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Home , Edwin McMillan, Polonium, Radiochemistry, Radium

after the Discoveries of and

by Robert Guillaumont and 1875, scandium in 1879, and germanium in 1886. In 1895, Bernd Grambow 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 he experimental chemistry of elements, sub- less developed than physics despite the existence of a stances which cannot be decomposed and chemical industry (, bases, , glasses, - Twhich combine in fixed ratios, was developed by lurgy, colorants, pharmacy, and perfumery), rapidly Antoine Lavoisier. Around 1805–1808, following John expanding chemical knowledge, and chemical theo- ’s work, a basic scientific concept emerged ries for certain fields. However, unifying and generally which held that each was ultimately accepted chemical concepts were still missing. composed of hard, particles () of specific, invariable mass (atomic weight), and that all sub- The Search for New Natural stances were composed of such atoms. The atoms Elements through Atomic Properties were too small to measure their weight directly, but relative atomic weights could be determined starting It was against this backdrop that in 1897 Marie with as the lightest one. However, the theory Skłodowska started her thesis on the origin and of atomism in chemistry was accepted with difficulty. properties of “uranic rays” discovered by . Curie promptly showed, by careful and systematic Significant advances were achieved by Dmitri quantitative measurement, that the intensity Mendeleev in 1869 and Julius Lothar Meyer in 1870 (linked to radioactivity) of many chemical compounds in ranking the nearly 60 known chemical elements was proportional to the quantity of in the according to a periodic law, linking relative atomic compound. She was surprised that certain natural, ura- weights of the elements to their chemical properties. nium-containing such as pitchblende, chalco- Mendeleev developed a chart showing that homologue lite, and autunite were much more radioactive than the elements have large differences in atomic weights and metallic uranium freshly prepared by . If different elements of similar atomic weight exhibit chalcolite was synthesized in the laboratory from pure large differences in properties. With a limited number uranium compounds, no such enhanced radioactivity of empty places in the chart, Mendeleev predicted the was encountered. This led to search in existence of yet-undiscovered elements, such as eka- these natural minerals for a small quantity of another aluminium and eka-silicium, and their expected prop- yet-unknown element, the source of these stronger erties. A final proof of the validity of the Mendeleev intensity rays (see excerpt next page). She invented concept was the discovery of the elements gallium in a new “radiochemical” method combining ordinary chemical analyses with the measurement of radioactivity. 1a 1b 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b 7a 7b 8 0 1 H He One substance she identi- 2 Li Be B C N O F Ne fied, polonium, had proper- 3 Na Mg Al Si P S Cl Ar ties similar to . In 1898, 4 K Ca Sc Ti V Cr Mn Fe Co Ni Pierre and Marie Curie couldn’t Cu Zn Ga Ge As Se Br Kr isolate a sufficiently large quan- 5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd tity of polonium to measure its Ag Cd In Sn Sb Te I Xe 6 Cs Ba La Hf Ta W Re Os Ir Pt atomic weight or to obtain the Au Hg Tl Pb Bi Po At Rn spectral signature. Today, we 7 Fr Ra Ac Th Pa U know that only about 6 nano- grams were isolated, beyond Periodic system at about 1895. All lanthanides were known except Pm (radioactive) any method of measurability and Lu discovered in 1907 (only La could be presented). In yellow are the missing non- available at the time; however, radioactive elements. Discovered were Ge in 1896, Ne, Kr and Xe in 1898, Hf and Re in measuring its “radioactivity” 1923 and 1925. In red are the missing radioelements with mass lower than uranium. was feasible. Pierre and Marie Curie didn’t immediately try to

