The DISCOVERY
of the
ELEMENTS. XIX. The RADIOACTIVE ELEMENTS* MARY ELVIRA WEEKS The University of Kansas, Lawrence, Kansas
In 1898 there was discovered a n element, radium, which continually and spontaneously emits light, heat, and other radiations. Investigation of these astonishing fihenomena by the Curies and many others has revealed about forty interrelated radioactive elements which, like radium, are unstable. They do not, however, occufiy forty places in the periodic system, but are crmded into only ten places. The explanation for the existence of these numerous so-called "radioactive isotopes" and their genealogi~al descent from uranium and thorium were discovered independently by K. Fajans, F. Soddy, A. S. Russell, and A. Fleck. Since the original literature on the radioactive elements embraces such a vast field of research, the following account of their discovery i s necessarily far from comfilete.
+ + + + + + "Radium i s not to enrich any one. I t i s a n element; i t is for all peafile" (1). "So the atoms in turn, we now clearly discern, Fly to bits with the utmost facility; They wend on their way, and, in splitting, disfilay A n absolute lack of stability" (2). ENRI BEG QUEREL,a member of a family renomedfor scientific achievemerit, noticed in 189,j that when a phosphorescent salt, such as sium wanyl sulfate, is placed near a photographic plate protected by black paper, theplate becomes fogged as though it had been exposed to light (5 (58). His later work showed t h a t all compoundsl even those which do not
H
PIERRE CURIE, 1859-1906 Professor of physics at the Sorbonne.
In collaboration with his brother, Jacques Curie, he discovered and He investigated pieza-electricity. introduced the concept of symmetry in physical phenomena, studied magnedc propetiiesas a f,&tion of ternperature, and collaborated with Mme. Curie in discovering radium, and investigating its properties. d
phosphoresce, give off penetratingrays which, like X-rays, darken a photographic plate and, by making the surrounding air a conductor, cause the gold leaves of a charged electroscope to lose their electrostatic charge andcollapse. These radiations are now known t o be of three kinds: alpha rays, which consist of helium a t o m s bearing two ANTOINE-HENRI BECQUERBL 1852-1908 units of positive French physicist and engineer. beta Disc~verer of the rays emitted by rays streams consisting of negative Of researches nran~um. on Herotatory carriedmagnetic out important polari,
' s; a n d gamma rays, which
zation, phosphoresence, infra-red spectra, and radioactivity. His grandfather, Antoine CCsar Becqnerel and his father. Alexandrea very (1788-1878), Edmond Becquerel (182&91), also metrating radiamade many important contributionr, p. t ~ o n of extremely to chemistry and physics.
wave-length. The amazingly rapid development of the science of radioactivity is largely due to the brilliant work of M. Pierre Curie and his wife, Mme. Marie Sklodowska Curie. The former was born in Paris on May 15, 1859, and was educated by his cultured parents. Many happy hours were spent on excursions to the country, and thus this city child grew up in intimate contact F t h nature, collecting plants and animals and enjoymg them in quiet contemplation. While serving as director of the laboratory under Schutzenberger at the School of Physics and Chemistry, Pierre Curie carried on researches on condensers, magnetism, piezoelectricity. and the principle of symmetry in nature. When in 1895 he received the degree of Docteur-essciences from the Sorbonne, Schutzenberger created K u s t r a t i o n s collected by F. B. Dains of The University of Kansas.
79
1
(JULES) HEN= POINCAR*.1854-1912 French mathematician, physicist. and astronomer. Prol5c and d t e d miter on mathematical analysis-analytical and celestial mechanicx, mathematical physics, and philosophy of sc~ence.
