Metallic hydrogen was first predicted in 1935 by Eugene Wigner and Hilliard Bell Huntington. The idea, as I understand it, is that hydrogen will take on metallic properties under extremely high pressure.
If you look at a periodic table, hydrogen is usually positioned just above the alakli metals. But hydrogen isn’t an alkali metal. It has one valence electron, like lithium and sodium do, but this has different implications for hydrogen than it does for the alkali metals.
See, hydrogen atoms only have one valence electron because they only have one electron period. And there’s only room for one more electron in its valence shell. So it’s half-full or half-empty, depending on your mood. By contrast, the alkali metals have valence shells that can hold eight electrons, so having just one valence electron is more meaningful. The atoms of alkali metals are eager to give up that electron, which leads to them forming positively charged ions. This informs a lot of their chemical properties.
For example when you have a lot of sodium atoms together, and there’s nothing else for them to react with, they wind up giving away their valence electrons to each other. But none of the atoms really want more electrons, they just want to give theirs away. This leads to what is sometimes referred to as a “sea of electrons”, a mass of metal cations held together by the constant exchange of their valence electrons. This would explain a lot of the properties found in metals. They’re pretty strong, you can pull them into wires, they conduct heat and electricity very well, and they’re shiny.
But hydrogen doesn’t work this way, because it’s valence shell is only half-empty instead of one-eighth empty like sodium. Giving up it’s lone electron isn’t as simple as it is for sodium, because sodium has ten more electrons to work with. Hydrogen just has the one, so giving it up would leave nothing but a hydrogen nucleus, which isn’t the most stable arrangement around. Since most hydrogen nuclei only contain one proton, hydrogen cations are sometimes referred to as “bare protons”. You’d have a much harder time getting a lattice of “bare protons” to hold still and exchange electrons freely, the way sodium atoms do. The protons would want their electrons back too badly.
Also, hydrogen atoms could just as easily accept a second electron for a full valence shell, like chlorine, or one of the other halogens on the other side of the periodic table. The half-fullness situation bears a certain resemblance to carbon, which has four valence electrons out of a possible eight. This is why hydrogen usually gets situated above the alkali metals, but also sort of spaced away, to indicate that it’s really in a class all by itself.
But, under immense pressure, perhaps the “bare protons” could be made to behave like sodium atoms do in ambient conditions. Maybe if you squeeze a bunch of hydrogen atoms really tightly, they don’t mind freely exchanging their electrons because they’re so closely packed together. If this could be done, then the lump of hydrogen atoms would start to resemble a metal and become shiny. This is what
Ranga Dias and Isaac Silvera claim to have achieved. They subjected a small sample of hydrogen to a pressure nearly 5 billion times that of Earth’s atmosphere, and they observed the sample becoming shiny like a metal. The experiment needs to be reviewed and duplicated, but if it turns out to be legitimate, this would be a big deal.
What I didn’t know until I read this article, though, is that it’s been speculated that metallic hydrogen might stay that way once it’s formed. In other words, you could reduce the pressure and it might not change back into the gaseous, diatomic form we’re accustomed to. Supposedly, this would have all sorts of useful technological applications, but that seems a little far-fetched to me. Even if it turned out to work that way, I’m not sure I’d feel safe around a device that uses metallic hydrogen wiring. If it somehow changed back into hydrogen gas, the device would explode from the sudden change in pressure. And one thing hydrogen and the alkali metals have in common is flammability. I would think even metallic hydrogen would be dangerous around an open flame.
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