As in the title.

We have the modern periodic table:

periodic table

In a potential near (or far) future setting humanity discovers a new element which serves as the basis for a cleaner, higher energy density fuel than anything we have today.

How this element is found is irrelevant, I don't care if it came from a comet smashing into earth or was found on an exploratory space mission to some other place.

  1. Are there potential gaps in the periodic table where such an element could exist?

  2. Where on the periodic table would such an element land to be the best fuel source?

Criteria for "best"

  • Higher energy density than Nuclear
  • Cleaner than all fossil fuel types
  • Preferably renewable.
  • 11
    $\begingroup$ What kind of detail can you expect for offering the bounty? The illustration you added already answers the question that there are no gaps, and I don't know how to explain that "better". $\endgroup$
    – JDługosz
    Mar 4, 2016 at 9:20
  • 1
    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ Mar 6, 2016 at 2:20
  • $\begingroup$ Still wondering... maybe answer before the time runs out? $\endgroup$
    – JDługosz
    Mar 7, 2016 at 16:53
  • $\begingroup$ @JDługosz I believe I had answered that but the comments seem to be gone. And if you read the bounty posting I explained why I put a bounty on it. $\endgroup$
    – James
    Mar 7, 2016 at 17:04
  • $\begingroup$ You mean “would like to ensure there are no other opinions outstanding that should be considered.” ? I read that as you want a different answer. $\endgroup$
    – JDługosz
    Mar 7, 2016 at 21:43

17 Answers 17



As we currently understand physical chemistry, all possible elements are known below whatever the state of the art number is now (Oganesson - 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, the number of protons contained in the nucleus. Just like there are no unknown integers between 1 & 118, there are no unknown elements in this range.

Chemical reactions occur by complicated interactions between electron spin, electrons filling (or not) orbitals (valence electrons), and the charge of the atom (ionic bonds). All of this is due to the quantity of electrons (strongly influenced by the number of protons through the electromagnetic force) and how they fill the electron orbitals. So if you have a nucleus with no protons, it isn't "element zero," it is a neutron. Since neutrons have no charge, they do not bind any electrons. If a nucleus has no electrons, then it cannot interact chemically with anything.

Magic Stable Island

However, there's a current hypothesis called Stable Island. It posits that certain, as yet to be discovered, elemental isotopes will exhibit more stability than the elements around them in the periodic table.

Some theoretical calculations show that some element's isotopes in the magic "stable island" could have half lives as high as $10^9$ (a billion) years. More recent calculations indicate they would possess much shorter half-lives on the order of hours or days. Since no isotope of any elements supposed to be in the magic island have ever been observed, scientific consensus is leaning strongly towards the lower estimates.

The Stable Island is circled in the graphic. It looks like the center of the island is around atomic number of 112 and nucleon number of 276 & 278 (Copernicium - Cm). There is a second island of less stable nuclei at around Atomic Number 125 (nucleon number 294+). We've created some isotopes of Element 112, but its half-life is so short, we do not know its bulk properties. We haven't created any isotopes of Element 125:
Stable Island Vertical axis is Atomic Number (number of $p$) and starts at 81 (Thallium). Each square represents an integer.
Horizontal axis is Nucleon Number (number of $p$ + $n$) and starts at around 205. Each square represents an integer.
Outlined boxes represent element isotopes already discovered or created.
Dashed black line shows the "optimal" $\frac{p}{n}$ ratio for a stable element isotope.

Finding an Unknown Element

I really like the idea of finding new elements, so let's assume that:

  1. The Stable Island exists
  2. There are undiscovered isotopes in one or more of these Stable Islands
  3. Some isotopes in the island have a half-life above $10^8$ years
  4. Supernovas make these elements

Plutonium (244Pu) has a half-life around $8 \cdot 10^7$ years. If we make some of it in a reactor, then for human purposes it sticks around forever. However, we have only once found any naturally occurring 244Pu (Do transuranic elements such as plutonium ever occur naturally?).

Just like Plutonium, a high-Z element with a half-life of $10^7$ - $10^8$ years would seem very stable to humans and exist for perhaps billions of years. All materials native to the solar system were create about 4.6 billions years ago in a supernova that cause the collapse of the dust cloud that formed our solar system. Since then radioactive elements have been decaying, so our proposed isotope would be short-lived enough that we should find very little or none of it in the materials native to our Solar System.

Interestingly, the Sun is sitting in a galactic feature called The Local Bubble. A series of supernovae which occurred from 10-50 million years ago blew the interstellar gases out of this region making the density of intergalactic gas in this region particularly low.

More importantly, the timing and location of these supernovae mean it is conceivable that a chunk of material from one of them could have made the trip to our solar system over the last 30-50 million years. Since these supernovae occurred only $10^7$ years ago, our proposed radioactive isotope should still have a very high percent of the original undecayed isotope remaining.

So imagine that humanity sees a body heading through the Solar System on a hyperbolic trajectory. This means that the body originated outside of our solar system and it will sail right through the Solar System unless we divert it. We would have to intercept and deflect it to pass near one of the gas giants to impart enough of a momentum change to capture it. Only one of these planets could deflect it enough to keep it in our Solar System. After deflection, we could find it is coated with a heavy elements (platinum group metals, uranium, plutonium, gold, and other materials that are rare on Earth).

What to use it for

Even if it burns more energetically with Oxygen than any other known chemical reaction (though physical chemistry suggests 118 is a Noble Gas, 112 will be close to a Noble Metal, and 125 will be a Rare Earth metal - so realistically you should expect any of these to react weakly or not at all with Oxygen), would humanity actually use it as a chemical fuel?

Certainly not.