a chair of physics pass through l i e for him (3). together hypnoMarie Sklodow- tizedinourdreams: ska, a daughter of your dream for Dr. Sklodowski,* your country; our aprofessor of phy- dream for humansics and mathe- ity; our dream for matics at the War- science." After saw gymnasium, their marriage in was born on No- 1895 Professor vember 7, 1867. Schutzenberger Because of t h e arranzed that they early death of hrr might work tdgifted mothrr, the gethrr in the lablittle f j r l grcw up oratory, and their in her father's l a b mutual devotion oratory and under to science once led his i n s t r u c t i o n . M. Curie to reShe soon devel- mark, "I have got ooed a assi ion ate a wife made expressly for me to love 'Ountry and joined a secret share all my presociety of students 0ccupati0nS" (5). who organized
evening for laborers and peasa n t s . However, because of the limited opportunities for advanced study, she decided to leave her beloved motherland and go to Paris. During the four years of her student life, she lived in a chilly little attic room, carrying the coal herself up the six flights of stairs, and cooking her simple meals over an alcohol lamp. This was Marie Sklodowska's introduction to the city which has ever since been her home (4), (68). When she enrolled at the Sorbonne, Henri Poincarg, the famous mathematical physicist, soon recognized her ability, and Professor Gabriel Lippmann also took great interest in her research. Her first meetincc with Pierre Curie was at the home of a Polish physicist i n Paris. Because of their mutual interest in scientific, social,and humanitarian subjects, there gradually developed a singleness of purpose that caused M. Curie to say, "It would . . .be a beautiful thing in which I hardly dare believe, to
POLONIUM AND RADIUM
GABXELLIPPMANA, 1845--1921 Professor of mathematical physics at the University of Paris. Inventor of the capillary electrometer and of a process of direct color photography. The phenomenon of piezo-electricity in crystals predicted by Professor Lippmann was first demonstrated experimentally by Pierre and Jacques Curie.
Professor Curie continued his researches on the growth of crystals, and his young wife prepared for her examinations. Many chemists consider her dissertation (55) to be the most remarkable thesis ever presented for the doctorate. She continued the work begun by Becquerel, and tested most of the known elements, including a number of rare ones loaned by Demar~ay and Urbain, for their ability to discharge a gold-leaf electroscope, and found that thorium and uranium were the only ones whose compounds caused the gold leaves to collapse (26), (54), (55). The radioactivity of thorium was discovered independently by Gerhardt Carl Schmidt, professor of physics a t the University of Miinster (25). Of a m u c h greater significance than this, however, was Mme. Curie's observation that the activity of the uranium mineral, pitchblende, is four or five times a s g r e a t a s one might expect it
-
* The feminine ends in -ska, the masculine in -ski.
Tlro LA~oRATORY' IN
WHICH
M.
AND
MME.CURIEDISCOVERED RADIUM
* Reproduced from an article by Jacques Dame, La Nature, 32 [I],217 (Mar. 5. 1904) by permission of Masson et Cie.. Paris.
to be from its uranium content (24). She concluded that the ore must contain another radioactive element in addition to uranium and that, since the composition of the ore was known, the active element must be present in extremely small amount and must therefore be very active indeed. Therefore it became necessary to work up large quantities of pitchblende and to make elaborate and tedious fractionations of this complex ore. The pitchblende was supplied by the Austrian goverument from its uranium mines in the Joachimsthal, Bohemia. As Mme. Curie examined each fraction with the electroscope, she found that a very active substance separated with the bismuth. After convincing herself in 1898 that this was a new element, she named it polonium in honor of her native country (27), (65). I t is also known as radium F. In 1902 Dr. Willy Marckwald of Berlin obtained a metallic deposit on a polished plate of bismuth immersed in a solution of the bismuth fraction from pitchblende. This deposit, which he called radiotellurium, was later shown to be identical with Mme. Curie's polonium (6), (29). After commenting on the discovery of gallium, scandium, and germanium (ekaaluminum, ekaborou, and ekasilicon), Mendel&& had written in 1891, "I foresee some more new elements, but not with the same certitude as before. I shall give one example, and yet I do not see it quite distinctly" (7). He had then proceeded to describe an undiscovered "dvi tellurium" with an atomic weight of about 212. Since polonium resembles tellurium and has an estimated atomic weight of about 210, it is probably the realization of Mendel6eff's "dvi tellurium." After the Curies, with the assistance of M. G. Bemont, had carried