Naming radon — and the lessons of protactinium

Posted on behalf of Brett Thornton and Shawn Burdette, who have penned the In Your Element article about radon in our September issue – Stuart

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Late in 2012, we were preparing what became the Nature Chemistry Thesis article The Ends of Elements, looking at both the history and the future of the suffixes on element names1. Chemists know (or implicitly know) that the -on in radon has a meaning: ‘noble gas’, just as the -ine in ‘astatine’ means ‘halogen’. While considering the details of the suffixes, we realized that there was rather a lot to say about how radon came to be named — it took decades for the element to be named radon.

The quirkiness of how radon was named became the radon In Your Element essay. In this blog post, we want to expand a bit on some things that did not fit in the short IYE piece. Like in the IYE article, we concentrate here on linguistics. Readers wishing to know more about the history of radon can find a number of excellent articles elsewhere2-5.

As we dug into the history of radon’s name, one thing that immediately popped out was the name thoron. Thoron (the 220Rn isotope), is relatively frequently mentioned in journals today. That’s more than slightly curious, because named isotopes are an artifact of the early 1900s, when the radioactive decay series were leading to names like ‘radium c-double-prime’ instead of the svelte ‘210Tl’ we’d use today. Today we’ve all but abandoned isotope names in favour of numbers. But thoron still exists in active use today — why?

We noticed something else odd about radon: phrases like ‘radon and thoron’ and ‘radon (222Rn) is a…’ are common — but aren’t these phrases a bit odd? To be clear: these phrases exist because radon has two meanings. Radon is both the element and the mass-222 isotope. So it is necessary to distinguish which you are talking about!

Double-definitions seem sloppy; why would chemists choose such a nomenclature? Actually, for many decades, the 86th element had another name: emanation, with the symbol ‘Em’. At that time, radon was only the mass-222 isotope: the isotope which came from radium. There was confusion early on: radon’s original (1899) name was emanation6, yet even in 1930, the name ‘emanation’ was described as ‘a general term for elements of atomic number 86’7. That’s our emphasis on the word ‘elements’. In modern parlance we’d say isotopes of atomic number 86.

By the late 1940s, when Walen reported the fourth natural isotope of radon8, many isotopes of many elements were known, and attempts to name them all had been abandoned.

If one counts the isotope names given the three early-discovered natural isotopes, element 86 has had many different names: 222Rn has been called radium emanation (RaEm), exradio, niton (Nt)9, radeon, radioneon, radon (Ro)10, radion, and radon (Rn)11. 220Rn has been called thorium emanation (ThEm), exthorio, thoreon, thoron (To)10, and thoron (Tn)11. 219Rn has been called actinium emanation (AcEm), exactinio, actineon, akton (Ao)10, acton, and actinon (An)11. Finally, the element has been called emanation (Em), emanon, and radon (Rn). That’s a lot of names for one element. By the mid-20th century, emanation was sometimes used for the element, though radon was more common, and most likely to appear on periodic tables.

But why didn’t IUPAC choose Rutherford’s emanation over radon? Were they following the herd of chemists and periodic-table designers, or was there something else to it?

It appears that the decision to back ‘radon’ parallels Kasimir Fajan’s disavowing of his own proposed name brevium in favour12 of protactinium for element 91. Fajans and Oswald Göhring had discovered brevium in 191313. Brevium (234mPa) has a short half-life of 1.17 minutes. When Soddy and Cranston, and Otto Hahn and Lise Meitner independently discovered the 32,000 year half-life protactinium (231Pa) five years later14,15 , and brevium was shown to be the same element, Fajans pushed the idea that the longest-lived isotope should be the name of the element. Fajans lived to see a problem with this, six decades later: one of his last published works was a defense of his priority in the discovery of protactinium16. History had begun to forget that ‘brevium’ was actually protactinium, and discovery priority was sometimes being assigned to the later discoverers of the longer-lived isotope.

Precisely the same thing happened to Rutherford and Owen’s discovery of thoron3, when the later-discovered radon became the name of the element. Overlooking that Rutherford was first, albeit with a shorter-lived isotope, seems likely due to these linguistic twists.

William Ramsay’s role in this story emphasizes his unique place in the discovery and naming of elements — he named neon, argon, krypton and xenon. (Sometimes the names were suggested to him by others). His seemingly peculiar –io suffix suggestion for radon’s isotopes made perfect sense in 1904. The three radioactive ‘emanations’ were like no elements seem before. Fresh from the discoveries of the lighter noble gases, and his successful lobbying for -on names for them, Ramsay saw the opportunity to define a new group of elements. It is interesting that his later niton suggestion was only for radium emanation. Likely, this was due to radium emanation being much easier to work with than thorium emanation or actinium emanation. The appearance of Ramsay’s niton on tables of atomic weights17 before Soddy coined the term isotopes18 and before Moseley’s revealing of atomic numbers likely set in motion the sequence that eventually led to ‘radon’ being elevated from isotope to element.

With the benefit of hindsight, it seems that emanation (or emanon, to maintain a more noble-gas sound) was the better name. Emanation follows Paneth’s influential 1947 guidelines for naming elements, which called for discoverer-priority in naming19. These guidelines were largely adopted by IUPAC later. Why radon was preferred over emanation remains somewhat unclear, though Fajan’s opinion was well regarded, and seems to have solidified the idea that longest-lived isotope names become element names (this rule was rarely invoked — it appears that protactinium and radon are the only examples). But brevium was cast aside relatively quickly; ‘radon’ existed for decades as only an isotope name before assuming its present dual role. Despite IUPAC’s guidelines, the literature is clear that radon is still used today to mean both the element and the isotope. The change from emanation to radon did not happen overnight; many papers were published still using ‘emanation’ through the 1960s, with some prominent workers and journals abandoning ‘emanation’ quite late20,21.

With and without IUPAC’s approval, there are now two elements on the periodic table with named isotopes in ‘active use’ in the scientific and common literature: hydrogen and radon. Interestingly, naming the hydrogen-2 isotope deuterium was the subject of another Nature Chemistry IYE essay22. One can’t expect the confusion about radon — element or isotope — to disappear, but it is interesting where the present situation originated from.

BRETT F. THORNTON1 and SHAWN C. BURDETTE2 are in 1Department of Geological Sciences (IGV) and Bolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden; 2Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609-2280, USA.
e-mail:; scburdette@WPI.EDU


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