It cannot be found on Earth, making it more valuable than any Terrestrial material you can think of (more valuable than Gold, Platinum, or even the most precious gemstones). If you were going to "burn" it, you wouldn't use the paltry energy release of chemical reactions (after all, how many people do you know who burn diamonds for heat?). Instead you'd go for the 1,000,000 $\times$ energy release of nuclear power.

Even so (and regardless of your "no nuclear" caveat), I suspect the material would be way too valuable to "burn" in fission reactors. With a $10^8$ish year half-life the only way we would get more would be if another such body flew through the Solar System. Don't expect it to be any sort of renewable energy supply. (If you wanted an SF analogy of the depletion of the Earth's fossil fuels, this might make for an interesting story though)

It would be used mostly for research - trying to discover just what the material could do. Or for the super wealthy, perhaps making some souvenir trinkets to wear (the material would be radioactive but not so radioactive as to be dangerous).

Edit 2/29/2016: So I was thinking about this and thought, hey what if we had a moderately large metallic asteroid on hyperbolic course through our solar system. We sent a probe to it and found it was chock full of elements from the stable island. If we could deflect its course, we'd have a huge quantity of the stuff available for all sorts of things (fission reactors, research, special material properties, etc.).

These materials would still not be used for their chemical reactions, the energy released would not be worth the energy investment to get the materials. It might be used for fission if it was a superior fission fuel (it would release more energy than the fission fuels we already use). Perhaps "burning" it would result in fission "ash" that were especially valuable elements (like platinum group metals), and released far fewer neutrons during the reaction. But that violates your no nuclear power criteria.

Regardless, this scenario also breaks your renewables scenario.

Other ways to get there

There are at least two other ways to get what you want.

Some form of fusion, preferably cheap and low energy (aka "cold fusion") would serve nicely. There's no plausible and economic mechanism for this to work at the moment but it would not be a complete violation of physical laws to assume some way of doing it was discovered.

Metastable Helium
A Helium atom has 2 electrons. The lowest energy orbital is the "1S" orbital. The "1S" orbital can hold up to 2 electrons. However, those two electrons must possess different values of "spin" (one "up" and one "down"). If you instead give the Helium two electrons with the same spin value (e.g. two with the "up" spin), then one will sit in the "1s" orbital but its presence prevents the second electron from also falling into the "1s" orbital. Instead it sits in the "2s" orbital. This is called Metastable Helium.

Metastable Helium could conceivably provide energies far higher than any chemical reaction and give specific impulses of up to 10x that of $2\text{H}_{2\text{(L)}} + \text{O}_{2\text{(L)}} \rightarrow 2\text{H}_2\text{O}$. Since this is a "Real Thing", you wouldn't be violating the laws of physics to include it in your world.

Metastable Helium has a half-life of about 2.3 hours but it can be catalyzed to decay ("burn") faster.

The two main drawbacks of Metastable Helium are:

  1. It is not a fuel, it is an energy storage mechanism (you still need power plants to make your energy).
  2. Metastable Helium is, well, metastable. It has a tendency to spontaneously release its energy. Helium switching from metastable to stable tends to catalyze surrounding Metastable Helium to do the same thing. If you have a large fuel tank, the large quantity and the high energy density of the substance tends to lead to an "Earth shattering Ka-Boom"

There's a variant of Metastable Helium that reduces of some of its problems (e.g. making it more stable and giving it a longer half-life). This is called diatomic metastable Helium. You bond a metastable Helium to a stable Helium then chill it until it forms a solid. The resulting material has a half-life measured in years but it releases its energy when exposed to heat.

Unfortunately, this halves the energy density of Metastable Helium - but that's still much better than typical chemical reactions.

Nuclear Isomers
Another possibility is a nuclear isomer.

Imagine a small but very elastic balloon with a wide mouth. Fill this balloon with 72 black ping pong balls (protons - $p$) and 102 white ping pong balls (neutrons - $n$) representative of the nucleus of Hafnium (Hf). Both $p$ and $n$ are nucleons. Each ping pong ball has a Mexican jumping bean in it. Shake your balloon until you get the minimum possible surface area - this is known as your minimum or ground energy state. Now carefully pull one nucleon out of its ground state and move it to the other side of the balloon so it sticks out. This is a nuclear isomer and represents the "excited state" of the nucleus.

If you just let the configuration sit for a while, the random energy supplied by the Mexican jumping beans will eventually cause the ping pong balls to suddenly shift back into their ground state. This will release a sound ("voomp!"). The balloon represents a nucleus. The ping pong balls represents the nucleons. The sound is the gamma ray ($\gamma$) released when the nuclear isomer releases its energy.

If you bang the balloon sufficiently hard, the nucleus will reconfigure to the ground state too. This is the what scientists are trying to prove in the lab.

While Metastable Helium uses excited electron states to store energy, nuclear isomers store energy in excited nucleons. Just as nuclear reactions are $10^6 \times$ more powerful than chemical ones, excited nucleons can store enormously more energy than electrons (on the order of $5 \cdot 10^5 \times$ more than most chemical reactions).

Similar to how the range of half-lives for radioactive elements range from picoseconds to more than tens of billions of years, the same is true for nuclear isomers. It is thought one isomer has a half-life of around $10^{15}$ years - it has never been observed to decay and has no practical use. Others have half-lives so short they are of no practical use either because all atoms in the sample appear to spontaneously decay. However, Hafnium has a nuclear isomer with a very convenient half-life of 31 years and might be useful.

There are a few problems with nuclear isomers. These include an inability to trigger the release of the energy (**more on this below). Energy is released from Hafnium as gamma rays (which requires shielding). As with Metastable Helium, nuclear isomers are not a fuel but are an energy storage mechanism. They are only renewable in the same way that batteries (and Metastable Helium) are renewable - they can be recharged and reused.

