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"One moon circles!"
– Alien message describing hydrogen, 2367 ("Night Terrors")

Hydrogen was a chemical element with the atomic number 1 on the periodic table. Its atomic symbol was generally H, but it also sometimes appeared as Cs. (TNG: "Rascals")

In real life, H is the atomic symbol for hydrogen while Cs is the atomic symbol for cesium, without exceptions.

Hydrogen was a highly inflammable diatomic gas with the molecular formula H2. It was in a class by itself as the most basic, and most abundant, element in the known universe, constituting 75% of the universe's elemental gas. Under standard pressure and temperature, hydrogen was a colorless gas. It was the least dense gas known and its very low boiling point of 20.7 kelvin reflected its nonpolar character and low molar mass. Its atomic structure consisted of one proton and one electron. Hydrogen had three isotopes, two of them stable (protium and deuterium) and one radioactive (tritium). Of the three isotopes, only the first two (often simply referred to as "hydrogen") were found in nature in measurable quantities. The H2 molecule had a "threshold" of 14.7 electronvolts. (VOY: "Relativity")

The Lothra were a hydrogen-breathing species. (DS9: "Melora")

In 2151, Commander Tucker referred to the risky job astronauts did with the use of rockets: "In the old days, astronauts rode rockets with millions of liters of hydrogen burning under their seats. You think they said, gee I'd love to go to the moon today but it seems a little risky?" (ENT: "Silent Enemy")

The Enterprise-D venting hydrogen

In 2367, the USS Enterprise-D released large quantities of hydrogen from their Bussard collectors to escape from a Tyken's Rift. The hydrogen, when combined with a calendenium-analog being released from an unknown alien vessel, created an explosion violent enough to rupture the rift, freeing both ships. The explosion released more energy than the combined firepower of Enterprise-D's phasers and photon torpedoes. (TNG: "Night Terrors")

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Background information

Hydrogen appeared twice in the Table of Elements seen in "Rascals", which was full of in-jokes and never intended to be legible on-screen. It appears both under "H" in its real-world position, as well as under "Cs", taking the place of cesium. A reproduction of the graphic appears in Star Trek: The Next Generation - The Continuing Mission (p. XIV).

The molecule "threshold" of 14.7 eV is respectably close to the ionization energy of hydrogen, which is 13.6 eV per atom.

Hydrogen has an emission spectrum which includes a 21-centimeter wavelength. In "Galaxy's Child", Geordi La Forge stated "All matter in space vibrates in a specific radiation band" to which Leah Brahms answers "21 centimeters." Due to the ubiquity of hydrogen in stellar matter, it's likely that refers to the presence of hydrogen.

The LCARS description read "Stored in the form of deuterium, used as principal reactant formatter/antimatter warp propulsion system, and as principal fuel source for the impulse propulsion system and auxiliary fusion reactors."

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