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January 2011.indd 24 1/3/2011 3:46:31 PM Separating Uranium from In non-pertubated uranium ores, 238U and 235U X) which all had the chemical properties of radium. are in secular equilibrium with their 23 main The question was how to classify them in the peri- daughters (alpha or beta emitters) with the total odic system? Only 12 spaces where left empty in the activity being 178 kBq/g of uranium. Only five of table. found the in calling them give easily detectable gamma rays. When these “elements” , which had all the same U is separated from ores by chemical processes, chemical properties and the same place in the periodic the remaining activity is 25 kBq/g of the original system, but differing in their the radioactive half life. activity content. Due to the ingrowth of the two Nevertheless, it took until 1935 until the complexity of short-lived daughters of 238U, it needs around chains was really understood. one year for the activity to reach the limiting value of 51 kBq/g. The emission of gamma rays The Way to a Unifying Concept for increases progressively. In Marie Curie’s co-pre- Chemistry cipitation experiments, the amounts of Po and Ra were around 70 ng/kg and 300 μg/kg of ura- , as well as Hans Geiger and Ernest nium, respectively. In ’s co-precipitation Marsden, used radium as a powerful source of alpha experiments of Ra, the amount of 228Ra was particles to probe the inner structure of the by around 400 ng/kg of . directing the beam of particles onto a thin foil of gold. This scattering experiment to the surprising result place polonium in the Mendeleev system. Since its that most of the was concentrated in a behavior was similar to that of bismuth, they may very small nucleus about 10 000 times smaller than have felt compelled, according to this system, to look the atom. It showed that atomic weight and nuclear for an eka-bismuth, but this element would have been charge are related. This key observation allowed heavier than uranium. It was not until 1906 that the Rutherford, in 1911, to develop a new atomic model of chemical similarity of polonium and was a positive nuclei with a charge roughly proportional to identified, giving polonium its place close to bismuth atomic mass. This nuclei, he theorized, was surrounded in the periodic system. In 1910, a weighable quantity of by moving around it in a yet unspecified way. about 100 micrograms of polonium was concentrated This model, in turn, was rapidly improved upon with in few milligrams of bismuth. the concept of (de Boer 1911; Mosley The other substance Marie Curie identified was 1913) and by ’s introduction, in 1913, of “ener- radium, which had chemical properties similar to bar- getic quanta,” which placed the electrons in a definite ium. Spectral analyses by Eugène Demarçay of isolated orbit around the nucleus. The path was now opened to “pure radium” salts confirmed the hypothesis that understanding periodicity and chemical bonding, such radium was a new chemical element. Gravimetrically, as in the work of Walther Ludwig Julius Kossel in 1916. Marie Curie initially obtained an atomic weight of 225; A new unifying concept for chemistry had formed, but in 1907 she obtained a weight of 225.9, close to the it would hardly have been possible if Marie Curie had correct value of 226. not isolated radium. Hence, polonium and radium are The position of radium in the periodic system not only the cornerstones of the science of radioactiv- was easily determined by the Curies. Indeed, radium ity as Marie Curie suggested in her Nobel lecture in is the higher homologue of in the family of 1911, but they are cornerstones for modern chemistry alkaline-earth and it could easily be entered as a whole. into Mendeleev’s chart in the corresponding column. Moving beyond Naturally Occurring Since 1899, many have tried to isolate new Radioelements radioactive elements from uranium- or thorium-con- taining compounds using the separation techniques The use of alpha particles as projectiles not only of Marie Curie. They were frequently surprised by the helped scientists probe the atoms inner structure, “emanations” and “active deposits.” In 1910, 44 “radio- but it led directly to a number of new discoveries. For active elements” were identified. For example, one example, in 1934 Irène Joliot-Curie and Frédéric Joliot could clearly distinguish three “radioactivities” asso- used very intense radioactive alpha emitters such as ciated with three supposedly new elements (called polonium, much stronger than radium, to discover at the time mesothorium I, X, and thorium the first artificial : radioactive phospho-

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January 2011.indd 25 1/3/2011 3:46:36 PM Chemistry after Polonium and Radium

rus. In irradiating a foil of aluminium of mass of 27 by supposed transuranic element. Proceeding by co-pre- a source of 80 millicuries of Po, they observed the cipitation with barium, it was impossible to increase emission of and of positive electrons; the the activity of the precipitate, i.e. to enrich it in radium. later were emitted in a delayed fashion because of the Was this because the “hypothetical radium” was an irradiation exposure event. Only 30 could imponderable quantity? (see excerpt). The answer have been formed, which must have been radioac- was no (supplementary experiences showed that an tive by emission. It was the separation and imponderable quantity of radium 228 could easily be identification of phosphorus 30 as phosphine, which enriched in a precipitate with barium; the laws of co- provided the first chemical proof that a transmutation precipitation were independent of concentration). One by a nuclear reaction had occurred producing a new had to conclude that the activity measured in the pre- type of radioactivity. cipitate was indeed radioactive barium and this could This discovery by Joliot-Curie of artificial radioac- only be explained by the hypothesis that the uranium tive matter motivated many chemists to look for new nucleus could break upon irradiation. The fis- radioisotopes. They irradiated light elements with sion of uranium had been discovered. Meitner’s rapid alpha particles and the more heavy elements with neu- calculation showed a gain of about 200 MeV from this trons. It took only three years to discover about 200 nuclear reaction, sufficient energy to change the fate new . New chemical elements were also of humanity. From there it all became clear. The neu- artificially produced. For example, was trons irradiating uranium produced barium and lantha- produced in 1937 by Casimir Perrier and Emilio Segré, num. The identification of hundreds of radionuclides, who bombarded molybdenum with deuterons and iso- isotopes of 30 chemical elements formed in the fission lated an irradiation product with chemical properties process of uranium 235, was a Herculean accomplish- similar to . ment for radiochemists. The procurement of radioisotopes for a large suite of chemical elements with periods ranging from a frac- Going beyond Uranium tion of a second to several years has enabled their use in areas as diverse as chemistry, geosciences, material Even though early attempts failed to produce “trans- science, biology, , industry, and agriculture. uranic elements” by the neutron irradiation of ura- has become a new tool for studying nium due to the predominance of fission, the initially mechanisms in all these fields. intended nuclear reaction did occur, although with a It was soon recognized that the neutron transmuted probability about 15 times less, too small to be iden- one atom of mass A into a new atom of mass A + 1, tified in the background of fission. However, careful which, by beta emission, decayed to an atom with neutron irradiation of a thin foil of uranium allowed atomic number Z+1, thereby becoming the element the breakthrough. All fission products should have next to the irradiated one in the . So, escaped the foil due to their extremely high recoil it was the logical next step to irradiate uranium with energy. However, a newly produced radioactive sub- neutrons to search for new elements even heavier stance did not escape the thin foil. This was indeed the than uranium. The pursuit of these “transuranic ele- long-searched-for proof of a series of new elements ments” quickly led to a riddle. The best radiochemists heavier than uranium. This new chemical element, were unsure how to analyze the chemical behavior of discovered by Edwin McMillan and in the “new radioactivities” they encountered in light of 1939–1940, was named . It behaved like their supposed homologous elements such as rhe- uranium and was not homologous to rhenium, which nium, osmium, or platinum, or of heavy elements such was expected. It was the first evidence of a new family as radium, which might have originated from decay of elements. The is of mass of the supposed transuranic elements. Ida Noddack, 239, also a and much more simple to Irène Curie, and Pavel Savich (1938) found products separate from uranium than uranium 235. It was ini- with the properties of lanthanum, but they did not tially difficult to find its place in the periodic table. The believe in the presence of a radioactive lanthanum. modern version of this table contains the A crucial experiment was conducted by Otto Hahn, and the lanthanides. The periodic table now has 118 and Fritz Strassman in 1938–1939 in which elements (see figure next page). The search for new they tested the hypothesis that radium was the radio- chemical elements still continues. active irradiation product coming from the decay of a