There has been some controversial research indicating that a method of stimulating the release of energy from nuclear isomers has been found. So far though, the amount of energy required to stimulate them is more than the energy they can release.

Despite all those issues, nuclear isomers are theoretically possible energy storage mechanism and they could become a component of an awesome energy storage infrastructure. You would charge them up and then use them like batteries whose power slowly wound down.

Energy Densities

You won't find any non-nuclear energy source with energy densities greater than those of nuclear. It has to do with the type of forces involved (Nuclear Strong and Weak versus Electromagnetism) and is fundamental to the nature of the Universe.

Many people don't have an innate understanding of the relative magnitudes of the energy released between chemical and nuclear power. So let's use distance as representative of energy. If 1 cm correlates to the energy released by the most powerful chemical reactions, then 16 kilometers is the energy released by nuclear fission. Fusion is 100+$\times$ more powerful (represented by a distance 1,600 kilometers). Generating power using chemical energy is just not very effective compared to nuclear.

This is the relative energy densities of several materials:

  • Antimatter > $12 \times$ Fusion (Wiki value of $150 \times$)
  • Fusion > $4 \times$ Fission (Wiki value of $100 \times$)
  • Fission > $10^6 \times$ chemical (Wiki value of $1.6 \cdot 10^6 \times$)
  • Nuclear Isomer > $5 \cdot 10^5 \times$ chemical
  • Metastable Helium > $10-100 \times$ most other chemical (Wiki value $10 \times$)
  • Diatomic Metastable Helium > $5-50 \times$ most other chemical (Wiki value $5 \times$)
  • Most chemical fuels > $100 \times$ most renewables (direct comparison is difficult because renewable fuels are often "free" but their infrastructure is huge and costly)

Incidentally, you get the most energetic chemical reactions between elements by combining elements from the upper right of the periodic table (oxidizers like Fluorine/Oxygen) with those on the lower left (reducers / alkali metals). An as yet not created alkali metal Ununennium - Element 119 would satisfy your requirements. However, this element is not expected to have a half-life greater than microseconds and wouldn't survive a trip from the nearest supernova (the stellar event that creates such elements) to the Solar System.

You can create molecules with more combustion energy by many mechanisms. Most of these extremely powerful explosives are not stable and therefore not safe to use in most cases. Others (e.g. Octanitrocubane aka Cubane), are so difficult to create that they are too expensive to manufacture in quantity.

Octanitrocubane (molecular formula: C8(NO2)8) is a high explosive that, like TNT, is shock-insensitive (not readily detonated by shock).


Octanitrocubane is thought to have 20–25% greater performance than HMX (octogen).

But ultimately chemical reaction energies are limited by the energies available by chemical bond strengths.


Renewables at least as commonly conceived (e.g. wind & solar) possess extremely low energy densities. Consider a wind farm with 1000 of the large 1 MW wind turbines. It would cover many square miles and generate 1/5 or less of the energy produced by a single mid-sized 1 GW coal burning or nuclear plant which occupies a just a couple of acres of land (actual power generated and not installed theoretical capacity).

If you need energy density, then you need nuclear.

If you want renewables, then you have to live with extremely poor energy densities.

If you want both, then you need something like the Metastable Helium (see above) to store the energy produced by your low energy density power plant in a form that has high energy density (but you still have to live with the huge, low energy density wind farm).

Although it is currently popular to extol the virtues of "renewables", ultimately renewables come from sunlight and sunlight comes from the Sun which is a giant nuclear power plant. Why deal with all the middlemen (intervening processes) each of which has fairly steep efficiency loss. Why not work directly with the nuclear power?

How might this work?

Using the diatomic metastable Helium example...

Suppose scientists were examining cometary materials (how they were collected isn't important) and discovered one such compound contained metastable helium in some form that was much more stable than any we've discovered so far. Just knowing that a material like that existed and having some examples would lead to a huge new area of research.

Eventually, everything might run on the new energy storage media using the metastable helium compound as the battery and huge renewables spread across the planet (or the Moon) to power up those batteries.

We would not depend upon mining the materials from heavenly sources.

As an alternative if it is important in your story that we had to mine the fuel source, then you could always turn to mining the Moon and other astronomical bodies for 3He. That requires you to use nuclear though. On the plus side, 3He throws off significantly fewer neutrons than most other fusion reactions.

  • 56
    $\begingroup$ +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. $\endgroup$
    – IchabodE
    Feb 29, 2016 at 23:39
  • 1
    $\begingroup$ Fair enough. I did mean nucleon number and not Atomic Mass. $\endgroup$
    – Jim2B
    Mar 3, 2016 at 17:48
  • 1
    $\begingroup$ This is why I love Worldbuilding.stackexchange. We get to hear from knowledgeable people about interesting problems. $\endgroup$
    – Numeri
    Mar 4, 2016 at 1:42
  • 1
    $\begingroup$ The elements hace been named now! $\endgroup$
    – JDługosz
    Jun 15, 2016 at 3:16
  • 4
    $\begingroup$ Now Ununoctium is called Oganesson. $\endgroup$
    – Ender Look
    Jun 26, 2017 at 22:04

Short answer: No.

Elements are identified by their atomic number, i.e. the number of protons in the nucleus.

When Mendeleev invented the periodic table, there were a number of holes. For example, there was no known element between Calcium, atomic number 20, and Titanium, number 22. Mendeleev therefore proposed the existence of new, previously unknown elements. He predicted properties of these elements based on where they would fit in his periodic table. When these elements were found and indeed did have the properties he predicted, this was strong evidence that the theory behind his periodic table was correct.

