Why is hydrogen where it is in the periodic table?

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Gunnar Sporrong

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Gunnar Sporrong  asked the Naked Scientists:
Hi!

My name is Gunnar Sporrong. I wonder if you know if the metallic hyrdrogen in the center of Jupiter (and maybe Saturn) have anything to do with the place hydrogen occupies in the periodic table.

Gunnar

P.S. Love your show!
What do you think?
« Last Edit: 02/05/2008 19:35:35 by chris »

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Offline Kryptid

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Why is hydrogen where it is in the periodic table?
« Reply #1 on: 02/05/2008 03:14:15 »
Hydrogen is in the alkali metal column, but strictly speaking, its ability to become a metal at high pressure isn't caused by this. Hydrogen has 1 valence electron, just like the metals below it in its column. This is one of the few properties it has in common with these metals, though. Since the alkali metals have other electrons below their valence electrons, they have a "shielding" effect on that valence electron. The result is that the valence electron is held rather loosely, and can be easily lost via ionization. This also allows for metallic bonding. In hydrogen, there are no "kernel" electrons; just the single valence electron. Therefore, it is closer to the nucleus and not shielded from its charge. This makes it harder to ionize and more likely to form covalent bonds instead of metallic bonds. Also, all of the alkali metals react with water. Metallic hydrogen does not, since water molecules already contain as much hydrogen as they can hold.

Other nonmetals can become metallic if subjected to sufficiently high pressure as well. This has been done with xenon, sulfur and oxygen, for example. In fact, metallic oxygen and sulfur can be superconductors at low temperatures.
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Offline chris

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Why is hydrogen where it is in the periodic table?
« Reply #2 on: 02/05/2008 19:40:02 »
Terrific answer, thanks. But can you explain exactly what "metallic" hydrogen is please?

Chris
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Offline Kryptid

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Why is hydrogen where it is in the periodic table?
« Reply #3 on: 02/05/2008 22:34:38 »
Metallic hydrogen is basically an electrically-conductive phase of hydrogen created under intense pressure. The idea is that metals gain their stereotypical properties such as electrical conduction from the "sea" of electrons that permeates the metal. In metals, valence electrons do not belong to any one atom, but rather they flow around and between atoms. This mobility of the electrons allows them to carry an electric current.

In normal hydrogen, the nucleus holds on too strongly to the valence electrons to allow them to flow freely from atom to atom. Under very high pressure, however, the atoms (and hence the valence electrons) move much more closely to one-another. Since the valence electrons in one atom now feel the positive charge of another atom's nucleus more strongly, they are more loosely bound to their own atom's nucleus. This gives them extra mobility and hence an electrical conductivity. This can happen in both liquid and solid hydrogen.

That's how I understand it at least. You might want to do a little more research to confirm my explanation.
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Offline chris

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Why is hydrogen where it is in the periodic table?
« Reply #4 on: 02/05/2008 22:43:54 »
Thanks for that excellent explanation. Does the same thing happen with any other substances, or is it unique to hydrogen?

Chris
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Offline Kryptid

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Why is hydrogen where it is in the periodic table?
« Reply #5 on: 02/05/2008 23:10:59 »
Presumeably, the same principle that allows metallic hydrogen to exist also applies to metallic oxygen, metallic sulfur, etc. I've even read that water can theoretically be metallized, although I'm unaware of any successful attepts to do this.
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