No
As we currently understand physical chemistry, all possible elements are known below whatever the state of the art number is now (currently Ununoctium - Element 118).
The atomic number of an element (the number that determines which element it is) can only be an integer. It is, after all, t...
Anti-matter turns out to be a storage medium also. Producing it is difficult and inefficient. When you burn it at best you lose 1/3 of the energy in the form of neutral particles. We have no viable means of storing even electrically charged tungsten bars of the stuff as the stray antimatter tungsten atoms drifting away will generate more heat than we can pump away from the storage tank! Realistically we have no current idea how we might use antimatter (or black holes) as a fuel in an engine but it seems to be a standard trope in SciFi.
+1 for completeness, though I would like to add that if the question had been asked a few decades back the answer would be different. The beauty of Mendeleev's table was the fact that there were gaps. We knew there were elements to be discovered, and we knew some of their properties, such as, they'd be an alkaline metal. But unfortunately, they've all been filled now. And no miracle fuel.
@Mindwin , I attached a wiki reference and then compared my original answer to what Wikipedia suggests. Part of the discrepancy is that I was looking at fuels for spacecraft propulsion and most fusion reactions lose a bunch of energy to neutrons. This energy is lost from a propulsive perspective. There are similar problems with antimatter.
@Jim2B Of course, it's possible that natural sources of antimatter might exist in the universe, in which case it wouldn't be just a storage medium anymore. Not on Earth's surface, and probably not close and/or abundant enough to be practical for typical power generation, but it might be still useful for space travel, where fuel mass is a huge concern. For example, there's been plans to harvest antimatter from the Earth's (or, more ambitiously, Jupiter's) magnetic field - still vastly cheaper than whatever we can produce on Earth right now.
"There's no plausible mechanism for this to work at the moment" -- for the purposes of fiction you could (for example) handwave why it is that this element 128 can somehow catalyse hydrogen fusion and/or deuterium fusion. So it's not actually a fuel, but it opens up a known fuel to exploitation and you only need one lump of Unobtanium per reactor. I'm not aware of any genuine prediction that it will do this though (and if it did you have to wonder what would happen if some fell in a star). So likely this would be implausible to people who understand enough atomic physics to know why that is.
Since you've already mentioned metastable electron configurations, you might want to also include metastable nuclear isomers in your answer. While they might fall under the vague "no nuclear power" prohibition, they do have several potentially interesting properties, like the fact that their decay tends to be unusually "clean", typically consisting of just the nucleus emitting a high-energy photon.
I would include them except I don't know a way to get them to release their energy on demand. As far as I've read and understand it, you can only use them similarly to how you'd use a RTG type (nuclear decay).
On diatomic metastable Helium. This would be good for propellant. Mix with liquid hydrogen and you might get efficiencies similar to various nuclear core rockets without needing to carry the heavy and complex nuclear part.
@Luaan "it's possible that natural sources of antimatter might exist in the universe" No there really isn't any possibility of that. We call space, space, because there isn't much stuff in it, other wise we would call it stuff. Even so, there are quite a bit of stray stuff flying through it all the time (interstellar medium, intergalactic medium etc). That "stuff" would eventually hit your "natural antimatter" and produce light. Therefore any natural antimatter source would be a ridiculously bright glow in the sky. That isn't the case, I just checked.
Another issue with the Island of Stability solution is that this new element would be really heavy, somewhere around 50% denser than lead. This makes it more unlikely that it will have a useful energy density (when measured in energy per unit weight, which is usually the most relevant).
I like the idea of new isotopes being used for high density storage - energy is easy, there's lots about, the difficulty comes in saving it for when its needed and moving it to where its needed...
@IlmariKaronen , I added a section about nuclear isomers. Turns out some scientists claim they can reclaim the energy when they want but so far it takes more energy in (in the form of gamma or x-rays) than they've been able to extract from the isomer.
@Aron Due to space expansion the further away the glow is the less energetic it would be. The glow is visible, we have only been misinterpreting the so called cosmic background radiation all this time.
Also as an FYI, the Earth (and Saturn) do have some antimatter orbiting the planet. This band is naturally replenished by the interaction of cosmic rays with the Earth's atmosphere and then trapped by our magnetic field. But the amounts of antimatter are minuscule (e.g. scientists detected 28 antiprotons). sciencemag.org/news/2011/08/…
Discussion on answer by Jim2B: Are th…
Discussion on answer by Jim2B: Are th…