Rhyming hydrogen with Rutherford, or who needs praseodymium and promethium anyway?
Two interesting side stories from this month’s tritium In Your Element article deserve some expansion here. To fully appreciate how the story unfolded, you need to know something about the naming the hydrogen isotopes, and why their nomenclature was such a big deal to chemistry in the 1930s.
First, some pertinent background about the naming of the hydrogen isotopes. When the two heavy isotopes of hydrogen were discovered in the 1930s, scientists were transfixed by the unprecedented naming opportunity. In science, taxonomy is one of the first things that happens—we need to agree on a lexicon in order to effectively discuss an exciting new area. Naming disputes are common in science, and many chemical element discoveries were accompanied by naming disputes (e.g. ref. 1, 2), in part because naming elements is such an exquisitely rare opportunity3. But to some chemists (and physicists) in the 1930s, hydrogen-2 and hydrogen-3 appeared to even more important and special than a new element.
To understand why, it helps to know a bit about isotopes4. In the early 1900s, many new substances were discovered amongst the natural radioactive elements and in their daughter products. These substances appeared to be—and initially were named as—new chemical elements. Many of these ‘elements’ eventually were realised to be isotopes of known elements, and the unique names were retired5. In fact during the 1910s, most of the new discoveries were isotopes of elements, not elements. But Moseley’s pivotal work in 1913 put the ceiling on the number of elements via atomic number, and the greatest prestige was now in discovering unknown elements, not isotopes. For many chemists to this day, isotopes remain something of a sideshow. The chemistry of all the isotopes of an element are basically the same. Most isotopes are nearly inseparable through chemical processes, and for chemists there has never been a particular incentive to name newly discovered isotopes—except in the 1930s with hydrogen-2 and hydrogen-3. The heavy hydrogen isotopes were special, precisely because they broke the expectation that all isotopes of an element would share basically the same chemistry. Hydrogen-2 and hydrogen-3 did not. By the original definition of isotopes, deuterium and tritium seemed more like elements in their own right.
In fact, deuterium and tritium’s chemical separability broke the definition of an isotope so much that Frederick Soddy, who had received the 1921 Nobel Prize for Chemistry for the discovery of isotopy, argued that deuterium and tritium were not isotopes. In 1934, Soddy argued: “With regard to separability, the instance would appear to be much more analogous to the separation of a pair of homologues, such as zirconium and hafnium, rather than isotopes.”6 To Soddy, deuterium and tritium were too different, chemically, from hydrogen-1, so they could not be isotopes by his definition. Of course, the modern strict characterisation of isotopes depends only on the number of protons in the nucleus, so tritium and deuterium are clearly isotopes of hydrogen.
The tale of how hydrogen-2, deuterium, was discovered has been well-told by one of its principal architects, Brickwedde7. In addition to being a doubly heavy variety of the first element on the periodic table, hydrogen-2 was unique because:
1) Deuterium was relatively easy to separate from “ordinary” hydrogen. There had never been such easy access to a single isotope of a multi-isotopic element. Many previously unimaginable experiments were now possible for chemists. Within a couple of years, hydrogen-2 research exploded exponentially.
2) Deuterium did not exhibit the same chemistry as “ordinary” hydrogen (this of course made separating it relatively easy). Chemists had a new variety of hydrogen that acted “a lot like” hydrogen in many chemical reactions, and was actually hydrogen as defined by a single nuclear proton. At many times though, deuterium didn’t behave like ordinary hydrogen.
The immense excitement about hydrogen-2 amongst scientists in the 1930 is hard to explain today. Thousands of papers were published about hydrogen-2 within a few years. Deuterium was the most exciting topic for a broad range of chemists and physicists to study since the radioactivity craze three decades earlier. From the perspective of today’s chemists, who can choose to publish in many more journals and also see orders of magnitude more papers, the closest possible contemporary analogy might be the profusion of papers on CRISPR gene-editing technology. Also similar to CRISPR, the significance of deuterium was realised almost immediately. Harold Urey, who led the discovery of deuterium in 1932, received the 1934 Nobel Prize for Chemistry. Urey, with Brickwedde and George Murphy, were prodded to suggest a name for hydrogen-2, because chemists needed a taxonomy, a nomenclature, for their new centrepiece8,9.
Gilbert N. Lewis, Urey’s former PhD supervisor, provided Urey the suggestions that instigated the naming debate in May 1933. Lewis became enamoured with the name “dygen”, but Urey hated it. Urey’s replied that presently he was considering “pycnogen” and “barogen”—based on Greek prefixes for “thick” and “heavy”, respectively. By mid-May 1933, Urey also considered grafting the pycn- and bar- prefixes to hydrogen. Interestingly, all these ideas preserved the -gen suffix of hydrogen, which made chemical sense since the new substance appeared to be behave similarly, though not identically, to “ordinary” hydrogen10. Lewis aggressively pressed Urey about the names throughout May via telegrams, which led Urey to furiously demand that his group be allowed to name the discovery, not Lewis. One could speculate that Lewis’ pressure on Urey resulted in the rejection of many early names, and the -gen suffix. On May 29, 1933, Urey bluntly wrote to Lewis that they had picked the name “deuterium” for hydrogen-2, and “tritium” for hydrogen-3, “In case a 3H isotope is discovered”.
