This guy says he’s got one. Results have yet to be confirmed; he’s still writing it up.
Anyone know if their is any possibility of an ultra low density material, with lower density than Helium that can withstand air pressure and not be as explosive as hydrogen? I’m thinking of a rigid foam that contains pockets of empty vacuum that is strong enough to not collapse under 1 atmosphere pressure. Or else a Hydrogen containing foam that is sufficiently flame repellent that it cannot burn in our atmosphere.
( yes I’m an airship fan )
I’m not a chemist, so please correct me if I got the following horribly wrong in every possible way…
Generally, the heavier a molecule is, the more likely it is to be unstable, i.e. radioactive. However, it is theorized thay beyond the heaviest molecules we know today, with names like einsteinium and mendelevium and someotherobscurescientistium, lies a group of elements that are stable and of course incredibly heavy.
Being able to mass-produce stuff made of atoms with, say, 370 protons would be cool, though I’m not sure what it would be useful for.
Martin Wolf: I think you meant “nuclide” in place of “molecule”. And yes, there is a theoretical “island of stability” for very heavy nuclides, we haven’t found any stable isotopes yet. Even if we did manage to make one synthetically, there’s no guarantee it would be useful.
Bippy the Beardless: It’s not possible to have a solid material less dense than a gas, even if most of the volume of the solid is empty space. Even if you somehow had atoms of a material with the same mass as hydrogen, to form a solid they would have to be packed closely together. Any reasonable volume you could enclose with this miracle material would still end up weighing more than an equal volume of hydrogen.
dylan_73, Grey, et al.: Room-temperature superconductors would have a lot of applications, but the temperature isn’t the only important metric. The other factor limiting the usefulness of diamond films and other ceramic superconductors is critical current density. Basically, if you try to push too much current through a superconductor, it stops superconducting. That’s why the “high-temp” ceramic supercondcutors that operate at liquid nitrogen temps haven’t had any great commercial success.
Now, the things I think would be cool:
Carbon semiconductors: By varying the construction of a carbon nanotube, you can change its electrical properties. It’s possible that in a few years, we’ll have molecule-sized transistors and logic gates. Also look for carbon whiskers with incredible tensile strengths - more than 100 times as strong per unit mass than Kevlar.
Spray-on aerogels: Aerogels have been around for a while, but making them outside the lab is hard. If someone can come up with a way to make them cheap to manufacture in large quantities, they’ll be useful for all sorts of things. For one, they’d make great insulation. They’re also useful for chemical sampling.
Next-generation superalloys: There are some superalloys currently in use, but they are very expensive and most have problems with corrosion. If there was a way to improve the mixing of dissimilar metals (and some non-metals), we’d probably have a whole new set of alloys to work with.
Ultra-compressed gases: Some have mentioned metallic hydrogen, which would be interesting, but there are other gases we might want in an ultra-compressed state. Nitrogen, for instance, is thought to form a solid at extremely high pressures. If you had a way to turn it back into a gas very quickly, the amount of energy you could store in a cubic meter of the stuff is phenomenal.
My personal thoughts on so-called “designer matter”, where interesting tricks are pulled with subatomic particles, is that unless a way is found to make these things cheaply, they’re going to remain lab curiosities. You can’t do much with materials that cost $10,000/microgram, even if they do have near-magical properties.
While we can’t yet make spiderweb, we can now make nanofiber composites that are both stronger and tougher than spiderweb. But I think that’s only a few centimeters at a time, for now.
Bippy, the material you’re looking for is hydrogen. Plain, ordinary hydrogen. Yes, it’s flammable, but not nearly so much as it’s usually made out to be. The Hindenberg fire was the envelope burning, not the hydrogen.
And Martin, you’re thinking of atoms, not molecules: A single molecule can be arbitrarily large. There are hypothesized to be “islands of stability” at high atomic number, but they’re only relatively stable. As in, they last a few seconds, instead of a few microseconds like other elements up that high. I won’t say that such elements would be useless, but you’re not exactly going to be shipping any cross-country.
