Protactinium: Origins, Oxidation States, and the Discovery Saga

In this Chemistry World episode, protactinium is examined for its unusual origin and position in the periodic table, its four oxidation states, and the story of its discovery. The discussion weaves together Mendeleev's predictions, the shift from atomic weight to atomic number as the ordering principle, and the ore pitchblende where protactinium occurs with uranium. The host and guest highlight the isotope Pa-231 and the early isotope work that shaped the element’s naming, alongside practical notes on its scarcity and lack of commercial use.

• Protactinium sits between thorium and uranium, with atomic number guiding its placement despite a challenging weight ordering.
• The element shows a tantalum-like highest oxide Pa2O5 and a strong four oxidation state chemistry, paralleling thorium and uranium in some aspects.
• Early radioactive discoveries involved Uranium X, Uranium X1 and X2, leading to the isotope Pa-234 and the short-lived brevium naming, then Pa-231 and protoactinium naming by Meitner and Soddy.
• Protactinium occurs in pitchblende and remains highly radioactive and toxic with scientific interest, including paleoceanographic applications using Pa-231 to Th-230 ratios in ocean sediments.

Overview: Protactinium and the historical context

The podcast opens by presenting protactinium as an element whose origin and placement in the periodic table have complicated the early history of the table itself. The discussion centers on how Dmitry Mendeleev, in the 1870s, predicted an element with an atomic weight near 235 that would form a highest oxide X2O5, and how Protactinium later emerged near this position in the table. The speaker explains that when atomic number is used as the ordering principle, the so called pair reversals disappear, and Protactinium should be placed after thorium even though its atomic weight is lower. This sets the stage for understanding protactinium as a key piece in the development of modern periodic table organization.

Between thorium and uranium: the ordering problem resolved by atomic number

The narrative then delves into the idea that early chemists relied on atomic weights to order elements, which caused a handful of reversals. The discovery of atomic numbers clarified the ordering, and Protactinium’s placement is consistent with its higher atomic number despite its lighter weight relative to thorium. The episode emphasizes the horizontal and vertical analogies Protactinium shows with tantalum and uranium in its oxidation state behavior, highlighting how the element can resemble several neighbors in the table depending on the property considered.

Oxidation states and chemical analogies

The program notes Protactinium’s four oxidation states, a feature that Mendeleev did not anticipate, and discusses the analogy with tantalum in forming a Pa2O5 oxide, which points to structural similarities with group 5 elements. The horizontal analogy with thorium and uranium is also discussed, showing Protactinium’s capacity to engage in a high oxidation state similar to its neighbors, a detail that adds depth to its chemical portrait.

Occurrence in pitchblende and discovery timelines

The host describes Protactinium as occurring in pitchblende together with uranium, a fact Mendeleev correctly predicted. The element’s discovery occurred in the 20th century, with complexity arising from what counts as discovery: mineral identification versus actual isolation of a new element. The podcast outlines the complication by recounting early observations of a radioactive substance in uranium residues and the interpretive steps that led later researchers to recognize Protactinium as a distinct element rather than a mere decay product.

Uranium X, isotopes, and the naming saga

A key portion of the narrative covers the Crookes uranium X idea, which later split into Uranium X1 and Uranium X2. The X2 component was first isolated by Kazimir Fajans in 1913 and was a very short lived isotope of Protactinium, Pa-234, with a half-life of just over a minute. In 1917, Lise Meitner isolated a more stable isotope Protactinium-231, with a half-life of about 33 000 years. Meitner chose the name protoactinium, later abbreviated to protactinium, to reflect its status as the progenitor of actinium (element 89) that forms when Protactinium decays via alpha decay. Independently, Frederick Soddy and John Cranston in Glasgow isolated the same isotope and, in the process, helped coin the term isotope. The discussion also notes the complication that historically Pa-231 is the longest lived isotope, which guided the naming decision at the time.

Modern significance and scarcity

The episode closes with a candid note on Protactinium’s hazardous nature and lack of commercial applications. Nevertheless, Protactinium has scientific utility, including the measurement of Pa-231 to Th-230 ratios in ocean sediments that enable reconstructing historical movements of North Atlantic water during the last Ice Age melting. The British Atomic Energy Authority’s 1961 achievement in producing a substantial amount of Protactinium from treated waste demonstrates the element’s limited, niche role in research contexts and its ongoing appeal for isotope geochemistry and fundamental chemistry studies. The podcast ends with a reminder of Protactinium’s status as a highly radioactive, toxic element worth studying for its historical and scientific significance.