the center of the island rests, i.e. the most stable of these transactinides, is uncertain, but is generally thought to lie around elements 112 or 114, in the vicinity of 184 neutrons, i.e. isotopes with atomic weights just below 300 (for reference, uranium’s most common isotope has a weight of 238).
Exactly how stable these isotopes are is uncertain; estimates for their half-lives range from minutes, in which case they’d just be scientific curiosities, to possibly hundreds of millions of years, comparable to uranium-235’s half-life, in which case they’d likely be very useful in a wide variety of advanced nuclear applications!
The really interesting possibility is if the longer predictions hold true, and this is what I’ve penciled in for my alternate-history/science-fiction/space-opera fictional universe. It’s worth noting that in my world nuclear explosives are in widespread civilian usage, and a nuclear bomb has been posited as the only viable way we have with current or near-future technology to synthesize the isotopes on the island of stability: after all, the most concentrated dose of neutrons we can deliver to anything is via a neutron bomb. Assuming the super-heavy elements on the island of stability have such long half-lives, they likely will be well known by the mid to late 20th century and synthesized in bulk quantities in my universe starting around that time.
Energetic processes in nature might produce these isotopes, but if so they would have to be found only in minute quantities, less than one part in one trillion relative to their stable homologs. It’s thought that there may be no naturally-occurring process that can synthesize these elements in bulk even if they are relatively stable; nevertheless, there are tantalizing hints here and there that they might indeed be naturally occurring in minute quantities, so watch that space.
Even more speculatively, it has been suggested there might not be one but two or perhaps even more islands of stability further along the periodic table. The location of these additional islands, assuming they exist at all, is highly uncertain, but regions centering around elements 126, 154, and 164, and atomic masses 354, 472, 482, 342, and 462, have been suggested. Their half-lives are also highly uncertain, but it’s possible these additional islands might have half-lives comparable to the isotopes on the first island of stability!
Quark Matter and the End of the Periodic Table
These possibilities raise the question of where exactly the periodic table ends. The lowest estimates posit an end soon after the first island of stability, around element 126, but on the flip side some top chemists have suggested that there is no end to the periodic table, with many others suggesting numbers in between for the truly final elements.
A much-more out-there possibility, first suggested in a 2018 study, is that there’s not just an island of stability but a whole continent of stability out there just beyond the isotopes we’ve synthesized to date. Specifically, once nuclei grow beyond an atomic mass of around 300, i.e. just beyond the peak of the island of stability, the nucleons (protons and neutrons) break down and the nucleus becomes a free-flowing mass of their constituent particles, up and down quarks, the state known as “up-down quark matter”, or udQM.
For heavy nuclei this is speculated to be a lower energy state than the quarks being bound within protons and neutrons; significantly, this state of matter is posited to be entirely stable, devoid of radioactive decay, stretching up perhaps forever across a much wider range of charges than conventional matter. The continent of stability in the 2018 study was calculated to extend through an atomic mass of 3000, up to a net positive charge (i.e. an element number) of around 1000 and down to a net negative charge of around 200.
The posited “continent of stability”, chart by Mrmw of Wikipedia.
Now there’s some superheavy elements that would be useful, particularly in a futurological or science-fictional context. Given that it’s an entirely different state of matter and the enormously wide range of masses and charges it encompasses, up-down quark matter would arguably be less an extension of the periodic table and more the beginning of a whole ‘nother periodic table.
Nevertheless, if you count up-down quark matter and its continent of stability as part of the periodic table, it seems likely that the periodic table will never end, that atomic masses and charges extend to infinity. Indeed, it’s possible that many stars we think of as neutron stars are actually quark stars, filled not with neutrons but rather with such up-down quark matter, in which case the entire core of it would arguably count as one big atomic nucleus.
For my space-opera universe’s worldbuilding I think I’ll include the continent of stability, since stable superheavy quark matter just beyond the island of stability is just too cool an idea to pass up.
The Periodic Table: beyond Quark Matter
Even more speculatively, it has been theorized that quarks are not the most fundamental particles in the universe, and that the quarks themselves are composites of even smaller particles, usually called “preons”. Such theories have little if any evidence behind them, and have a host of problems, but they do have certain interesting properties that would explain a lot about our universe; the rishon model, in particular, elegantly explains the abundance of matter over antimatter: all the antimatter is bound up in preon-level structures. So for worldbuilding purposes I’ve included preons along the lines of the rishon model in my setting.
One upshot of this is that there might be stars even more exotic than the quark stars: preon stars, stellar remnants held together by preon degeneracy pressure instead of neutron degeneracy or even quark degeneracy. Like the quark stars they would superficially look much like neutron stars, but they’d be smaller and denser. It’s even been proposed that preon stars formed primordially in the Big Bang might be what dark matter is made of!
Another upshot, and one even more speculative, is that if there’s a certain atomic mass beyond which quark matter is a lower-energy state than hadronic matter, might there likewise be an atomic mass beyond which preon matter is a lower-energy state than quark matter? A second continent of stability? If there is such a second continent of stability, where matter is in the state of free-flowing preons rather than being bound up in quarks, the range of stable charges might be even larger than it is for quark matter, in principle extending outward infinitely.
As near as I can tell, quark matter would be far denser than any form of hadronic matter, and as a stable non-radioactive material would find all manner of practical applications. You could probably do things with it like shield yourself from instantly-lethal radiation with a material as thin and flexible as paper (enabling far more compact and powerful reactor systems for small-scale purposes like personal vehicles). Armor made out of it may well be impenetrable. Preon matter might be denser still, and even more useful.
That’s the sort of development that could come out of left field in terms of the progression of technology, and be very scientifically interesting to boot. This is especially true in a science-fictional context, like the universe I write in, but this is all based on real science: this very well could be the future in real life! Super cool.