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Vol. 33 No. 1
January-February 2011
 
Chemistry after the Discoveries of Polonium and Radium

by Robert Guillaumont and Bernd Grambow

The experimental chemistry of elements, substances which cannot be decomposed and which combine in fixed ratios, was developed by Antoine Lavoisier. Around 1805–1808, following John Dalton’s work, a basic scientific concept emerged which held that each chemical element was ultimately composed of hard, solid particles (atoms) of specific, invariable mass (atomic weight), and that all substances were composed of such atoms. The atoms were too small to measure their weight directly, but relative atomic weights could be determined starting with hydrogen as the lightest one. However, the theory of atomism in chemistry was accepted with difficulty.

Significant advances were achieved by Dmitri Mendeleev in 1869 and Julius Lothar Meyer in 1870 in ranking the nearly 60 known chemical elements according to a periodic law, linking relative atomic weights of the elements to their chemical properties. Mendeleev developed a chart showing that homologue elements have large differences in atomic weights and different elements of similar atomic weight exhibit large differences in properties. With a limited number of empty places in the chart, Mendeleev predicted the existence of yet-undiscovered elements, such as eka-aluminium and eka-silicium, and their expected properties. A final proof of the validity of the Mendeleev concept was the discovery of the elements gallium in 1875, scandium in 1879, and germanium in 1886. In 1895, 80 elements already had been identified (see figure below). Still, this classification was purely empirical.

Until this point in late 1895, chemistry was still much less developed than physics despite the existence of a chemical industry (acids, bases, salts, glasses, metallurgy, colorants, pharmacy, and perfumery), rapidly expanding chemical knowledge, and chemical theories for certain fields. However, unifying and generally accepted chemical concepts were still missing.

Periodic system at about 1895. All lanthanides were known except Pm (radioactive) and Lu discovered in 1907 (only La could be presented). In yellow are the missing nonradioactive elements. Discovered were Ge in 1896, Ne, Kr and Xe in 1898, Hf and Re in 1923 and 1925. In red are the missing radioelements with mass lower than uranium.

The Search for New Natural Elements through Atomic Properties
It was against this backdrop that in 1897 Marie Skłodowska Curie started her thesis on the origin and properties of “uranic rays” discovered by Becquerel. Curie promptly showed, by careful and systematic quantitative measurement, that the radiation intensity (linked to radioactivity) of many chemical compounds was proportional to the quantity of uranium in the compound. She was surprised that certain natural, uranium-containing minerals such as pitchblende, chalcolite, and autunite were much more radioactive than the metallic uranium freshly prepared by Henri Moissan. If chalcolite was synthesized in the laboratory from pure uranium compounds, no such enhanced radioactivity was encountered. This led Marie Curie to search in these natural minerals for a small quantity of another yet-unknown element, the source of these stronger intensity rays (see excerpt below). She invented a new “radiochemical” method combining ordinary chemical analyses with the measurement of radioactivity.

Separating Uranium from Ores
In non-pertubated uranium ores, 238U and 235U are in secular equilibrium with their 23 main daughters (alpha or beta emitters) with the total activity being 178 kBq/g of uranium. Only five of them give easily detectable gamma rays. When U is separated from ores by chemical processes, the remaining activity is 25 kBq/g of the original activity content. Due to the ingrowth of the two short-lived daughters of 238U, it needs around one year for the activity to reach the limiting value of 51 kBq/g. The emission of gamma rays increases progressively. In Marie Curie’s co-precipitation experiments, the amounts of Po and Ra were around 70 ng/kg and 300 μg/kg of uranium, respectively. In Otto Hahn’s co-precipitation experiments of Ra, the amount of 228Ra was around 400 ng/kg of thorium.

One substance she identified, polonium, had properties similar to bismuth. In 1898, Pierre and Marie Curie couldn’t isolate a sufficiently large quantity of polonium to measure its atomic weight or to obtain the spectral signature. Today, we know that only about 6 nanograms were isolated, beyond any method of measurability available at the time; however, measuring its “radioactivity” was feasible. Pierre and Marie Curie didn’t immediately try to place polonium in the Mendeleev system. Since its behavior was similar to that of bismuth, they may have felt compelled, according to this system, to look for an eka-bismuth, but this element would have been heavier than uranium. It was not until 1906 that the chemical similarity of polonium and tellurium was identified, giving polonium its place close to bismuth in the periodic system. In 1910, a weighable quantity of about 100 micrograms of polonium was concentrated in few milligrams of bismuth.

The other substance Marie Curie identified was radium, which had chemical properties similar to barium. Spectral analyses by Eugène Demarçay of isolated “pure radium” salts confirmed the hypothesis that radium was a new chemical element. Gravimetrically, Marie Curie initially obtained an atomic weight of 225; in 1907 she obtained a weight of 225.9, close to the correct value of 226.

