Trihydrogen cation
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| Identifiers | |
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| Properties | |
| H+ 3 |
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| Molar mass | 3.02 |
| Related compounds | |
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Other anions
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hydride |
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Other cations
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hydrogen ion, dihydrogen cation, hydrogen ion cluster |
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Related compounds
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trihydrogen |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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| Infobox references | |
The trihydrogen cation, also known as protonated molecular hydrogen or H+
3, is one of the most abundant ions in the universe. It is stable in the interstellar medium (ISM) due to the low temperature and low density of interstellar space. The role that H+
3 plays in the gas-phase chemistry of the ISM is unparalleled by any other molecular ion. The cation is also the simplest triatomic molecule, since its two electrons are the only valence electrons in the system. It is also the simplest example of a three-center two-electron bond system.
H+
3 was first discovered by J. J. Thomson in 1911. While studying the resultant species of plasma discharges, he discovered something very odd. Using an early form of mass spectrometry, he discovered a large abundance of a molecular ion with a mass-to-charge ratio of 3. He stated that the only two possibilities were C4+ or H+
3. Since C4+ would be very unlikely and the signal grew stronger in pure hydrogen gas, he correctly assigned the species as H+
3.
The formation pathway was discovered by Hogness & Lunn in 1925. They also used an early form of mass spectrometry to study hydrogen discharges. They found that as the pressure of hydrogen increased, the amount of H+
3 increased linearly and the amount of H+
2 decreased linearly. In addition, there was little H+ at any pressure. This data suggested the proton exchange formation pathway discussed below.
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