out many laborious fractionations of barium chloride, they found that the most insoluble fractions were the most radioactive. In the course of her experiments Mme. Curie had learned that radioactivity is an atomic property depending solely on the quantity of active element present. For this reason the presence of another active element was suspected, and the radioactive barium chloride was therefore submitted to M. Demarfay for spectroscopic examination. He detected a new line in the ultra-violet region of the spectrum, and certain other lines, all of which were most distinct in the most radioactive preparations, and, as fractionation proceeded, the barium lines became fainter and fainter (23), (28), (52). While tracing down the new element, the Curies often wondered how its salts would look, and hoped that perhaps they might display beautiful colors. The radium chloride which they finally obtained proved to be a white salt, however, but it was even more beautiful than their brightest dreams: it glowed in the dark! Radium, like phosphorus, is a giver of light, and this property was to them, as it had been to Brand and Kunckel, a source of surprise and delight. "One of our joys," writes Mme. Curie, "was to go into our workroom at night; we then perceived on all sides the feebly luminous silhouettes of the bottles
or capsules containing our products. I t was really a lovely sight and always new to us. The glowing tubes looked like faint fairy lights" (8), (60). The new substance was named radium, the giver of rays, and, were it not for t h i s property, it might still be numbered among the missing e l e m e n t s . hAlthough it gives a distinct soectrum. the methbd of de: tecting it with an electrometer is five hundred thousand times more
Ir
il
IIm., 110,,,#~1
MME. MARIE SKLODOWSKA CURIE, 1867-
Professor of radioactivity at the University of Paris. With her husband. Professor Pierre Curie, she discovered radium and polonium, and founded the science of radioactivity.
sensitive than the spec t r 0 s c 0 p i method (9). Professor Georges Urbaiu once said,
I was certainly privileged, for I saw with my own eyes the birth of radium. Pierre Curie, who was my teacher, rendered me the incomparable honor of according me his confidence and friendship. I saw Mme. Curie work like a man at the dBicult treatments of great quantities of pitchblende. I saw the first fractionations of the bromides of barium-radium. I saw the radium-bearingcrystals shine in the dark before the radium spectrum could be observed in them. Every Sunday we used to go with Langevin, Perrin, Debierne. Cotton, and Sagnac to the little Curie home, which was thus transformed into an intimate academy. There the master, with his customary simplicity, explained his ideas to (74). us and deigned to discuss ours.
..
The late Wilhelm Ostwald gave in his autobiography the following account of his visit to the birthplace of radium: At my urgent request the Curie laboratory, in which radium was discovered a short time ago, was shown to me. The Curies themselves were traveling. I t was a cross between a honestable and a potato-cellar, and, if I had not seen the worktable with the chemical apparatus, I would have thought it a practical joke (10).
When Pierre Curie was urged in 1903 to accept the decoration of the Legion of Honor, he wrote, "I pray you to thank the Minister, and to inform him that I do not in the least feel the need of a decoration, but that I do feel the greatest need for a laboratory." Nevertheless, Mme. Curie looks back upon the years spent in this dingy old shed as "the best and happiest" ones of her life (8). THE URANIUM SERIES
In 1900 Sir William Crookes prepared a solution containing a uranium salt and a small amount of a
ferric salt. When as uranium Y (4G), (50), (%), (59), which was dishe added to this an covered in 1911 by G. N. Antonoff, who was working excess of a solution under Sir Ernest Rutherford at the University of Mancontaining ammo- chester. He afterward returned to St. Petersburg. nium hydroxide and Uranium Y, like uranium 2, belongs to a subordinate ammonium carbon- branch of the family. Soddy attributed Antonoff's sucate, he found that cess, not to the special chemical process adopted, the resulting ferric but "to the lapse of a suitable period of time between hydroxide precipi- successive separations" (75). Thus in the uranium tate was intenselv series uranium 1 breaks down to form uranium X,. .. radioactive. After and this in turn disintegrates to form the successive studying the radio- products: uranium Xz, uranium 2, uranium 2, and active properties of uranium Y. the su+take which THE RADIUM SERIES ~recloltates w ~ t h In 1907 the late Professor Bertram Borden Boltthe iron, he said, "For the sake of wood of Yale University discovered an element which lucidity the new he named ionium and which he found to be the parent body must have a substance of radium (39). Professor Boltwood had
