Recalling Radon's Recognition

Home , Radon, William Ramsay

in your element Recalling radon’s recognition Brett F. Thornton and Shawn C. Burdette look back at the discovery — and the many different names — of element 86.

n 1899, Pierre and Marie Curie noted an Thoron, unlike radon, requires no such “induced radioactivity” — left behind by clarification, and220 Rn is routinely called Iradium, distinct from its own radioactivity. thoron today. Thoron is far easier to say Ernest Rutherford and Robert B. Owens also than ‘radon-two-twenty’, perhaps explaining reported that year on a radioactive substance why the annual count of scientific papers 220 ( Rn, t1/2 = 55.6 s) emitted by thorium, which mentioning thoron has increased over twenty- they called emanation. In 1900, Friedrich fold since thoron was ‘disallowed’ in 1957. Dorn realized that the Curies had observed a Distinguishing between 222Rn (the 222 220 unique substance ( Rn, t1/2 = 3.8 d), similar isotope called ‘radon’) and Rn (thoron) is to emanation. In 1904, André-Louis Debierne not of idle linguistic and historic interest. 222

found a third radioactive particle; this one © SUPERSTOCK/ALAMY Rn can persist indoors, whereas the short- 219 produced from actinium ( Rn, t1/2 = 4 s). lived thoron cannot. Not all home radon These were at first regarded as elements and actinon (An) for the three isotopes; detectors (pictured) are sensitive to thoron, and became colloquially known as thorium names suggested by Elliott Q. Adams. and thoron-sensitive detectors must be emanation, radium emanation and actinium An official statement on a name for all placed with care because thoron does not emanation, but today we recognize them three isotopes — that is, a name for the travel far from its source. 222Rn tests cannot as radon isotopes. William Ramsay and element itself — would take seven more years. necessarily rule out the presence of thoron7, J. Norman Collie suggested1 a unique set of In a 1931 paper5 that included Marie Curie, which occasionally leads to uncertainties in names (exradio, exthorio and exactinio) for Rutherford and Debierne as co-authors, radon risk assessments. these ‘elements’: “it appears advisable to devise Rutherford’s original 1899 element name Very few isotopes hold special names a name which should recall its source, and, at emanation (Em) was finally formalized, in in modern chemistry: deuterium, tritium the same time, by its termination, express the addition to the three isotope names. Despite and thoron exist as near-element footnotes radical difference which undoubtedly exists this consensus of three early researchers of the on the periodic table. Actinon has nearly between it and other elements.” element, periodic tables and lists of elements vanished from the scientific literature, likely The specific –io suffix names were largely most often chose the name radon — the due to its short half-life and comparatively ignored, but the concept of amalgamated most stable isotope — instead of emanation. negligible health impacts. In 1948, the fourth names resonated within the scientific IUPAC’s 1957 Nomenclature of Inorganic natural isotope of radon was found (218Rn,

community. As experiments revealed Chemistry elevated radon from isotope name t1/2 = 35 ms). No one, apparently, thought of similarities to the noble gases, Ramsay and to element name6, an implicit change because naming this final natural radon isotope — a Robert Whytlaw-Gray suggested niton (Nt), only radon appeared on the element list. β-decay product of 218At — ‘astaton’. ❐ a name with the noble gas –on suffix, for the IUPAC’s amended rules stated: “All isotopes luminescent radium emanation. In 1911, of an element should have the same name. BRETT F. THORNTON1 and SHAWN the International Committee on Atomic For hydrogen the isotope names protium, C. BURDETTE2 are in 1Department of Weights added the ‘isotope’ niton to their list deuterium and tritium, may be retained, but Geological Sciences (IGV) and Bolin Centre of elements2, inadvertently intertwining the it is undesirable to assign isotopic names for Climate Research, Stockholm University, linguistics of isotopes and elements. This was instead of numbers to other elements.” 106 91 Stockholm, Sweden; 2Department before 1913, the year Moseley showed that Despite this declaration, radon had been of Chemistry and Biochemistry, Worcester each element has a unique atomic number, designated an isotope for nearly four decades, Polytechnic Institute, Worcester, and Frederick Soddy coined the term ‘isotope’, and was a poor choice for an element name. Massachusetts 01609-2280, USA. so this early misstep is understandable. Emanation was replaced by the awkward e-mail: [email protected]; Names for the emanations that evoked need to specify if the word radon referred to [email protected] both the noble gases and the emanations’ the element or the isotope. Using radon as the parent elements eventually appeared, element name also muddled history. Dorn, References including Marie Curie’s suggestion3 of radion who identified radium emanation (a specific 1. Ramsay, W. & Collie, J. N. Proc. Roy. Soc. Lond. 73, 470–476 (1904). 2. Clarke, F., Thorpe, T., Ostwald, W. & Urbain, G. J. Am. Chem. Soc. or radioneon. Eventually, in 1923, IUPAC isotope of radon) is today often mistakenly 33, 1639–1642 (1911). adopted4 the names radon (Rn), thoron (Tn) credited with discovering the element 3. Wilson, D. Rutherford: Simple Genius (MIT Press, 1983). radon. Dorn’s work even cited Rutherford 4. Aston, F. W. et al. J. Am. Chem. Soc. 45, 867–874 (1923). 5. Curie, M. et al. J. Am. Chem. Soc. 53, 2437–2450 (1931). and Owens’ earlier discovery of thorium 6. Bassett, H. et al. J. Am. Chem. Soc. 82, 5523–5544 (1960). emanation, the radon isotope thoron. 7. Janik, M. et al. J. Radiat. Res. 54, 597–610 (2013). Bi Po At Rn Fr Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 804 NATURE CHEMISTRY | VOL 5 | SEPTEMBER 2013 | www.nature.com/naturechemistry