(Note: Mendeleev built his table using atomic weights rather than atomic numbers, so his methods were not quite as rigorous as we have today. But that's a side point.)

Today we have identified elements for every possible atomic number from 1 (Hydrogen) to 118 (tentatively named ununoctium). That is, we know elements 1, 2, 3, 4, 5, 6, etc, every possible integer up to 118. So any unknown elements must have atomic numbers higher than 118.

All known elements with atomic numbers 84 or higher (84=Polonium) are radioactive and unstable. As Jim2B mentions, some physicists theorize that there may be stable elements with higher atomic numbers, but no such elements have ever been found in nature or synthesized in the lab, so at this point it's all theory.

  • $\begingroup$ Actually, Bismuth and higher have no stable isotopes. Uuo os not a tenative name but a placeholder. $\endgroup$
    – JDługosz
    Mar 1, 2016 at 11:03
  • $\begingroup$ RE Bismuth: Wikipedia says it was thought to be stable until 2003. So okay. $\endgroup$
    – Jay
    Mar 1, 2016 at 14:34
  • $\begingroup$ I'm not sure what distinction you're making between "tentative" and "placeholder". If "placeholder" is the official term, ok. Point is, the powers that be have not yet decided what the official name will be, so they're calling it ununoctium until such time. $\endgroup$
    – Jay
    Mar 1, 2016 at 14:36
  • 7
    $\begingroup$ tentative means they chose a name but it's still waiting for final approval and might not end up being that but presumably will. Un un oct is understood as not a name at all, just a cool way to say "element 118". $\endgroup$
    – JDługosz
    Mar 2, 2016 at 0:29
  • 2
    $\begingroup$ @JDługosz RE tentative: Ok, I'll buy that. "Placeholder" is a better word. $\endgroup$
    – Jay
    Mar 2, 2016 at 3:32

People have already covered the parts of the question about the periodic table so I'd like to just talk about one of the side-conditions for a moment.

The two desired properties of renewable and higher energy density than nuclear are, essentially, mutually contradictory. Something being renewable means that the substance can, essentially, come into existence spontaneously. In turn, that means that it can't require huge amounts of energy to construct because, whatever spontaneous process creates the substance from other sources would have to somehow put that energy in. In particular, it would have to concentrate that energy to the energy density of whatever stuff it's making.

That very much limits the energy density of renewable resources. Any spontaneous process that tried to pack so much energy into a small place would probably find it easier to light itself on fire than to produce unobtainium. Not least because there are so many different ways of catching fire, but only a couple of ways of synthesizing unobtanium.


You've answered your own question by posting an image of the Periodic Table - clearly there are no gaps between 1 and 118.

However, since this is WB and not Physics, let's speculate. The nucleus is usually depicted as a bunch of grapes, forming a spherical structure, since that is the lowest energy state. But is it...

Nuclei are bound together by the Strong Force. This is very short range - essentially binding nucleons to their nearest neighbours. Fighting against the Strong Force is the electrostatic repulsion of the protons, which is long range. Any one proton feels repelled by all the others in the nucleus. As the nucleus grows, the repulsive force becomes greater while the Strong Force doesn't really change. Adding neutral neutrons helps because this pushes the protons further apart, thus weakening the electrostatic force. This is why the proton-neutron mix starts off about equal and becomes increasingly neutron-rich as we go up. By the time we get to a few hundred nucleons, it is getting difficult to keep the thing together and we get radioactive isotopes that keep falling apart.

However, what if there is a region of stability in the very large atomic number range? Maybe when we get to, say, 1000 nuclei, structures can form that lead to it being stable. For example, you could have an outer shell of alternating protons and neutrons (like the pattern on a football) with a central core of neutrons. That would keep the protons well apart but allow you to have a massive nucleus.

Its chemical properties would be weird - a giant nucleus would play havoc with the electron orbital radii. It would be very dense - maybe two or three orders of magnitude denser than existing elements. If you perturbed the proton-neutron lattice, it would decay into a spray of lighter elements and release rather a lot of energy, I would imagine.

Such a thing would be unlikely to form naturally since nuclei are usually made by squishing together lighter nuclei. However, you might be able to engineer such a nucleus: make a 2D $p-n$ lattice, then wrap it round a blob of neutronium. Simples!

  • $\begingroup$ Unless there's a new fundamental force of the Universe, no you can't have stable nuclei at atomic mass of 1000. Strong force is attractive but very short ranged. Weak force is very strong but weaker than the Strong force and always repulsive.. It's range is limited but not as limited as the strong force. What happens in nuclei to make them fall apart (radioactive decay) is that they get too big for the Strong force to hold them together. The weak force + electromagnetic repulsion push bits of the nuclei out. $\endgroup$
    – Jim2B
    Mar 3, 2016 at 2:16
  • $\begingroup$ From what I understand, neutrons and protons in the nucleus are best treated as part of a "liquid drop," rather than as fixed points in a lattice. So I don't think "an outer shell of alternating protons and neutrons" really makes sense. $\endgroup$
    – zeta
    Mar 4, 2016 at 1:41
  • $\begingroup$ It's not that simple: your model predicts that adding more nutrons is always an improvement. But note that this is not the case, and a bunch of neutrons (only) don't stick together at all! $\endgroup$
    – JDługosz
    Mar 4, 2016 at 9:29
  • $\begingroup$ @Jim2B Yeah... but I was wondering if some funny organised structure could make it work. I was imagining a kind of buckyball with alternating protons and neutrons at the nodes and a pure neutron filling. BTW, this is WB, not Physics :-) $\endgroup$ Mar 8, 2016 at 12:50
  • $\begingroup$ @JDługosz Come to think of it, why don't neutrons stick together? But my idea wasn't for a random giant nuclei - they have to be arranged in a very unlikely structure for it to work. $\endgroup$ Mar 8, 2016 at 12:52

Lots of good answers, but here's my $0.02 (before tax).

