Hydrogen has three naturally occuring isotopes. They are ¹H₁, ²H₁ (also called "deuterium") and ³H₁ (known as "tritium"). Of the three, ¹H₁ is the most abundant, with deuterium occuring in about a 1:6500 ratio with ¹H₁, and tritium in about a 1:10¹⁸ ratio.

Tritium is the most interesting since it's radioactive. Tritium has a half-life of 12.4 years and breaks down like this:

³H₁ --> ³He₂ + ⁰e_-1

where ⁰e_-1 is a beta particle (electron) and ³He₂ is helium.

You can produce tritium at home by bombarding lithium with neutrons to make tritium and helium.

⁶Li₃ + ¹n₀ --> ³H₁ + ⁴He₂

This is one of the reactions that take place when a hydrogen bomb explodes.

H-5 discovered

This is an update on the current state of the art concerning hydrogen.

The commonly known isotopes of hydrogen are:

• Hydrogen-1, the "normal" and most abundant variety,
• Hydrogen-2, known as deuterium, and
• Hydrogen-3, known as tritium.

Deuterium and tritium are constituents of heavy water. Tritium is radioactive. Both are much rarer than H-1 and play a role in the development both of the hydrogen bomb and of nuclear fusion reactors.

Atom-smashing equipment, in particular particle accelerators, enable physicists to create some isotopes which do not occur in nature. Apparently they've already produced H-4, a synthetic isotope with 3 neutrons in the nucleus, but I haven't come across any exciting information about this. The big news is H-5.

Late in 2001, an international group of researchers collided a beam of helium-6 nuclei into a target of cryogenic (frozen) hydrogen. In the ensuing nuclear reaction, it's possible for the He-6 to transfer all 4 of its neutrons to the hydrogen, turning it into - Hydrogen-5. Of course the experiment, which took place at the RIKEN detector in Japan, produced only trace quantities of the highly unstable isotope; and rather than collecting some of the product, the researchers obtained only indirect evidence of the production of this isotope. Still, it's considered a major breakthrough in nuclear physics, as the search for H-5 had been ongoing for 40 years previously.

• http://www.spiegel.de/wissenschaft/mensch/0,1518,151441,00.html
• http://physicsweb.org/article/news/5/8/15