The position of radium in the periodic system was easily determined by the Curies. Indeed, radium is the higher homologue of barium in the family of alkaline-earth metals and it could easily be entered into Mendeleev’s chart in the corresponding column.

Since 1899, many chemists have tried to isolate new radioactive elements from uranium- or thorium-containing compounds using the separation techniques of Marie Curie. They were frequently surprised by the “emanations” and “active deposits.” In 1910, 44 “radioactive elements” were identified. For example, one could clearly distinguish three “radioactivities” associated with three supposedly new elements (called at the time mesothorium I, actinium X, and thorium X) which all had the chemical properties of radium. The question was how to classify them in the periodic system? Only 12 spaces where left empty in the table. Frederick Soddy found the solution in calling these “elements” isotopes, which had all the same chemical properties and the same place in the periodic system, but differing in their the radioactive half life. Nevertheless, it took until 1935 until the complexity of radioactive decay chains was really understood.

The Way to a Unifying Concept for Chemistry
Ernest Rutherford, as well as Hans Geiger and Ernest Marsden, used radium as a powerful source of alpha particles to probe the inner structure of the atom by directing the beam of particles onto a thin foil of gold. This scattering experiment lead to the surprising result that most of the atomic mass was concentrated in a very small nucleus about 10 000 times smaller than the atom. It showed that atomic weight and nuclear charge are related. This key observation allowed Rutherford, in 1911, to develop a new atomic model of a positive nuclei with a charge roughly proportional to atomic mass. This nuclei, he theorized, was surrounded by electrons moving around it in a yet unspecified way. This model, in turn, was rapidly improved upon with the concept of atomic number (de Boer 1911; Mosley 1913) and by Niels Bohr’s introduction, in 1913, of “energetic quanta,” which placed the electrons in a definite orbit around the nucleus. The path was now opened to understanding periodicity and chemical bonding, such as in the work of Walther Ludwig Julius Kossel in 1916. A new unifying concept for chemistry had formed, but it would hardly have been possible if Marie Curie had not isolated radium. Hence, polonium and radium are not only the cornerstones of the science of radioactivity as Marie Curie suggested in her Nobel lecture in 1911, but they are cornerstones for modern chemistry as a whole.

Moving beyond Naturally Occurring Radioelements
The use of alpha particles as projectiles not only helped scientists probe the atoms inner structure, but it led directly to a number of new discoveries. For example, in 1934 Irène Joliot-Curie and Frédéric Joliot used very intense radioactive alpha emitters such as polonium, much stronger than radium, to discover the first artificial radionuclide: radioactive phosphorus. In irradiating a foil of aluminium of mass of 27 by a source of 80 millicuries of Po, they observed the emission of neutrons and of positive electrons; the later were emitted in a delayed fashion because of the irradiation exposure event. Only phosphorus 30 could have been formed, which must have been radioactive by positron emission. It was the separation and identification of phosphorus 30 as phosphine, which provided the first chemical proof that a transmutation by a nuclear reaction had occurred producing a new type of radioactivity.

This discovery by Joliot-Curie of artificial radioactive matter motivated many chemists to look for new radioisotopes. They irradiated light elements with alpha particles and the more heavy elements with neutrons. It took only three years to discover about 200 new radionuclides. New chemical elements were also artificially produced. For example, technetium was produced in 1937 by Casimir Perrier and Emilio Segré, who bombarded molybdenum with deuterons and isolated an irradiation product with chemical properties similar to rhenium.

The procurement of radioisotopes for a large suite of chemical elements with periods ranging from a fraction of a second to several years has enabled their use in areas as diverse as chemistry, geosciences, material science, biology, medicine, industry, and agriculture. Radiochemistry has become a new tool for studying chemical reaction mechanisms in all these fields.

It was soon recognized that the neutron transmuted one atom of mass A into a new atom of mass A + 1, which, by beta emission, decayed to an atom with atomic number Z+1, thereby becoming the element next to the irradiated one in the periodic table. So, it was the logical next step to irradiate uranium with neutrons to search for new elements even heavier than uranium. The pursuit of these “transuranic elements” quickly led to a riddle. The best radiochemists were unsure how to analyze the chemical behavior of the “new radioactivities” they encountered in light of their supposed homologous elements such as rhenium, osmium, or platinum, or of heavy elements such as radium, which might have originated from decay of the supposed transuranic elements. Ida Noddack, Irène Curie, and Pavel Savich (1938) found products with the properties of lanthanum, but they did not believe in the presence of a radioactive lanthanum.