First, there's no room to throw new elements into the mix unless you can find some interesting high-mass element that breaks all the known rules. No gaps to fill except with some odd isotopes and such.

That said, let's look at your three conditions in reverse order:

#3: Renewable

This is going to depend on your source, I guess. In general this means either that the fuel and/or any precursor materials are not a static resource (no mining it from asteroids or similar). Fossil fuels are not renewable, plant-derived alcohols and bio-diesels are. This one might be fairly simple to resolve, might not.

#2: Clean

Not quite as simple as it sounds. If you just mean that it produces a minimum of environmental impact when used as a fuel source, without regard to the environmental impact of its manufacture, then we can maybe throw a lot of interesting things into the mix. Both sides of that are problems for nuclear energy since both the refining and use of nuclear materials produces waste products that are likely to be a problem for millennia.

#1: Better than Nuclear

The reason I reversed the order, this is both the hardest criteria to satisfy and the most direct influence on the possible answers. No matter what sort of obscure atomic isotopes or fictional element you come up with, chemical reactions simply do not release the sort of energy that nuclear ones do. Period. Chemical bonds are orders of magnitude less strong than nuclear ones, and that's never going to change no matter what material you're reacting.

Which to my mind leaves one actually real material: anti-matter.

Specifically, Antihydrogen composed of a nucleus of 1 antiproton with a single positron. This was first synthesized (in ultra-tiny proportions, of course) at CERN back in the 90s and is still being studied. If the predictions are true it will act exactly like normal Hydrogen and form the anti-matter equivalent of H2 gas.

Take 1 part Hydrogen and one part Antihydrogen, mix in an appropriate reactor - taking care to never let your antimatter come in contact with 'normal' matter - and the resultant energy release (gamma rays from the electron/positron reaction plus various pions, muons, neutrinos, positrons and electrons from the proton/antiproton) is as close as you can currently get to complete liberation of energy from mass.

Apart from the neutrinos escaping pretty much any currently available containment the result is pretty close to the highest density fuel it is possible to make, almost pure E=MC2. By fuel mass a Matter/Antimatter power plant should produce around 2 billion times as much power as a diesel engine. If you wanted to power a land vehicle with it (assuming you can build a compact reactor) a gram of Anti-Hydrogen will last for a freaking long time.

From the 'clean' perspective, M/AM reactions have fairly safe byproducts if you can capture the gamma rays and so on. There's no waste gasses from combustion, no water, etc. If you could just use positrons all you'd get is a whole bunch of gamma, but storing them is even harder when you don't have an antiproton for them to latch to. Effectively though the only real emissions from the reactor will be neutrinos, which aren't all that interested in reacting with anything. At high enough densities there could be some odd effects, maybe the occasional transmutation or unplanned ionization in surrounding matter... nothing big.

Antimatter can be synthesized in small quantities at the moment by using stupendous amounts of power, which is a bit of a drawback. But if you can solve the synthesis problem and have access to sufficient energy from other sources then you could perhaps produce a steady supply. That covers renewable.

The problem of course is that you have almost zero chance of avoiding the inevitable uncontained M/AM reaction and wiping your species off the face of the planet. Maybe only use this stuff in space where an explosion is less likely to tear holes in the planetary crust.

All of the above relies on a very high technology level - think Space Opera - and before all of the kinks are worked out it might be possible to create a device which liberates energy directly from matter. Personally I'd be happier to see a matter-to-energy converter than M/AM reactors, but both are way out of our reach right now.

  • $\begingroup$ Exactly why this was posited as the future power source in the Star Trek universe.... $\endgroup$
    – Eli Skolas
    Mar 7, 2016 at 6:52
  • 1
    $\begingroup$ Which is odd because they also apparently have matter-to-energy-to-matter conversion in their replicators - albeit at a limited resolution that doesn't appear to be capable of producing living products - and in the transporters. Why not simply use the energy from that instead of carrying around dangerous antimatter? $\endgroup$
    – Corey
    Mar 7, 2016 at 22:48

Suppose 128ium were a catalyst for a low temperature fusion reaction - like common isotope A fuses with common isotope B on the surface of a 128ium nucleus to result in unstable isotope C which decays rapidly by beta decay into stable byproduct D, generating lots of heat and no dangerous by-products. You'll have to comb through a nuclear decay chart to find a friendly reaction, or make one up that's off the chart but plausible.

  • $\begingroup$ Welcome to the site cranhike $\endgroup$
    – James
    Mar 1, 2016 at 22:04
  • $\begingroup$ So, something similar to the way "Dilithium" is used in Star Trek. There, it's supposed to be a way of containing and regulating a matter-antimatter reaction, not exactly a catalyst. $\endgroup$ Mar 3, 2016 at 1:00
  • $\begingroup$ This is the answer I wanted to post - as others have pointed out, ultra-rare elements will not be worth burning as a fuel. But what if they are catalysts or enable some other useful reactions, be it cold fusion, an FTL drive, or what-have-you? $\endgroup$
    – Peter S.
    Mar 7, 2016 at 10:44

Yes, perhaps

Look into multipositronic systems.


The short version is that you can have bind positrons (i.e. anti-electrons) to negative ions to create new, fancy atoms. Thus in between the elements of the periodic table you can have new 'elements'. To my knowledge not much about the properties of such things are known, but that's where I would go for mystery matter.


899 number line

This xkcd cartoon sucked me in to a few hours wasted time with the note you see between 3 and 4, "Gird— Accepted as Canon by orthodox mathematicians".