Oddly, most of the names under discussion for hydrogen-2 and eventually hydrogen-3 were derived from words meaning “two” or “double” or “three” and “triple”9. Conspicuously, these names were not based on the properties of the isotopes, their place of discovery, or any other common name derivation for elements. No other elements or isotopes had numerological names, which was incongruous at the time, and even more bizarre to us with more distance in time and connection from those discussions. How could any other element’s isotopes possibly be named using this scheme? To give one example of thousands, both cadmium and tin have an isotope of mass 114. Clearly numeric-derived names would have to be limited to isotopes of hydrogen, not of any other element, to avoid confusion9. For elements, the numerically derived names have rarely even been considered, though centurium was bandied about for element 100.
Ernest Rutherford entered the debate in a letter to Urey on 20 June 1933 in which he suggested both “deuterogen” or “diogen” as possible names for hydrogen-2, but his preference was “diplogen”. He also recommended the symbol D, which conveniently could also be applied to Urey’s deuterium. In Urey’s response to Rutherford on July 6, he noted that diplogen had already been suggested by his co-worker Brickwedde, but rejected8.
A long series of loud, spiraling and sometimes chaotic naming debates followed. Urey’s choice of deuterium was so despised that many people began to insert themselves into the discussion, not just to side with “deuterium” or “diplogen”, but to assert competing claims, as well as to suggest names or symbols for the new isotope9. Perhaps the two heavy hydrogen isotopes had more unsolicited “public suggestions” than any element on the periodic table—at least until 2016 when traditional and social media increased proposals for the “last” 4 elements exponentially. As we noted in the tritium IYE article, the discovery by Rutherford’s research group of hydrogen-3 in early 1934 exacerbated the controversy11. Rutherford now had a second reason to add his voice to hydrogen isotope nomenclature clamour—why should Rutherford and his co-workers accept Urey’s “tritium” name for their discovery? But though Urey’s preemptive “tritium” proposal clearly agitated Rutherford, none of his early papers on hydrogen-3 suggested any alternative name. Publicly, Rutherford kept quiet on hydrogen-3’s name for some time, but ultimately he coveted the opportunity to name it.
Rutherford also convened a “Discussion on Heavy Hydrogen” at the Royal Society in 1934 to find the “best” names for the hydrogen isotopes. Although the assembled British scientists generally liked and sided with Rutherford’s choices, especially “diplogen”, they were quite aware of the difficulties in dislodging Urey’s proposed names. They also were clear about the basic importance of naming the heavy hydrogen isotopes. N.V. Sidgwick explained the importance of naming hydrogen-2 thus:
“You must remember that this is not like the discovery of a rare earth. It does not much matter to any of us what we call No. 61 because we rarely have occasion to refer to a substance like that; but the isotope of hydrogen is going to be one of the most important substances in the coming development of chemistry.”6 Pity poor promethium! They went so far as to suggest “Pr” as the symbol for hydrogen-1, protium, because, after all, who needs praseodymium either?
Rutherford refused to concede defeat on his preferred -gen suffixed names. Despite chemical societies in America and Britain endorsing deuterium, the nomenclature debate continued unabated through much of 1934. Urey tired of the deuterium nomenclature fight, as well as the intrusions of Lewis and Rutherford, so he submitted a paper using Rutherford’s diplogen, only to earn a rejection based on the aberrant nomenclature. Urey wrote a letter to Rutherford on 6 August 1934, politely asking if Ruther-ford would reconsider deuterium, since his attempts to use diplogen had been rebuffed because “deuterium has been used by many more people.” Rutherford relented on deuterium, while also seeking a euphonious name for hydrogen-3, which wasn’t Urey’s tritium. “Triterium”, which matched syllables of deuterium, seemed the obvious choice12, but as we recounted in the tritium In Your Element article (link to IYE), Rutherford’s quest to name the isotope he discovered ended in frustration.
Urey’s last word on deuterium to Rutherford: “I am writing, therefore, to ask whether you and your group would not consider making the change to hydrogen and deuterium (I wish that the names rhymed)”9. As longtime supporters of proper suffixes in chemistry, so do we, Harold, so do we10,13.
Brett F. Thornton is in the Department of Geological Sciences (IGV) and Bolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden.
Shawn C. Burdette is in the Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609-2280, USA.
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