{{Isotopes table/footer}}

 

[[File:Hydrogen.svg|thumb|right|Protium, the most common [[isotope]] of hydrogen, consists of one proton and one electron. Unique among all stable isotopes, it has no neutrons. (see [[diproton]] for a discussion of why others do not exist)]]

{{Main|Hydrogen atom}}

The proton has never been observed to decay, and hydrogen-1 is therefore considered a stable isotope. Some [[grand unified theories]] proposed in the 1970s predict that [[proton decay]] can occur with a half-life between 10²⁸ and 10³⁶ years.<ref>"Grand Unified Theories and Proton Decay", Ed Kearns, Boston University, 2009, page&nbsp;15. http://physics.bu.edu/NEPPSR/TALKS-2009/Kearns_GUTs_ProtonDecay.pdf</ref> If this prediction is found to be true, then hydrogen-1 (and indeed all nuclei now believed to be stable) are only [[Stable nuclide#Observationally stable|observationally stable]]. To date, experiments have shown that the minimum proton half-life is in excess of 10³⁴ years.

 

{{Main|Deuterium}}

'''²H''' (atomic mass {{val|2.01410177811|(12)|ul=Da}}), the other stable hydrogen isotope, is known as ''[[deuterium]]'' and contains one proton and one neutron in its nucleus. The nucleus of deuterium is called a deuteron. Deuterium comprises 0.0026–0.0184% (by population, not by mass) of hydrogen samples on Earth, with the lower number tending to be found in samples of hydrogen gas and the higher enrichment (0.015% or 150&nbsp;ppm) typical of ocean water. Deuterium on Earth has been enriched with respect to its initial concentration in the Big Bang and the outer solar system (about 27 ppm, by atom fraction) and its concentration in older parts of the Milky Way galaxy (about 23 ppm). Presumably the differential concentration of deuterium in the inner solar system is due to the lower volatility of deuterium gas and compounds, enriching deuterium fractions in comets and planets exposed to significant heat from the Sun over billions of years of solar system evolution.

 

Deuterium is not radioactive, and does not represent a significant toxicity hazard. Water enriched in molecules that include deuterium instead of protium is called [[heavy water]]. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for ¹H-[[NMR spectroscopy]]. Heavy water is used as a [[neutron moderator]] and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial [[nuclear fusion]].

 

{{Main|Tritium}}

'''³H''' (atomic mass {{val|3.01604928199|(23)|ul=Da}}) is known as ''[[tritium]]'' and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into [[helium-3]] through [[β− decay]] with a [[half-life]] of 12.32 years.<ref name="Miessler">

{{cite book

 

== Hydrogen-4 ==

{{cite journal

|author=G. M. Ter-Akopian|year=2002

 

== Hydrogen-5 ==

{{cite journal

| author = A. A. Korsheninnikov| year = 2001