On the explains page I linked you see links to related values: Bleem, Derf, Bleen, SCP-033 (which nearly lost me my tablet), and Sorf. You can start there and find online stories (and even a short film now) for The Strangest Number, etc.

So having bleem jelly beans is a silly trick. Why do the overloards not want us to notice this integer? Maybe having bleem protons in an atom is far more of a serious matter.

If you have an extra number you have overlooked, then you will have a corresponding element, and some isotopes of a few other elements.


Are there potential gaps in the periodic table where such an element could exist?

As everyone else has said: No.

While you might have elements beyond 118 (and Jim2B has terrifically addressed the possibility of some of them being stable), there are clearly no existing gaps in the periodic table, and nothing will change that.

Where on the periodic table would such an element land to be the best fuel source?

First, we have to discuss what it means to be an element, and what it means to be a fuel.

An atom of a given element can be thought of as a nucleus surrounded by a cloud of electrons. To act as a fuel, these atoms must react in such a way that they produce energy. When atoms react, they can do so in one of two ways. First, they might interact superficially, through the surrounding cloud of electrons, creating and breaking chemical bonds with other atoms in a process known as chemical reactions. Secondly, they could react at a deeper level, fusing or breaking apart their nuclei -- a process known as (wait for it...) nuclear reactions.

Thus, by the very definition of an element, you're automatically limited to either weaker chemical reactions, or more energetic nuclear reactions. But you've a priori ruled out nuclear reactions as not powerful enough, so it's not clear what kind of reaction you're looking for. There really isn't much of a third option here -- either you involve the nucleus in the reaction or you don't (thereby forfeiting their larger amount of energy).

I'll give two options, both of which technically break your rules.

Nuclear Redux

You don't explain why you don't want nuclear, except that you want a higher energy density. As mentioned, nuclear reactions are the higher density reactions. However, not all "nuclear" is equal. Is it OK if it's nuclear power, but with a higher power density than current nuclear plants?

The main categories of nuclear power are fission and fusion. But even within these categories, there is a huge variety of designs with staggering differences between them. In fact, the amount of energy you can get out of a given amount of fuel is largely determined by the reactor design, not just the fuel.

For example, most of the nuclear reactors in use today are "Generation II" reactors, designed and built between the 60's and the 90's. A few of the newer ones are Generation III, which are incremental improvements on Gen II designs, but are still based on solid fuel rods and pressurized water cooling. However, there are a number of Generation IV reactor designs -- for example, various molten salt reactors (MSRs) -- that look as if they can deliver "100-300 times more energy yield from the same amount of nuclear fuel". Where current Gen II or III designs only extract a small portion of a fuel's energy and leave behind a large amount of waste, these newer designs are capable of squeezing out the majority of a fuel's energy, leaving behind very little waste that is relatively short-lived. For your purposes, does this count as being more power dense than nuclear?

The best fuels for fission end up being as massive as possible and radioactive, yet stable enough to not quickly decay into something else. The elements that best fit that bill are things like Uranium and Thorium.

With MSRs especially, there is a debate about whether to stick with Uranium fuel, or switch to Thorium, which apparently is more power dense than uranium, and arguably more common. While not quite renewable, both elements are in abundance on our planet, and would essentially last indefinitely (the ocean is saturated with Uranium that enters it from rivers, and Thorium is an extremely common byproduct of certain types of mining that currently gets thrown away; it's also common on the Moon). Uranium and Thorium are the only significantly radioactive elements that exist on our planet in large amounts.

And this is all before we even start to consider fusion. In fusion, since you're trying to make the nuclei go fast enough to bump into each other without being deflected, the best fuel is lightweight -- hence the desire to use Hydrogen or Helium (atomic numbers 1 and 2). Some fusion reactors use an isotope of Helium known as Helium-3.


This is not an element, but a different type of matter, with opposite electric charge from regular matter. When matter and antimatter meet, they annihilate each other, and produce the maximum possible amount of energy for a given amount of mass. This is your absolute limit on the amount of energy you can extract from matter. Technically, this is also a form of nuclear reaction, since you're annihilating the nucleus.

The problem is that antimatter is not known to occur in nature, and is very difficult to make, so you end up losing energy in the process. This is where I'm going to stray a bit from known science and speculate for the purpose of fiction. I can think of a few possible sources where you might find antimatter.

Interstellar Medium

This is probably false, but there's a small chance that you might happen to run into trace amounts of antimatter in the vacuum of deep space, much like interstellar hydrogen. This could conceivably be scooped up and harvested with some sort of Bussard Ramjet. I assume this is how the USS Enterprise gets at least some of its antimatter.

Black Holes

While black holes are usually thought of as things from which nothing can escape, Stephen Hawking actually argued that they evaporate, emitting so-called Hawking radiation in the process. This can be thought of as a particle/anti-particle pair forming from the vacuum near an event horizon, and then one member getting sucked into the black hole, while the other is freed. You might think about trying to manipulate with electromagnetic fields which member of the pair is devoured and which is emitted, it's probably just easier to harvest the resulting Hawking radiation directly (again, speaking in a purely speculative manner).

Or, as long as you've got a black hole, you could go for some of the other options listed here: https://physics.stackexchange.com/questions/20813/how-would-a-black-hole-power-plant-work

Magnetic Fields

Taking a step back from the exotic physics of black holes, it turns out that a similar particle/antiparticle process occurs when cosmic radiation strikes a planetary atmosphere. And if there's a magnetic field present, this can trap the antiparticles. According to this page, it's belived that there could be on the order of several hundred micrograms of antimatter stored in Saturn's radiation belts, and a good amount around Earth as well. How you go about extracting it is another question entirely. The good thing is that it will be renewed by more cosmic radiation.

