Radon Rn is the 86th element of the periodic system. It is the heaviest of the noble gases. It is highly radioactive and its natural abundance is so low that it could not be identified when W. Ramsay and M. Travers discovered other inert elements. Only application of the radiometric method made possible the discovery of radon.
What we know as radon at present is the combined name for the three natural isotopes of the element No. 86, which were discovered one by one and called emanations. Their appearance heralded a new stage in the studies of radioactivity as they were the first gaseous radioactive substances.
At the beginning of 1899 E. Rutherford (who lived at the time in Canada) and his collaborator R. Owens studied the activity of thorium compounds. Once Owens accidentally threw open the door to the laboratory where a routine experimenter was performed. There was a drought and the experimenters noticed that the intensity of radiation of the thorium preparations suddenly dropped. At first they ignored this event but later they observed that a slight movement of air seemed to remove a larger part of the activity of thorium. Rutherford and Owens decided that thorium continuously emitted a gaseous radioactive substance, which they called the emanation (from the Latin to flow) of thorium, or Theron.
By way of analogy, it was suggested that other radioactive elements could also evolve emanations. In 1900 the German physicist E. Dorn discovered the emanation of radium and three years later Debierne observed the emanation of actinium. Thus, two new radioactive elements were found, namely, radon and action. An important observation was that all the three emanations differed only in their half–lives–51.5 s for thoron, 3.8 days for radon, and 3.02 s for action. The longest–lived element is radon and therefore it was used in all studies of the nature of emanations. All the other properties of emanations were identical. All of them lacked chemical manifestations, that is, they were inert gases (analogues of argon and other noble gases). Later they were found to have different atomic masses. But there was just a single slot for these three elements in the periodic system, immediately below xenon.
Such exclusive situation soon became a rule. Therefore, we shall have to discuss briefly some important events in the history of radioactivity studies. Now we must finish the story of radon. This name remained because radon is the longest–lived element among the radioactive inert gases. Ramsay suggested to name it niton (from the Latin for glowing) but this name did not take root.