A crucial experiment was conducted by Otto Hahn, Lise Meitner and Fritz Strassman in 1938–1939 in which they tested the hypothesis that radium was the radioactive irradiation product coming from the decay of a supposed transuranic element. Proceeding by co-precipitation with barium, it was impossible to increase the activity of the precipitate, i.e. to enrich it in radium. Was this because the “hypothetical radium” was an imponderable quantity? (see excerpt). The answer was no (supplementary experiences showed that an imponderable quantity of radium 228 could easily be enriched in a precipitate with barium; the laws of co-precipitation were independent of concentration). One had to conclude that the activity measured in the precipitate was indeed radioactive barium and this could only be explained by the hypothesis that the uranium nucleus could break upon neutron irradiation. The fission of uranium had been discovered. Meitner’s rapid calculation showed a gain of about 200 MeV from this nuclear reaction, sufficient energy to change the fate of humanity. From there it all became clear. The neutrons irradiating uranium produced barium and lanthanum. The identification of hundreds of radionuclides, isotopes of 30 chemical elements formed in the fission process of uranium 235, was a Herculean accomplishment for radiochemists.

Going beyond Uranium
Even though early attempts failed to produce “transuranic elements” by the neutron irradiation of uranium due to the predominance of fission, the initially intended nuclear reaction did occur, although with a probability about 15 times less, too small to be identified in the background of fission. However, careful neutron irradiation of a thin foil of uranium allowed the breakthrough. All fission products should have escaped the foil due to their extremely high recoil energy. However, a newly produced radioactive substance did not escape the thin foil. This was indeed the long-searched-for proof of a series of new elements heavier than uranium. This new chemical element, discovered by Edwin McMillan and Philip Abelson in 1939–1940, was named neptunium. It behaved like uranium and was not homologous to rhenium, which was expected. It was the first evidence of a new family of elements. The decay product is plutonium of mass 239, also a fissile material and much more simple to separate from uranium than uranium 235. It was initially difficult to find its place in the periodic table. The modern version of this table contains the actinides and the lanthanides. The periodic table now has 118 elements (see figure below). The search for new chemical elements still continues.

Periodic table showing radioelements and artificial elements (fission products). Blue symbols (like Po) are naturally occurring radioelements. Red symbols are man made radioelements. Light blue boxes indicate fission products (artificial elements with special isotopic composition) and green boxes indicate actinides found in spent nuclear fuel (over 50 g/metric ton), the most radioactive material that exists today.

Radiochemistry Becomes Part of Chemistry
Since Marie Curie’s discoveries, a new branch of chemistry dealing with the chemical properties of radioactive matter has progressively emerged. Such matter is in perpetual renewal due to the radioactive decay of radionuclides and the emission of ionizing radiation. Radiochemistry is based on its own methodology. It allows scientists to look at many processes beyond the scope of chemistry and it has become a key discipline for understanding actinide behavior—so important in nuclear industry and environmental science. In this regard, we know how to extract plutonium, a fissile material, from spent nuclear fuel. However, we have yet to find an ultimate solution for isolating the radioactive waste associated with this endeavor.

Conclusions
The era of radioactivity and radiochemistry, which started between 1896 and 1898, led to discoveries that have profoundly influenced chemistry. Until 1915, only a few teams of researchers—in Paris, Cambridge, Berlin, Vienna, and Montreal—had worked with radioactive material. The isolation of radium and polonium allowed these teams to probe the structure of the atom, and from this a unified concept of chemistry emerged. From that point forward, chemists have used the properties of radionuclides to understand chemical reactions and transport mechanisms in all areas of the science. The chemical knowledge gained from radiochemistry was decisive in many fundamental discoveries: radioactivity as an atomic property, artificial radionuclides, the completion of the periodic table, nuclear fission, and transuranic elements. Today, radioactive matter is used by radiochemists for fundamental research in many fields, especially medicine and energy.

The discovery of polonium and radium and the course of chemistry and society would have been different were it not for the extraordinary patience, determination, and curiosity of Marie Curie as she searched for the origin of the strong radiation from uranium compounds. Her unwavering believe in the hypothesis of radioactivity as an atomic property and her spirit of adventure and readiness to pursue unorthodox thinking, changed the course of history.

Robert Guillaumont is an honorary professor of chemistry (University of Paris-Sud, Orsay) and a member of the French Academy of Sciences. His research field in radiochemistry focused mainly on tracer scale chemistry and on thermodynamics of actinide chemistry. He is a member of several committees on radioactive waste management.

Bernd Grambow is a professor of radiochemistry and head of Subatech Laboratory, a mixed research unit of the Ecole des Mines, the university, and the IN2P3/CNRS in Nantes. He obtained his Ph.D. in chemistry at the Free University of Berlin. Principal research interests are in the thermodynamics and kinetics of chemical reactions involving radionuclides and in the radiochemistry of nuclear waste disposal.

See the References section for works cited in this article.


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