SteelWolf A foam can be made lighter than a gas, that is what a helium balloon is! (OK it’s a one bubble foam, so glue a few together…) Using pure hydrogen always has the problem that hydrogen tends to escape from whatever envelope it is put in, and in escaping the space it took is replaced by air leading f not countered to an explosive mixture.
Has anyone made a lighter than air foam that is more rigid than glued together helium balloons?
Flexible, efficient conducting polymers.
Low voltage, efficient electroactive polymers.
UV resistant switchable windows.
There’s a hypothetical arrangement of atoms called “beta-carbon nitride” that in theory would be even harder than diamond. But no one has been able to synthesize any yet.
Another fun theoretical material would be “diamond ether”. This is a hypothetical solid with an empirical formula of CO2. Picture the sturcture of diamond- each carbon atom linked to four others tetrahedrally. Now modify this by inserting an oxygen atom into each carbon-carbon link. Viola! A stable solid with the same molecular formula as carbon dioxide.
I’ve heard that monopolar magnets exist - in theory, but nobody has ever managed to find or build one, although plenty of scientists are trying. I’ve no idea what use they would be.
There’s an interesting book called Nanosystems that answers this question on about every page. II haven’t opened the text in a couple years, so some of the structures discussed may have been made by now.
I do believe that, in the former example, neutronium would be useless, as it’s A: superfluid, and B: will expand rapidly when not under intense gravitational pressure.
And the latter example… are you referring to a quark-gluon plasma? I would imagine it would also be requisite to remain in a high-gravity/high-temperature environment.
I´d like to add silicon monocrystals made for semiconductors; the process is quite simple, dip a single silicon crystal on a crisol of melted silicon and start pulling it up slowly, the atoms align with the original cristal as they stick to the lower part of the cylinder. It´s the spittoon effect. 
This is what I think about a few of the materials that are being discussed:
fullerenes: Fullerenes are the first thing that comes to mind when you think of a novel, fascinating-but-still-impractical material. Fullerenes with caged atoms and ions have been around since 1985, and practical uses for them are beginning to appear.
artifical blood: The first thing I thought of wasn’t perfluorocarbons but some kind of heme-like molecule with a lot of furan rings that could supposedly bind dioxygen reversibly. Apparently much of the current work in ‘artificial blood’ is focused on genetically engineered hemoglobins designed to be more efficient than natural hemoglobin. It would be possible to design an oxygen carrier that was, for example, immune to CO poisoning.
magnetic monopoles: Probably not – that’s all I’m going to say.
room-temperature superconductors: Almost certainly possible and certainly of great practical value. Likewise optical semiconductors, and carbon semiconductors too.
spider silk: Again, almost certainly possible and of great value because of its high tensile strength. Zut mentioned artificial clam shell; there’s also research being done into synthetic abalone shell, a ceramic-like material much stronger than any current ceramic.
supercritical fluids: No one’s mentioned this either, but supercritical fluids (which exist, but aren’t well-known to the general public) are promising as reaction solvents and also for separation.
novel catalysts: I know it’s not a material, strictly speaking, but I think this is a good place to mention that the economic potential of a highly efficient catalyst for decomposing water into hydrogen and oxygen would be immense. As if that’s not obvious.
biological macromolecules: Refusal said that manufacturing large molecules like DNA is slow and difficult, but probably not as slow and difficult as most people think. It’s possible to make oligonucleotides and oligopeptides of up to, say, 50 or 100 bases/residues quite efficiently. Larger macromolecules are possible even now, and with some refinement of the technique we can expect complex designer proteins and designer DNA in the relatively near future.
Optical semiconductors? C’est quoi?
Well, actually, Transparent Aluminum does exist. Sort of anyways, - its actually specially made aluminum oxide, but it is see though, and very strong - possible uses for it include bullet proof windows. Close enough for me. 
