Protactinium discovery

The isolation and identification of 4 radioactive elements in minute amounts took place at the turn of the century, and in each case the insight provided by the periodic classification into the predicted chemical properties of these elements proved invaluable. Marie Curie identified polonium in 1898 and, later in the same year working with Pierre Curie, isolated radium. Actinium followed in 1899 (A. Debierne) and the heaviest noble gas, radon, in 1900 (F. E. Dorn). Details will be found in later chapters which also recount the discoveries made in the present century of protactinium (O. Hahn and Lise Meitner, 1917), hafnium (D. Coster and G. von Hevesey, 1923), rhenium (W. Noddack, Ida Tacke and O. Berg, 1925), technetium (C. Perrier and E. Segre, 1937), francium (Marguerite Percy, 1939) and promethium (J. A. Marinsky, L. E. Glendenin and C. D. Coryell, 1945). 

It was first identified and named brevium, meaning brief, by Kasimir Fajans and O. H. Gohring in 1913 because of its extremely short half-life. In 1918 Otto Hahn (1879—1968) and Lise Meitner (1878-1968) independently discovered a new radioactive element that decayed from uranium into (actinium). Other researchers named it uranium X2. It was not until 1918 that researchers were able to identify independently more of the elements properties and declare it as the new element 91 that was then named protactinium. This is another case in which several researchers may have discovered the same element. Some references continue to give credit for protactinium s discovery to Frederich Soddy (1877—1956) and John A. Cranston (dates unknown), as well as to Hahn and Meitner. 

The first actinide metals to be prepared were those of the three members of the actinide series present in nature in macro amounts, namely, thorium (Th), protactinium (Pa), and uranium (U). Until the discovery of neptunium (Np) and plutonium (Pu) and the subsequent manufacture of milligram amounts of these metals during the hectic World War II years (i.e., the early 1940s), no other actinide element was known. The demand for Pu metal for military purposes resulted in rapid development of preparative methods and considerable study of the chemical and physical properties of the other actinide metals in order to obtain basic knowledge of these unusual metallic elements. 

The probable existence of protactinium was predicted as early as 1871 by Mendeleev to fill up the space on his peiiodic table between thorium (at, no. 90) and uranium (at. no, 92). He termed the unconfirmed element ekatantalum. In 1926, O. Hahn predicted the properties of the element in considerable detail, including descriptions of its compounds. In 1930, Aristid v. Grosse isolated 2 milligrams of what then was termed ekatantalum pentoxide and showed that element 91 differed m all reactions with comparable amounts of tantalum compounds with exception of precipitation by NH3. However, credit for the discovery of protactinium generally is attributed to Lise Meitner and Otto Hahn in 1917,... 

O. Hahn and L. Meitner, Die Muttersub-stanz des Actiniums, Physikalische Zeit-schrift 19 (1918) 208-18 R. L. Sime, The Discovery of Protactinium, Journal of Chemical Education 63 (1986) 653-57. 

At the time, Soddy was seeking evidence that lead from thorium ores had different atomic weights from normal lead. When Soddy announced the discovery of a sample of lead of atomic mass 207.74, he acknowledged the contribution of Hitchins for the separation and analysis work. Thus, Hitchins precise and accurate measurements on the atomic masses of lead from different sources were among the first evidence for the existence of isotopes.51 In addition, Hitchins took over the research on protactinium from Cranston when the latter was drafted for the First World War. 

Meitner and Hahn found protactinium while searching through the products of a nuclear reaction that had only recently been discovered. In fact, the ability of Hahn and Meitner to unravel the nature of that reaction proved to be even more important than the discovery of protactinium. 

Twenty-nine isotopes of protactinium with measured half lives are known. All are radioactive. (A more detailed explanation of protactinium isotopes can be found in the Discovery and Naming section.)... 

Hahn began a 30-year collaboration with Dr. Lise Meitner, who came to Berlin from Vienna. They worked on investigations on beta rays, discovered protactinium, and Hahn discovered the fission of uranium and thorium. In 1944 he was awarded the Nobel Prize in chemistry for his discovery of the fission of heavy nuclei. ... 

