Element Radon, Rn, Noble Gas

In 1899, Robert B. Owens, an American physicist, observed that some new substance appeared during thorium decay. This new substance may be removed from thorium solutions by air flow. Rutherford, who was teaching at McGill University in Montreal, Canada, concluded that thorium compounds continuously emit a radioactive gas which retains the radioactive powers for several minutes. He called this gas "emanation", from Latin "emanare" - to elapse and "emanatio" - expiration. In 1900, Friedrich Ernst Dorn in Germany and Andre-Louis Debierne in France confirmed the findings of Rutherford as radium emanation. Rutherford, Ramsay and Soddy showed that thorium emanation is a new element of the noble gas family named radon.

Originally, Radon was called Niton. The name niton was given by Gray and Ramsay to radium emanation. Its history is, naturally, closely involved with that of radium, but it may be mentioned here that its discovery was due to Dorn. It was early recognised that this product of the radioactive disintegration of radium was a gas, and resembled the inert gases in its indifference to platinum- and palladium-black, lead chromate, zinc and magnesium powder, and magnesium and lime at a red heat, to phosphorus burning in oxygen, and to sparking with oxygen.

The density was determined and the molecular weight thereby found to be about 223, it became evident that, were it a monatomic gas, it would find a place in the periodic classification in the group of inert gases and in the same series as radium. It has since been found that its other physical properties accord well with those to be expected in an inert gas of atomic weight 223, and niton is now, by general consent, given a place in Group 0.

Noble Gas Radon is one of the rarest of natural elements. The outer 1.6 km deep layers of the Earth's crust only 115 tons. Being formed in radioactive ores it gradually comes on to the Earth surface, in hydrosphere and atmosphere. Average concentration in atmosphere is 6x10^-17 mass %, in sea water until 0.001 pCi/l. Radon contributes the main part of air natural radioactivity.

Radon doubtless occurs in all radium minerals; but as it undergoes radioactive change and only a minute amount can be present in equilibrium with the small quantity of radium, it is hardly surprising that it has not hitherto been detected.

It is undoubtedly the case that some radioactive substance is present in the atmosphere, because a negatively charged wire exposed to air acquires a radioactive deposit which can be removed thence by solution in ammonia or by rubbing. Observations on the rate of decay of this deposit indicate that it is due to the presence in the atmosphere, of the emanations of thorium and radium derived from thorium and radium contained in the soil. The relative proportions of radon and thorium emanation present in the lower levels of the atmosphere appear to be very variable, and it has been found that at Sestola, in the Apennines, at a height of 3000 feet, the proportion of the total atmospheric activity due to thorium varies from 29-73 per cent. In general it may be said that thorium emanation preponderates in the lower levels of the atmosphere and radon in the higher levels.

The amount of radon present per cubic metre of air is approximately that which would be in equilibrium with 1.0×10^-10 gm. of radium.

Isolation. - Radon is continuously formed by the radioactive change of radium and its salts under all conditions. Solid radium salts always retain the whole of the radon - presumably in a state of solid solution, - but the greater part of the gas may be driven out by heating to about 800°. The proportion retained decreases as the temperature is raised and is independent of the duration of heating, but varies much with different preparations. The radon may be pumped away directly, but the amount usually handled is so minute that it is necessary to wash it out with another gas, e.g. oxygen.

When an aqueous solution of a radium salt is kept in a closed flask radon is evolved, together with oxygen and hydrogen produced by the decomposition of the water. The amount of radon increases rapidly during the first few days and afterwards more slowly until, after about a month, the maximum amount which can be in equilibrium with the radium present is reached.

The hydrogen produced is about 5-10 per cent, in excess of that equivalent to the oxygen present - this is probably due to the action of the penetrating rays of radium, which bring about the conversion of water into hydrogen peroxide and hydrogen. On exploding the gas, all the oxygen and most of the, hydrogen are converted into water. If the residual gas be passed through a copper or glass spiral cooled in liquid air, the whole of the radon is condensed, and the residual hydrogen, together with any helium produced by the decomposition of the radon during the time it has been in the flask, may be pumped off. A similar method may be used to condense radon from air. The gas thus prepared usually contains a small amount of carbon dioxide produced by the oxidation of adventitious organic matter, but this can be removed by prolonged contact with baryta.

The amount of radon which can be obtained from radium is very small, as will be readily realised from the fact that the maximum amount that can be in equilibrium with 1 gram of radium is 0.585 cubic mm. at N.T.P.