“a catalyst for separating water into hydrogen and oxygen” may be a little misleading to some. It’s possible, but no catalyst can split water at anything resembling “normal conditions”
A) catalysts don’t driver reactions, they only lower the activation energy barrier (the speed bump) between states,
B) without that speed bump, the reaction will proceed in the spontaneous direction, to the same equilibrium - only faster
C) Water is lower energy than a mixture of oxygen and hydrogen (which is why O2 and H2 will combine explosively and give off all that extra energy).
You can have a catalyst that will separate water into oxygen and hydrogen under conditions that are suitable -conditions where water spontaneously falls apart- but such conditions are very high energy, and the activation energy is negligible by then anyway. (You get the activation energy back - and more- when the reaction is finished anyway, so it only affects speed, nothing else.
In complex systems catalysts may seem to do counter intuitive things by, say speeding only one, normally rare, reaction until it outpaces the uncatalyzed reactions that normally take place. Within the catalyzed reaction itself, the equilibrium constants and relative energies are unchanged.
A catalyst might save you a little waste heat loss, by allowing a usable rate of production at a lower reaction termperature. e.g. If you’re using the old standby industrial process for hydrogen production (forcing steam though iron powder or wool at 550F, so the iron rusts so avidly that it yanks the oxygen off the water molecule) then the catalyst might let you do the reaction at 400F instead of 500F, but if the termperature or oter conditions changed, and the hydrogen want its oxygen back, the catalyst would help make a bigger boom (the right catalyst for this process would be a iron-oxygen catalyst, not hydrogen-oxygen).
If you are electrolyzing water, it would take about the same voltage and current to pry the atoms apart, with or without a catalyst
BTW, unless it has some restrictions on the conditions where it is an effective catalyst (like high temperature) any catalyst that can break water will cause hydrogen and oxygen to recombine even faster than they ordinarily do. If you reduce the speed bump of activation energy, the traffic goes faster in both directions. This could have somes uses, if hydrogen becomes a widespread fuel. Hydrogen and oxygen hook up plenty fast without a matchmaker, but it could be useful to scavenge the inevitable hydrogen seepages by instantly oxidizing any stray hydrogen atoms before they could accumulate.
(A hydrogen ion is a proton - not an atom or ion as we usually think of them. H+ is unique because it has no electrons and hence none of the electron shells that make up 99.99999…% of the volume of any atom. A hydrogen ion -a single naked subatomic particle- can drift between the molecules of any material. The diffusion is slower through some materials, but it’s always there. Fortunately, in a cylinder of H2, very little gas is in the ionized form at any one time)
Flashing ink.
I’ve been talking about this stuff for nearly two decades and I still believe it is technically possible - there are a number of chemical reactions that cycle back and forth between two states - the problem is that the cycles are not uniformly synchronised throughout the medium.
Antimatter (antihydrogen in particular)- the ultimate battery.
If a relatively efficient way can be found of manufacturing it from sunlight - there is a lot of sunlight available, nearer the sun…
SF worldbuilding at
http://www.orionsarm.com/main.html
A plastic or paint-like material that converts radio waves to visible light at room temperature.
Buckminster Fuller was an architect who was best known for the invention of the geodesic dome. While he did more than just architecture, he was not a chemist, and had nothing to do with the discovery of buckyballs.
Two chemists from Rice University, Robert Curl and Richard Smalley, along with a third chemist from University of Sussex in England, discovered a form of carbon that “consists of 60 atoms of carbon arranged in hexagons and pentagons that resemble a soccer ball or a geodesic dome.”
They named these compounds buckminsterfullerenes (commonly referred to as buckyballs) in honor of the architect. Buckminster Fuller died in 1983, two years before the discovery of the molecules that bear his name.
[sub]By the way, Robert Curl was my Physical Chemistry prof at Rice (after he had done the work that earned him the Nobel, but 8 years before he was awarded the prize).[/sub]