Metallic Hydrogen

[Previous Community Member]

A friend of mine recently learned of Metallic Hydrogen, such as what we theorize lies beneath the clouds of Jupiter, and thought that hydrogen should be catagorized as a metalloid on the periodic table. We discussed this for some time, with me stating that the periodic table only pertains to elements at room temperature and sea level pressure, and that hydrogen in that state is very non metallic, sharing few, if any, properties with metals. But I wasn't able to convince him, so I have a few questions that I feel should come straight from the experts.

1: What are your thoughts on Metallic Hydrogen, and how similar is it to other metals or metalloids?

2: What causes metallic hydrogen to behave in this way? Hydrogen surely shouldn't have any spare electrons, so what causes the conductivity?

3: Does the periodic table only apply to elements at room temperature and sea level pressure? And if not, could hydrogen be considered a metalloid due to its metallic state?

4: How common is this sort of behavior in elements? Do other elements take on metallic properties or loose them if they are metals/metalloids or do they mostly remain in the same group(ie, alkaline and halogen, or non metal and metals) thoughout their states?(Is gaseous Iron a metal?)

5: The requirements for metallic hydrogen seem to be fairly intense, so even if it could be considered a metalloid, would it ever be made one due to the difficulty in creating the metallic properties?

Thanks, Jack.

P.s. No need for lay terms, I'm fairly well versed.

[scooke]

Dear Jack,

Here’s just a few of my thoughts. First of all, science is a continuing adventure, trying to figure out how the universe works. We use what is useful, but should never be really expecting to find (or hold on to) “absolute facts”! Thus, discussions of observations and interpretations are great, but we really shouldn’t ever let them become debates to “win”.

1: What are your thoughts on Metallic Hydrogen, and how similar is it to other metals or metalloids?

That’s what the experiements and questions are for! You are already on a “slippery slope” when you include a term like ‘metalloids’. Is it even necessary to strictly define “metallic” properties, and what if there are anomalous characteristics of some elements? Personally, I think that “Metallic Hydrogen” is a fascinating concept, and I enjoy reading about the experiments that try to create and characterize it.

2: What causes metallic hydrogen to behave in this way? Hydrogen surely shouldn't have any spare electrons, so what causes the conductivity?

I’m not sure of what you mean by “spare electrons”, as hydrogen has only one in its ground state, and thus can act as either an electron donor or acceptor. It makes for fun chemistry! Exactly “why” metallic hydrogen acts in one way or another I don’t know personally, and that is why I do love to read about other researchers experiments and observations.

3: Does the periodic table only apply to elements at room temperature and sea level pressure? And if not, could hydrogen be considered a metalloid due to its metallic state?

No. And that was another possible error in your “argument” about elements. The Periodic Table is a list of elements grouped by common characteristics – and there are many variations on it! It does NOT require or imply any particular physical environmental conditions for any elemennt, and those that are listed are for convenience and utility in the use of the Table.

4: How common is this sort of behavior in elements? Do other elements take on metallic properties or loose them if they are metals/metalloids or do they mostly remain in the same group(ie, alkaline and halogen, or non metal and metals) thoughout their states?(Is gaseous Iron a metal?)

Extreme states of matter (high/low pressures, temperatures) often reveal unique and often unexpected properties. Despite how much information we accumulate, it is still most probably true that we do not know much more than we know. Again, that is one of the allures of scientific studies! Also, while humans try to make patterns in order to make sense of our surroundings, be careful about any labels and focus on accurate descriptions instead of titles. Even something as ubiquitous as acids and bases have gone through several definition iterations! Each is an improvement in utility and predictive assessment, and none negated a previous one. But, “definitions” such as “metal” or “nonmetal” only have real use in actual utility. Don’t get constrained by a label and miss the reality of life.

5: The requirements for metallic hydrogen seem to be fairly intense, so even if it could be considered a metalloid, would it ever be made one due to the difficulty in creating the metallic properties?

As with some of the theoretical heavy elements, there is a wide range of motivation in scientific inquiries. Some are very theoretical, and others tend to be more practical. We usually discover particular properties before we find any particular use for them. I wouldn’t worry too much about what to call something until we have a bit more of it to study, and more people have had a chance to observe it under different conditions.

ENJOY the learning process! It’s always about our own growth, and not about establishing particular “rules” or conditions that may be inappropriately administered in any case.

Cheers,

Steven

[tessier]

The position of hydrogen in the periodic table has been a point of concern for a while. (See Marshall W. Cronyn “The Proper Place for Hydrogen in the Periodic Table” J. Chem. Ed. 2003, 80, 947-951.) Hydrogen is most commonly placed above Group 1 in the periodic table. This is probably because it can function as H⁺, one of its more important functions but definitely not the only important one. In some periodic tables, hydrogen is placed over both groups 1 and 17. This is in agreement with a redox rule of thumb (ROT) taught in introductory courses that states that H bound to a nonmetal is H⁺ and when bound to a metal it is H^-. When I first heard of this ROT, I wondered why hydrogen is not considered a metalloid. The introductory ROT that concerns the acidity of oxides also suggests that hydrogen is more like the metalloids than either Groups 1 or 17. The ROT is: oxides of metals are usually basic whereas the oxides of non-metals are acidic. The main oxide of hydrogen, water, is a source of both H⁺ and OH^- and therefore it is amphoteric. Another argument that places hydrogen closer to the metalloids is electronegativity (EN). A ROT that I teach is that the EN of hydrogen is roughly in the middle of the Pauling scale, which ranges 0.7 for cesium to 4.0 for fluorine. If you take the average of 0.7 and 4.0 you get 2.35, a little higher than that of hydrogen at 2.1. In addition, the EN of hydrogen is similar to the ENs of the metalloids at 1.8-2.0. On the basis of the EN arguments, the first year chemistry text that I teach from places H somewhere above B and C in the periodic table (Silberberg, M. S. Chemistry, 8^th ed.; McGraw Hill: New York, 2017, p. 391.). 

[RENE_VERNON]

Elements are normally classified on the basis of their properties in ambient conditions. So hydrogen is effectively universally regarded as a nonmetal. While its electronegativity is intermediate, its ionisation energy is too high to be regarded as a metalloid, not to mention it not looking like a metal. Metalloids look like metals but chemically behave predominately as (weak) nonmetals.

1. It will presumably look like a metal and exhibit good electrical conductivity. The elements commonly recognised as metalloids are semi-metals or semiconductors. On its way to being crushed into metallic form, hydrogen might pass through these phases.
2. Compare the ratio of the force holding an individual atom's valence electrons in place with the forces on the same electrons arising from interactions between the atoms in the solid or liquid element. When the interatomic forces are greater than, or equal to, the atomic force, valence electron itinerancy is indicated and metallic behaviour is predicted.
3. The periodic table is usually taken to apply in ambient conditions.
4. Quite common. Oxygen, for example, becomes a metal at 100 GPa. Sodium turns into a transparent nonmetal at ~ 200 Gpa. Gaseous iron would be regarded as a nonmetal. Liquid carbon may or not be a metallic conductor, depending on pressure and temperature.
5. See my answer to 1. Various claims have been made for the synthesis of metallic hydrogen. The technology required appears to be within reach. See the Wikipedia article on metallic hydrogen.