By Ron B. Davis Jr., Georgetown University
The primordial nature of uranium and thorium led to their discovery much earlier than any of the remaining actinide elements. The name of the radioactive element, uranium, was originally given to a uranium oxide compound, isolated in 1789 by the German chemist, Martin Klaproth. Thorium, however, was discovered in its oxide form a generation later, in 1828, by Jons Jacob Berzelius.
Thorium
Earlier on, some elements were mistakenly identified as thorium prior to the acceptance that transmutation accompanied many forms of radiation. In a few instances, before the discovery of neutrons and the isotopes they create, thorium itself was also mistaken for a new element. This happened in 1908, when it was determined that element 88, radium, forms from the alpha decay of a then-unknown larger nucleus.
The unknown parent atom for radium was hastily named ionium by its creators. Only after discovery of the neutron, two decades later, was it realized that so-called ‘ionium’ was simply a second, far-less-common isotope of thorium; thorium-230. This isotope has a half-life of 75,000 years and is itself produced by the nuclear decay of the long-lived uranium-238.
Thorium-230
What makes ionium’s story special is that although ionium ultimately proved not to be a new element, its name for the isotope thorium-230 stuck. That’s in part because of ionium’s special uses in radioactive dating. Thorium, unlike uranium, is not soluble in water. So, when dissolved uranium-238 decays by alpha emission, uranium stays in the water, but the resulting thorium-230 precipitates out of solution and becomes part of the sediment.
Being deposited in sediment allows the much shorter half-life of ionium to be useful. By measuring the ratio of ionium to ordinary thorium, scientists have a useful way to date ocean sediments. So even today, thorium-230 is often referred to as ionium, to distinguish it from primordial thorium-232.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Emitting Radiation
Klaproth had isolated his uranium compound from the mineral pitchblende. By the late 1800s, it had already been discovered that uranium samples had the curious property of emitting a type of radiation that could expose photographic plates. French physicist, Henri Becquerel, had theorized that uranium, much like other phosphorescent materials he had been studying, had the ability to absorb sunlight and convert it into a much higher-energy form of light, which was re-emitted from the uranium atoms.
He tested his idea by placing uranium samples into sunlight, then exposing photographic paper to the uranium, finding that an image was produced. So far, so good. But one cloudy day in 1896, Becquerel decided to forgo his experiment for the day and instead placed his experiment in a dark desk drawer.
To his surprise, when he developed the photographic paper, he again found a distinct image, just as he had in the samples exposed to sunlight. His observation became an important part of his scientific inquiry—the control experiment.
By repeating his test identically in every way but one, removing the sunlight he thought was the source of uranium’s radiation, disproved his own hypothesis that uranium’s high-energy emissions were merely re-emitted light derived from an external source. The only explanation now was that the radiation originates within the uranium atoms themselves.
Marie Curie
The discovery of radiation in uranium led almost immediately to the discovery of many other radioactive elements as well.
As the radioactivity of uranium came into better focus, Marie Curie noted that, the radiation coming from samples of the mineral pitchblende was too intense to be accounted for only by the uranium it was known to contain. She concluded that there must be additional radioactive elements hiding within the pitchblende mineral, which had been used as a source of uranium for over a century. She was right.
Isolating New Radioactive Elements
Marie Curie’s astute observation set in motion many further attempts to isolate new radioactive elements that appeared to be hiding in pitchblende.
These efforts led to the discovery of more intensely radioactive elements with atoms smaller than the actinide series—polonium and radium in 1898, and radon in 1900. However, these efforts also led to the discovery of elements from within the actinoid series, starting with actinium—in 1899, by Andre Debierne, one of Marie Curies’ proteges. This was followed by so-called ‘Proto-actinium’ in 1913.
Andre Debierne
Debierne’s work is especially impressive, as he successfully isolated the only known naturally occurring actinium isotope—actinium-227. And it wasn’t easy!
Pitchblende itself is about 85% uranium by mass, but this isotope of actinium forms as a decay product of uranium-235, which itself makes up only about 0.7% of all uranium atoms in nature. With a half-life of just 21.7 years, actinium-227 is also fairly quickly consumed as the decay chain continues on its way to lead-207.
Because of the low abundance of its parent nucleus uranium-235, and the short half-life of actinium itself, one ton of pitchblende is expected to contain only about 150 milligrams of actinium.
Element 91
Element 91 first went by another name when it was discovered in 1913. Its discoverers found it as an intermediate in the uranium-238 decay chain, and suggested the name ‘brevium’ in recognition of its 1.17 minute half-life. Over the next several years, the name brevium and the elemental symbol ‘Bv’ were widely adopted and actually published in more than a few tables.
Just a few years later, in 1917, Lise Meitner uncovered the much more stable, longer-lived isotope of element 93, that precedes actinium in the uranium-235’s decay chain.
The isotope that would have been called ‘brevium-231’ actually has a much longer half-life of over ten thousand years. Since this was not so ‘brief,’ and indeed it was much longer-lived than any isotope of actinium, Meitner and collaborator Otto Hahn published a paper suggesting another name.
Proto-Actinium
They named it ‘proto-actinium’, to recognize its position prior to actinium in the same U-238 decay chain. Ultimately, the name was shortened to protactinium, which is the name still in use today.
Thereafter, proto-actinium was quickly adopted as the preferred name of element 91, as can be seen in a periodic table authored by HG Deming and published in the USA by Wiley in 1923. It uses our current elemental symbol ‘Pa’ but still reports only the larger atomic mass of its most stable isotope, 234.
Common Questions about Discovering New Radioactive Elements
By measuring the ratio of ionium to ordinary thorium, scientists have a useful way to date ocean sediments.
Marie Curie’s efforts led to the discovery of more intensely radioactive elements with atoms smaller than the actinide series—polonium and radium in 1898, and radon in 1900.
Andre Debierne’s work is especially impressive, as he successfully isolated the only known naturally occurring actinium isotope—actinium-227.