  • $\begingroup$ Thanks Caleb. There are definitely some good/useable ideas in here. $\endgroup$
    – James
    Mar 9, 2016 at 14:43

No , but perhaps an isotope of a preexisting element can discovered

Due to the nature of the periodic stable , the can not be any gaps in it, however , some super fuel isotope can be discovered like deuterium or uranium 235


Why an element?

Are you sure that you want your fuel to be a new element? As others have noted, we've pretty well covered the element space. New elements are just increasingly unstable clumps of protons and neutrons (with electrons circling them) that only exist in rare situations.

We already know ways to make use of higher density fuels. For example, nuclear fission is so energy dense that the contaminants in coal would provide more energy if used as uranium than if the coal were burned. We don't use that because it's expensive.

Another, even higher density fuel, is anti-matter. We don't use that as a fuel now because we don't have a source for it and don't really have the equipment to manage it if we did. It's not even as far along in the development process as fusion. So if the people in your world discover a plentiful source of anti-matter and figure out a way to harvest it, that would fill much the same space as a new element. And it could operate entirely in keeping with what we know about the universe.

What about lower level particles? Why restrict yourself to just atoms? You could instead be talking about something made from quarks other than the more standard proton, neutron, electron, neutrino, and their anti-particles. This wouldn't fit into the periodic table, but it would act more like a new element.

Another possibility would be an alternate version of energy. Currently we can store energy as potential energy, kinetic energy, or electromagnetic energy (photons). What if you discovered a more manageable version of a photon? Easier to store as energy?

Or figure out a way to use the basic underpinnings of gravity to store energy. We don't know how gravity is transmitted. We speculate a lot. Perhaps one branch of speculation is suitable for your purposes. Perhaps the secret is gravitons. Or something else. If that sounds interesting, I'd suggest a new question where someone more knowledgeable than I could give more concrete suggestions.

I think that if you let go of the periodic table, it will be easier to accomplish your real goal.

  • $\begingroup$ Ah, shades of John Campbell's space strain drive from Islands of Space, storing energy in a warp in space and extracting it as necessary by controlled collapse of the warp. $\endgroup$
    – Corey
    Mar 7, 2016 at 22:51

(1) There is no logical gap in the periodic table in which you could insert a new mystery element for use as a fuel.

(2) Even so, do not rule out solutions with ions and molecules. Neither type is an element. Nuclear fuel is good for a number of purposes, but it decays over time. Also, the best energy source depends on the use it is to be put.

I speculate that what you really want is a fuel that plausibly can remaining in a high potential state while it travels light years. For a science fictions story, it might be helpful if the fuel can be renewed, or recharged, in orbit around a distant sun.

Consider a device that utilizes ions, molecules, and magnetohydrodymanics and can be recharged by solar power. You can call the device one of the spacecraft's fuels, but disclose to your readers that it is more than just elements.

  • $\begingroup$ Welcome to the site and thanks for the answer. $\endgroup$
    – James
    Mar 7, 2016 at 16:49

Nyes. Or Yeno. Or perhaps mayes, or nobe. Or any other variation of yes-no-maybe.

As many posters here have expertly said, there are no gaps in the periodic table.

The number of protons has to be an integer number.

Except, why an integer number? Why not one and a half protons?

We now know that protons are not indivisible. They are made up of even smaller particles. So an atom that is made up of an unusual proton? One that is heavier or lighter than a normal proton? Perhaps it only has two quarks, or perhaps six? There is no reason to believe that it could not be an element distinct from one with normal (three quark) protons. And since it might just have a more or less positive charge than a traditional proton, the number of electrons needed to balance the charge would produce an interesting situation. How would you fit such an element into the periodic table, except to say that it filled a gap? Not an isotope, but an element with the same number of protons, only the protons were different? Call it, say, element 91(a) and 91(b), or 91 and 91.5?

I could not even begin to imagine the energies that could be available under such a circumstance, and how they could be released. Perhaps in some nuclear conversion back to a traditional proton? But that is the beauty of fiction. You need not explain any more than is necessary for the plot. "Feasible" is good enough.


There are, evidently, no gaps in the periodic table due to the integer nature of elements and the plethora of other reasons from other answers. However, it doesn't look like anyone has mentioned neutronium, which can give you some wiggle room at the head of the periodic table rather than the end.


Certainly a hypothetical substance, it's composed entirely of neutrons. There are some nuclear models that permit this, and others that explicitly exclude it. In your universe, it may be these first models that are correct and allow neutronium as an element. Given how little we know about it, you'll have more or less free reign about what exactly its properties are and how it can be used as a fuel source.


Although the question of whether there are gaps in the chemical table has been answered thoroughly here, and there has been some discussion of alternate fuel sources besides chemical combustion, I would like to expound upon that a bit. The reason why is because, although there are no gaps in the periodic table, there is plenty of room for new stuff in the standard model, which is the domain of high-energy physics, and high energy is precisely what you seem to be interested in. Better, since so little is known on the fringes of the model, there is an incredible amount of room to make stuff up.

All energy production, simplified

(Bear with me)

To simplify the general problem of "energy production", it is helpful to recall that energy can be neither created nor destroyed. What this means is that energy isn't actually produced.... it was there all along. The mechanism by which we harness said energy is by moving a system from one stable state at a relatively high energy to another stable state with lower energy. Often, by use of some clever mechanism, the mover can harness the difference.

For example, when one burns coal, the reaction C + O_2 -> CO_2 occurs. It is a happy coincidence of our universe that, although having separate C + O_2 is at a relatively high energy, and CO_2 is at a lower energy, the state of matter "in between" those two situations is even higher energy. This prevents all of the C and the O_2 from simply spontaneously collapsing into CO_2. In order to overcome this "barrier" of the even higher energy state, we have to input some amount of energy. However, one gets that energy back when the reaction finally settles at CO_2.

