In General > s.a. early-universe nucleosynthesis [deuterium]; molecular physics.
* Idea: The simplest, and the most abundant element in the universe, by far; Hydrogen makes up 75 percent of all the visible mass in the universe, and more than 90 percent of all the atoms.
* Properties: It easily forms compounds with nearly all non-metallic elements on the periodic table, and its bonds with oxygen and carbon are the reason why we exist; Hydrogen auto-ignites at 500 ºC.
@ General references: PW(95)jul issue; Rigden 02.
@ In quantum mechanics: Ogilvie a1603; > s.a. composite quantum systems.
@ Related topics: Blanford et al PRL(98) + pn(98)apr [anti-H]; Korsheninnikov et al PRL(01) + pn(01)aug [5H], PRL(03) [7H, considered inconclusive]; news pw(05)aug [hydrino state]; news pn(07)jul [7H]; news PhysOrg(11)oct [producing 99% pure H in a single device, without a separate purification step]; Stodolna et al PRL(13) + Smeenk Phy(13) [direct observation of excited states]; news(17)may hydrogen bonds detected].

Phases > s.a. high-temperature superconductors.
* Metallic form: At high temperatures compressed hydrogen becomes a metallic liquid; 1935, Existence of this state proposed by Wigner (at pressure of 250,000 bar); Initial attempts made with crystalline structure; 1968, Ashcroft proposal that solid metallic H could be a high-temperature superconductor; 1996, Obtained in disordered state, at pressures > 1.4 Mbar [@ LLNL 1996 page; Weir et al PRL(96) + pn(96)mar]; 2011, Solid hydrogen can exist in several different phases, phase IV obtained at 2.2 Mbar, arranged into graphene-like sheets; 2016, Most of the planetary mass of the solar system may be in the form ofmetallic hydrogen, mainly in the interiors of the giant planets; Several labs are still trying to obtain solid metallic hydrogen, at 3–4 Mbar.
@ High-pressure behavior: Strobel et al PRL(09) + Ashcroft Phy(09); news sn(12)mar [new high-pressure phase, honeycombed atoms layered with free-floating molecules]; news PhysOrg(12)apr; McMahon et al RMP(12) [properties under extreme conditions]; news sa(16)jun [metallic H inside planets]; news sn(16)aug [the search for solid metallic H]; news sn(17)jan, csm(17)jan [debate on metallic form]; Celliers & Eggert Phy(17) [surprises].

H phases

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