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Under normal conditions hydrogen gas is a mix of two different kinds of
molecules which differ from one another by therelative spin of the nuclei. These two forms are known as ortho- and para-hydrogen (this is different from isotopes, see below). In ortho-hydrogen the nuclear spins are parallel (form a triplet), while in para they are antiparallel (form a singlet). At standard conditions hydrogen is composed of about 25% of the para form and 75% of the ortho form (the so-called "normal" form). The equilibrium ratio of these two forms depend on temperature but since the ortho form has higher energy (is an excited state), it cannot be stable in its pure form. In low temperatures (around boiling point), the equilibrium state is comprised of almost only para form.
The conversion process between the forms is slow and if hydrogen is cooled down and condensed rapidly, it contains large quantities of the ortho form. It is important in preparation and storage of liquid hydrogen since the ortho-para conversion produces more heat than the heat of its evaporation and a lot of hydrogen can be lost by evaporation in this way during several days after liquifying. Therefore, some catalysts of the ortho-para conversion process are used during hydrogen cooling. The two forms have also slightly different physical properties. For example, the melting and boiling points of parahydrogen are about 0.1 K lower than of the "normal" form.
Hydrogen is the only element that has different names for its isotopes. The symbols D and T (instead of 2H and 3H) are sometimes used for deuterium and tritium, although this is not officially sanctioned. (The symbol P is already in use for phosphorus and is not available for protium.)
The most common isotope of hydrogen, this stable isotope has a nucleus consisting of a single proton; hence the descriptive, although rarely used, name protium.
The other stable isotope is deuterium, with an extra neutron in the nucleus. Deuterium comprises 0.0184-0.0082% of all hydrogen ( IUPAC); ratios of deuterium to protium are reported relative to the VSMOW standard reference water.
The third naturally-occurring hydrogen isotope is the radioactive tritium. The tritium nucleus contains two neutrons in addition to the proton.
Hydrogen-4 was synthesised by bombarding tritium with fast-moving deuterium nuclei.
In 2001 scientists detected hydrogen-5 by bombarding a hydrogen target with heavy ions.
Not yet discovered.
In 2003 hydrogen-7 was created (article) at the RIKEN laboratory in Japan by colliding a high-energy beam of helium-8 atoms with a cryogenic hydrogen target and detecting tritons—the nuclei of tritium atoms—and neutrons from the break up of Hydrogen-7, the same method used to produce and detect Hydrogen-5.
Hydrogen is a highly flammable gas burning at concentrations as low as 4%. It also reacts violently with chlorine and fluorine, forming hydrohalic acids that can cause damage to the lungs and other tissues. When mixed with oxygen, hydrogen explodes on ignition.
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