A common trope in science fiction is the discovery of “an element that does not appear on the periodic table.” This is of course nonsense… if the element has three protons, it’s lithium, full stop. And we’ve discovered – or invented – every element up to Element 118, Oganesson. These higher elements are quite unstable, generally having half lives of microseconds to minutes… but there is a suspected “island of stability” with some of these heavier elements if you can nail the isotopes correctly. Which nobody has.
But even if you can reliably manufacture the stable versions of these heavy elements on an industrial scale, the likelihood of them being useful to make spaceships or battle armor is low… and they’d still appear on the periodic table. Their physical properties probably won’t be that terribly interesting. Oganesson, for example, melts at a boring 52 C/125 F, and has a density of only 7.2 gm/cubic cm.
But Amazing Materials are terribly useful in science fiction. Witness Star Trek’s “dilithium,” a crystal that somehow or other allows matter-antimatter reactions that are controllable and useful. In Babylon 5 there was Quantium 40, necessary for the construction of jumpgates. So what the hell are these?
I kinda recall that “Quantium 4o” was just bog-standard potassium-40 that had been close enough to a supernova to be *somethinged,* and changed at the quantum level *somehow,* resulting in a material that does Amazing Things. How about dilithium? Well, dilithium is a real substance…. just a molecule with two lithium atoms bonded together in the gas phase. It’s clearly not the same thing as Trek’s “dilithium crystals,” and certainly not the later “trilithium resin” which has the interesting property that a few grams of the stuff can cause all fusion to cease in a star moments after it’s dropped in.
So… what the hell are Trekkian “dilithium,” or “vibranium” or “adamantium” or “byzanium” or “naquadah?” A century or more ago, fabulous new elements like “cavorite” could be thrown in without issue, because the list of known elements was massively incomplete. We can’t do that today.
In general, unless it’s important to the story, there’s neither need nor benefit to explaining such details in a story… it just exists. But now we know that there are no slots in the periodic table for new elements, so introducing them raises questions. So how to create actual new elements?
Honestly… I don’t know. But I’ve got an ill-formed probably crappy idea.
Take “dilithium.” Clearly “lithium” has something to do with it. Presumably *two* lithiums, given the name. If you simply stick two lithium atoms together, you get the previously mentioned uninteresting real-world “dilithium.” If you mash the lithiums together hard enough that the nuclei bond together, you no linger have lithium, you have carbon. This is just the way it is.
But while we can’t really posit new elements, we *can* posit the discovery, creation, utilization of new particles. We’re not done discovering such things. So *perhaps* there is a particle that can, say, replace neutrons in a nuclei, fullfilling the same function of gluing the protons into place. But they have a different effect on the electron shells. Perhaps to the point where two lithium nuclei so modified will actually stick together, but not actually fuse into carbon. Sort of a contact binary nuclei. Or these mystery particles are simply added to the neutrons and protons; being the same mass as a proton, if you add six of these new particles to a lithium atom you double the mass… thus di-lithium. These modified atoms would undoubtedly have very different properties from the regular stuff… properties the writer can simply declare. Adamantium, for instance, is a modification of helium; this turns it into a metal that, apparently, melts at a low temperature *once*, but when it solidifies it becomes not only insanely hard and tough, its melting temperature shoots way up.
In recent years one of the more popular fictional elements is “unobtanium” from “Avatar,” a room temperature superconductor. This *has* to be a new pseudo-element, rather than an alloy or compound. Because if it was the latter, a technological species with the ability to create antimatter-powered relativistic starships could simply synthesize the stuff. There’d be no need to mine it light years away. But if it was a material made from exotic particles… shrug.