SO to generalize the process of burning coal: Take a system at a high, but stable, energy state. Hope that that system has a lower, but not yet realized, energy state. Input energy to move the system from the higher to the lower energy state. Get your input energy back, plus some, by use of some energy-harnessing-device.

Nuclear energy works just this way: Uranium is a high energy state. Krypton and Barium (the byproducts of Uranium fission) are lower energy. The state "in between" is very high energy. One inputs energy to the Uranium to overcome the "in between" state, ending up with Krypton and Barium (the low energy state) and a crapboat of heat energy that can power, for example, a steam engine.

Hydroelectric energy works just this way: Water in a mountain is a high energy state. By releasing the floodgates on the dam, the water moves to a lower energy state in a valley. By use of a water wheel, we can harness the difference.

etc etc etc.

What does this have to do with anything get to the point geez

(Making up a new energy source)

Modern physics is full of weird fields that simply exist all around us, scientists have only just begun to explore some of this stuff experimentally, and there are plenty of speculative forms of we-know-not-what hypothesized to exist. This leaves lots of room for invention, since all you need is a high-energy state, a means to move it into a lower-energy state, and a mechanism by which you harness the difference. One can invent a new higher energy state for stuff, a new lower energy state for stuff, or both.

So, what does this mean for worldbuilding? Some immediate consequences:

1) Going from "ordinary" stuff to "ordinary" stuff: Modern physics has a decent handle on the energy states of the stuff that we see and feel around us. This means that if your high-energy state and your low-energy state BOTH involve "ordinary stuff", then it will be trivial to check your work. For example, regardless of the mechanism you use to harness the energy, the "nuclear energy" reaction: U -> Kr + Ba produces a fixed quantity of energy that can be calculated, and is independent of any fancy new device you conceive. In other words, if your new energy-producing mechanism looks like: Ordinary stuff -> Other ordinary stuff, then you will obtain an "ordinary" amount of energy, because we know all about how ordinary stuff works.

2) Going from "exotic" stuff to "ordinary" stuff: For example, you might hypothesize that dark matter is an incredibly high-energy state of matter, that there is some source of this stuff, that there exists some mechanism to transform dark matter into "ordinary" matter, and devise some device to harness the difference in energy. As a side effect, you would have a waste product of "ordinary" matter. You could generate basically infinite amounts of energy this way.

3) Going from "ordinary" stuff to "exotic" stuff: You might hypothesize that dark matter is an incredibly low-energy state of matter. However, since the starting "ordinary" matter energy-level is known, you can only harness so much energy this way. The upper bound is E=mc^2. However, mc^2 is a whole heck of a lot of energy. OTOH, the concept of negative energy might be able to overcome this limitation.

4) Going from "exotic" stuff to "exotic" stuff: The world is your oyster, but it might be difficult to distinguish your new energy source from magic.


I'll keep my answer very short: https://en.m.wikipedia.org/wiki/Exotic_matter
There are a lot of examples if you browse through the page, and there are a lot of unknowns regarding them. If you wave your hands really hard, you might be able to use one of these.
Edit: they're not necessarily gaps in the periodic table. They're more like extensions to what we call everyday matter. They can't be placed on the periodic table as we know it.


Like JIm2B said, there really is no conventional, non-exotic matter you could shove into the periodic table without breaking physics, and unless you create an island of stability, the atoms decay too fast to use as actual fuel the way you seem to want.

The point of this site isn't to let people tell you why your idea is impossible, it's to help you figure out how to make it work in your story. With that in mind, let's dive into some plausible (-sounding) ideas for naturally occurring sources of fuel that have not been discovered or created on earth as of now:

  • First, you could probably use positronium for something smart-sounding. Positronium is made of one electron orbiting its anti-particle, and according to thirty seconds on Wikipedia, is extremely common but has not been synthesized en masse on earth. It is also very unstable, but can likely be stored with electric fields and (what else?) magnets.
  • Secondly, tachyonic matter (along with other exotic particles) is a possible component for the Alcubierre drive. This works very well in terms of being “discovered”, as tachyons have not been proven to exist and have not been synthesized by humans. On the other hand, this is less fuel as it is structural components, but you could hand-wave this by saying that tachyons are naturally hard to contain or must be used up to provide negative energy density to the warp field. Finally, you couldn't have physical stockpiles of tachyons waiting around to be exploited; perhaps they are produced by certain kinds of quasar or black hole?

Or you could simply use antihelium and helium as reactants for a antimatter drive, but that wouldn't necessitate discovering anything and, more importantly, not be much fun.

  • 1
    $\begingroup$ I'm sorry, but none of the ideas you suggest involve the periodic table, besides the antimatter suggestion. $\endgroup$
    – HDE 226868
    Feb 29, 2016 at 22:18
  • 1
    $\begingroup$ Apart from the content, you could put some efforts in the expression. Sentences, grammar, clear paragraphs, etc. really help. I tried to improve it for the last point. $\endgroup$ Mar 1, 2016 at 10:12
  • $\begingroup$ It might be hard to produce positronium en mass, as it has almost no mass ;-) $\endgroup$ Mar 6, 2016 at 14:28
  • $\begingroup$ Regarding antimatter, there is a lot that would be needed in the way of discoveries to both synthesize and contain it. This is largely hand-waved away in popular fiction, with the occasional containment field failure for dramatic effect. Don't count it out, M/AM reactions are about the highest density fuels in terms of energy yield besides pure matter/energy conversion. $\endgroup$
    – Corey
    Mar 7, 2016 at 22:55

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