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January 2011.indd 26 1/3/2011 3:46:37 PM Chemistry after Polonium and Radium

1 2345678 9 101112131415161718 sd p 1H He 1 2 2Li Be B C N O F Ne 3 4 5 6 7 8 9 10 3Na Mg Al Si P S Cl Ar 11 12 13 14 15 16 17 18 4K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 5Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 6Cs Ba La* Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 7 Fr Ra Ac** Rf Db Sg Bh Hs Mt Ds Rg Cn 87 88 89 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118

* Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 4f 58 59 60 61 62 63 64 65 66 67 68 69 70 71 ** Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 5f 90 91 92 93 94 95 96 97 98 98 100 101 102 103

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 ), the most radioactive material that exists today.

Radiochemistry Becomes Part of chemical reactions and transport mechanisms in all Chemistry areas of the science. The chemical knowledge gained from radiochemistry was decisive in many fundamen- Since Marie Curie’s discoveries, a new branch of chem- tal discoveries: radioactivity as an atomic property, istry dealing with the chemical properties of radioac- artificial radionuclides, the completion of the periodic tive matter has progressively emerged. Such matter is table, , and transuranic elements. Today, in perpetual renewal due to the radioactive decay of radioactive matter is used by radiochemists for fun- radionuclides and the emission of . damental research in many fields, especially medicine Radiochemistry is based on its own methodology. It and energy. allows scientists to look at many processes beyond the The discovery of polonium and radium and the scope of chemistry and it has become a key discipline course of chemistry and society would have been for understanding behavior—so important in different were it not for the extraordinary patience, nuclear industry and environmental science. In this determination, and curiosity of Marie Curie as she regard, we know how to extract plutonium, a fissile searched for the origin of the strong radiation from material, from spent nuclear fuel. However, we have uranium compounds. Her unwavering believe in the yet to find an ultimate solution for isolating the radio- hypothesis of radioactivity as an atomic property and active waste associated with this endeavor. her spirit of adventure and readiness to pursue unorth- odox thinking, changed the course of history. Conclusions Robert Guillaumont is an honorary professor of chemistry (University of Paris-Sud, The era of radioactivity and radiochemistry, which Orsay) and a member of the French Academy of Sciences. His research field in started between 1896 and 1898, led to discoveries radiochemistry focused mainly on tracer scale chemistry and on thermodynamics that have profoundly influenced chemistry. Until 1915, of . He is a member of several committees on only a few teams of researchers—in Paris, Cambridge, management. Berlin, Vienna, and Montreal—had worked with radio- active material. The isolation of radium and polonium Bernd Grambow is a professor of radiochemistry and head of Subatech Laboratory, allowed these teams to probe the structure of the a mixed research unit of the Ecole des Mines, the university, and the IN2P3/CNRS in atom, and from this a unified concept of chemistry Nantes. He obtained his Ph.D. in chemistry at the Free University of Berlin. Principal emerged. From that point forward, chemists have research interests are in the thermodynamics and kinetics of chemical reactions used the properties of radionuclides to understand involving radionuclides and in the radiochemistry of nuclear waste disposal.

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