The hydrogen isotope whose nucleus is a bare proton is called protium and its symbol is H¹. This is the only atomic nucleus which contains no neutrons at all. (Another unique property of hydrogen!) Add a neutron to this single proton and the result is the nucleus of the heavy hydrogen isotope called deuterium (H² or D²). Protium is far more abundant in nature than deuterium, consti­tuting over 99 per cent of all the hydrogen.

   But there is a third variety of hydrogen, with two neutrons in its nucleus; this is tritium (H³ or T³). It forms continuously in the atmosphere under the action of cosmic rays. It forms only to disappear again rather quickly. It is radioactive and decays into a helium isotope (helium-3). Tritium is a very rare element: its content in all the atmosphere of the Earth is only 6 grams. There is only one atom of tritium in every 10 cubic centimetres of air. Just recently still heavier isotopes of hydrogen H⁴ and H⁵ have been obtained artificially, but they are very unstable.

   The mere fact of its having isotopes does not distinguish hydrogen among the chemical elements. What does distinguish it is that hydrogen isotopes differ noticeably in properties, primarily in physical properties. Isotopes of the other elements are almost indiscriminable.

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