He was one of the first to conclude in 1912 that some elements can exist in forms that are chemically identical, and save only as regards the relatively few physical properties which depend on atomic mass directly, physically identical also. He called them isotopes. Later he promoted their use in determining geologic age. He is credited (with others) with the discovery of the element protactinium in 1917. 

All these new discoveries, of course, verified Seaborg s theory, and the transuranium elements, along with thorium, protactinium and uranium, are now called the actinide elements. They all fit in the Periodic Table between actinium and the element eka-hafnium. Eka-hafnium is the tentative name given to the undiscovered element with the atomic number 104 which lies directly below hafnium in the Periodic Table and which is expected to have chemical properties similar to those of hafnium. 

After the discovery of uranium radioactivity by Henri Becquerel in 1896, uranium ores were used primarily as a source of radioactive decay products such as Ra. With the discovery of nuclear fission by Otto Hahn and Fritz Strassman in 1938, uranium became extremely important as a source of nuclear energy. Hahn and Strassman made the experimental discovery Lise Meitner and Otto Frisch provided the theoretical explanation. Enrichment of the spontaneous fissioning isotope U in uranium targets led to the development of the atomic bomb, and subsequently to the production of nuclear-generated electrical power. There are considerable amounts of uranium in nuclear waste throughout the world, see also Actinium Berkelium Einsteinium Fermium Lawrencium Mendelevium Neptunium Nobelium Plutonium Protactinium Rutherfordium Thorium. 

U/Ac ratio was found to be constant, but the amount of actinium present was nevertheless less than would be expected if it were a direct disintegration product of uranium. This was the reason for assuming it to lie in a separate chain. By the Group Displacement Law protactinium should belong to Group v and thus resemble tantalum. It was this consideration that led to its discovery. 

Element 43 technetium (Tc, discovered 1939) 61, promethium (Pm, 1945) 75, rhenium (Re, 1925) 85, astatine (At, 1940) 87, francium (Fr, 1939). Lewis book was quite up-to-date— Hafnium (Hf) was discovered in 1923, the year White Lightning was published, and one can imagine the author happily updating the title of Chapter 72 in the galley proofs. Chapter 86 is titled Niton (now Radon) Chapter 91 is titled Brevium (now Protactinium). For a brief table on the discovery of the chemical elements, see A.J. Ihde, The Development of Modern Chemistry, Harper Row, New York, 1964, pp. 747-749. 

The work was inteniipted by World War I. Hahn was conscripted and assigned to the chemical warfare unit, but Meitner continued the work on her own. Letters written by Meitner to Hahn from this period have been preserved and indicate a superficially formal relationship (Meitner always used a formal title when addressing Hahn), but under this was an easygoing exchange. From the letters we can surmise that Hahn was anxious for results, and when the war drew to a close, Meitner had them ready. In 1918 they published the discovery of protoactinium (meaning before actinium), though the name was shortened to protactinium in 1949-... 

We have already discussed the history of discovery of two natural radioactive elements, that is, uranium and thorium, in Chapter 4. These elements can fairly easily be found in minerals with chemical analysis since their content is sufficiently high. Other natural radioactive elements (polonium, radon, radium, actinium, and protactinium) are among the least abundant elements on Earth. Moreover, they exist in nature only because they are the products of radioactive transformations of uranium and thorium. 

Now let us turn back to the last decades of the 19th century. When Mendeleev developed the periodic system of elements it contained many empty slots corresponding to unknown elements between bismuth and uranium. These empty slots were rapidly filled after the discovery of radioactivity. Polonium, radium, radon, actinium, and finally protactinium took their places between uranium and thorium. Only eka-iodine and eka-cesium were late. This fact, however, did not particularly trouble scientists. These unknown elements had to be radioactive since there was not even a hint of doubt that radioactivity was the common feature of elements heavier than bismuth. Therefore, sooner or later radiometric methods would demonstrate the existence of elements 85 and 87. 

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