OP, Say you have a particular "element" as you say (whether carbon, iron, gold or the like). Those elements have a specific number of protons. Each atom (say gold) has a specific number of protons (gold == 79).
BUT the number of neutrons can vary. That's called an isotope. So gold usually has 118 neutrons, but occasionally you can get 116 neutrons and a few other variants.
(Most isotopes are not stable - they just evaporate quickly.)
As someone pointed out, you may be thinking of a "material" or "compound" more than an "element". However, taking your question at face value...
It is no longer possible to discover a "new element" - in short, we have fully found and investigated every one of them. (We can even build some from scratch, which is cool.)
But in a handy-wavey sense, you could say that on planet X there exists an isotope of Iron (or Gold, Carbon - whatever base element you want) which is stable and has the very special properties you mention.
For some reason, the isotope - ultra-iron! - which is unknown to us, and difficult/impossible for us to manufacture in labs - for some reason it appears on your planet X. How could that be? Maybe there was (or still is) a black hole or even something more exotic such as a pulsar, magnetar (!) or even something more fanciful such as a "dark energy concentration" (!!) .. nearby your planet X.
Recall that, in short, "elements" as you ask for are made in stars, so exotic galactic phenomena kind of add up for you.
So, that is a bit more scientific than the usual "Krypton" is a special "thing" that comes from a certain planet - you know?
A challenge now would be to read up on isotopes (and indeed the elements for that matter), and these days you have wikipedia a click away. Maybe it will give you some ideas.
Here's an plot point for you :) ...
So, Your Planet is quite close, let's say 300ly, to the Crab Nebula, which for us is just a pretty dot in the sky. (We are 7000 ly away from the Crab Nebula.)
At that relatively short distance (unknown to our physicists today), the Crab pulsar's spinning neutron star emits huge quantities of neutralinos, which are in real life one of the current subatomic particles, which, our scientists are desperately trying to create or spot. It's extremely difficult/impossible to see neutralinos here, but on Planet Mystere, they are bathed in them. (Neutralinos are indeed harmless to life and matter).
It turns out, a neutralino bath creates a unique stable isotope of Iron. As described in the above answer. So that's why Planet Mystere has the amazing "crab-iron" or "neutralino-iron" or "heavy-iron" as our scientists come to call it. You can build spaceships, AI, and all sorts of wild stuff with neutralino-iron, that cannot be built with what we think of as materials. (Surely, it would be superconducting to begin with, it would form macroscopic single-molecule nano fibers easily, etc etc - naturally!)
But check this out...
As the material moves away from the vicinity of the crab nebula, there are less and less neutralinos, and .... the isotope falls apart and it becomes normal iron again.
So those jokers can build FTL ships, and build true AI - but - as soon as they fly, oh, 100 ly further away from the Crab Nebula - the super-substance just turns in to a lump of iron, rather like a 70s Chevy small block.