Protactinium’s Discovery Redux: Kasimir Fajans and Oswald Göhring discovered element 91 in 1913

The great Periodic Table of Videos series on YouTube a few weeks ago published a video on element 91: protactinium. Protactinium is an element that is (a) obscure and rare (b) has a confusing discovery story, and (c) had linguistic issues with its name. Given our interest in discovery, isotopes and nomenclature, we found a few details in the video that should be revisited. For an element such as protactinium, which has few applications today, an element’s entire “story” concerns how it was discovered.

The Periodic Table of Videos series is exceptional, but things go slightly awry beginning around 3:00 in the protactinium video with the still frame of Frederik Soddy & John Cranston. These two British scientists, along with Lise Meitner & Otto Hahn of Germany, are credited with the discovery of protactinium. That’s incorrect: Meitner & Hahn published their successful precipitation of element 91 in 1918, but this was not the discovery of the element—it wasn’t even the first precipitation of the element. The independent work of Soddy & Cranston, also published in 1918, also was not the true discovery.

Starting around 4:00, the professor explains that the isotope ²³⁸Pa, which has a 32,760 year half-life, was discovered by Meitner & Hahn and “a few years earlier” Kasimir Fajans discovered ²³⁴Pa. ²³⁴Pa has a much shorter half-life and was named “brevium” (Fajans’ co-discoverer Oswald Göhring is not mentioned).

Shouldn’t Fajans & Göhring be the credited as the discoverers of element 91? They beat Meitner & Hahn and Soddy & Cranston by five years! In 1913, Fajans & Göhring had three separate publications on element 91 including the precipitation of ²³⁴Pa (K. Fajans & O. Göhring, Phyik. Z., 14, 877, 1913). This work is discussed explaining “[Fajans'] was the first discovery of protactinium, but it turns out that there is a custom if several different isotopes of an element are discovered, whoever discovers the longest lived isotope, wins, and they’re deemed the discoverer.” Wait, what??? That sounds like redefining the word “discovery”!

What is explained in the video is only half the story. The other half is that Kasimir Fajans pushed the idea that the elements should be given the name of the longest lived isotope (in his 1919 book Radioaktivität und die Neueste Entwicklung der Lehre von den Chemischen Elementen). In the 1910s, isotopes were a very new idea and it was still unclear whether or not they should be treated as separate elements. Meitner & Hahn’s 1918 paper clearly claims a new “element” even though they knew they knew that their “element” had the same atomic number as Fajans & Görhing’s “element” discovered 5 years earlier. Meitner & Hahn were using the definition of the word “element” as was common in 1918 when it was still unclear how to classify isotopes. When chemists say “element” today, they clearly mean all isotopes with the same atomic number. Fajans never intended to give away the credit for element 91’s discovery to Hahn & Meitner or Soddy & Cranston. In fact, one of Fajans’ last papers defends his discovery priority.

There is one other element that was renamed based on its longest-lived isotope: radon. Radon’s discoverers wanted to call the element emanation. For decades, “radon” was only the name of the longest-lived isotope of emanation. After the name was changed, history began to forget who had actually discovered radon. Just like protactinium, people preferred to give credit to the individual or group who had discovered and named the longest-lived isotope of the element. We wrote quite a bit about the linguistic twists of radon last year in Nature Chemistry. In an accompanying blog post, we explored protactinium’s naming and discovery confusion. There is also an excellent Nature: In Your Element piece that delves into the protactinium story.

Is there is a lesson here for chemists? If you get to name something, you might get credit for discovering it, even if you didn't. That can happen both inside and outside of chemistry, which might be a more than a little disconcerting.

It’s incorrect to say that “the discover of the stablest isotope gets to name the element” as a general rule, especially today. Since most superheavy elements are created as highly unstable, neutron-poor isotopes first, only later are (slightly) more stable isotopes created. No one is ceding naming or discovery priority to anyone who years later makes the most stable version element 112 for example. The Periodic Table of Videos project is great and everyone should follow their work, but examine the protactinium discovery a little more closely to get the complete story.

- Brett F. Thornton with editorial assistance by Shawn