To see why, let's take a look at the periodic table, which summarizes what we know about the elements. To begin with we need to have a definition of elements themselves. Basically, all atoms have an integer number of protons in their nucleus, known as their atomic number. Atoms with the same number are the same element, and have the same attributes.
|An element's atomic number uniquely identifies it.|
And thus we see the problem that a fictional element (in a moderately hard science fiction setting) must overcome: all the elements up to a hundred and eighteen have been discovered already. Having a new element without further explanation feels to me like someone trying to tell me that they've discovered a new integer between 1 and 118; my immediate response is always going to be, “But we already know all of those integers.”
Now, while this is a problem, there are two approaches that I can see to justifying it:
- The easy way: give it an atomic number higher than 118. The problem with this is that everything we know about nuclear physics says that anything with such a number should decay so fast it'd be nearly impossible to detect. But this is science fiction we're talking about here. It's pretty trivial to throw in a section discussing why what we thought we knew about physics was in fact wrong, allowing stable high-atomic-number elements to exists. At the very least, lampshade it by having someone remark how they have no idea how such an element is stable, but it'll give the physicists and chemists something to work on! I've seen this done in a free game set in the Half-Life universe released recently called Transmissions: Element 120. They went with the route of explicitly disclaiming knowledge of how element 120 (the putative source of the game's gimmick) was so stable, but it showed to me that they had at least acknowledged that a problem existed and had thought about it, and it didn't break my immersion the way simply announcing some new element would have. (They even used what would be the correct chemical symbol for element 120, Ununbium: Uub).
- The slightly odd way: have the fictional element replace a real one. So this universe is like ours, except that, say, ruthenium is replaced by fictitium. I've never seen this used, and it would really only sort of paper over the problem, but it could be interesting if the replacement element is judiciously chosen. Elements that are uniformly highly radioactive and decay quickly might be good choices to replace; no one's really going to notice if astatine didn't exist, whereas history would be very different if iron didn't exist.
Of course, it's painfully obvious that most of the fictional elements that most badly break my immersion were only added to their respective settings to drive the plot (usually through opposing factions warring over it) and had no thought whatsoever given to them. To those needing such a plot driver, might I suggest a third path? Use an existing element, and give it a new property that explains why it's in such high demand. It's much easier to accept that an obscure element has suddenly become much more sought after due to a recent discovery than a totally new, never-really-explained element. Plenty of elements that were simply intellectual curiosities to their 19th-century discoverers are now critical components of modern items and infrastructure due to discoveries in the intervening time that could scarcely be imagined at the time they were discovered. (Various rare-earth metals being important in making high-end electronics, for one example.)
Note that this whole rant really only applies to science fictional settings of at least a moderately hard nature where it can be assumed that the periodic table exists. Mithril in The Lord of the Rings doesn't bother me because it's a fantasy setting. (You could also rationalize it as maybe being some real element that was unknown to any but the dwarves at the time. A light, strong, silvery metal could apply to aluminum, which is normally found in compounds but can very rarely occur in pure form as nuggets, explaining why it's only found in the mines of Moria. Just a fun thought exercise.)
Similarly, I don't mind if all the elements in a series are fictional. Star Wars goes this route, with the Expanded Universe (before Disney disavowed it) having probably dozens of fictional elements and minerals. That's interesting, and it gives you a lot of latitude in coming up with interesting elements. It's mixing real and fictitious elements (without justifying the latter) that really starts the smoke coming out of my ears as my brain tries to reconcile the fictional element with reality (for settings where it's assumed things are like reality unless specifically noted).
To put it simply, in science fiction stories, don't mix your real and fictitious elements without comment or justification, please. One or the other, not both.
Well, I don't know how interesting any of that was, but it's been buzzing about in my head for a while and I wanted to get it out. Sorry if I've now retroactively spoiled anyone's favorite story. A hui hou!