Finding Francium Eric Scerri Recounts the Story of Element 87, Which After a Number of False Starts Was Finally Tracked Down in France — and Named in Its Honour
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in your element Finding francium Eric Scerri recounts the story of element 87, which after a number of false starts was finally tracked down in France — and named in its honour. ne of the most remarkable things itself — rather than its radioactive daughter about element 87 is the number of isotopes — and thereby discovered a weak, Otimes that people have claimed to yet significant, branch in one of the three have discovered it after it was first predicted main radioactive decay series. by Mendeleev in 1871 and given the Her analysis of the data revealed a provisional name ‘eka-caesium’. new element with a half-life of 21 minutes. It was recognized early on that the When she was later asked to name the periodic table more or less fizzles out element she chose ‘francium’ to honour after element 83, bismuth. All subsequent the country of her birth2. It was also an elements are radioactive and therefore appropriate choice in marking the continuing unstable, with a few notable exceptions contribution made by French scientists to such as uranium and thorium. But this fact the study of radioactivity. Not only was the did not deter a number of scientists from phenomenon itself discovered by Becquerel, searching for element 87 among natural but other landmark achievements in the sources and, in many cases, from claiming field included the isolation of the radioactive to have isolated it. For example, Druce elements polonium and radium by the and Loring in England thought they had Curies, as well as actinium by Debierne — all identified the element by using the classic both of which were missing at the time. of which happened within a few years, and method developed by Moseley for measuring Dozens of papers were published on this all in France. the Kα and Kβ lines of any element’s X-ray effect, including a number of studies As it turns out, francium was one of the spectrum. But it was not to be. arguing that it was spurious. These days the last natural elements to be discovered, and Allison effect is often featured in accounts is the second rarest after astatine. Estimates Estimates suggest that there is of pathological science, alongside the claims of the abundance of francium suggest that for N-rays and cold fusion1. there is only about 30 g in the whole of the only about 30 g of francium in The next principle claim came from Earth’s crust. It is one of a very few elements the whole of the Earth’s crust. Paris and was supported by Jean Perrin, the that has no practical applications, mainly physicist who is perhaps best known for because it has such a short half-life. confirming Einstein’s theory of Brownian Nevertheless, the fact that the francium In the 1930s it was the turn of Fred motion and, as a consequence, provided atom has the largest diameter of any Allison from the Alabama Polytechnic supporting evidence for the existence of element, at a huge 2.7 ångströms, and the Institute (now Auburn University). Allison atoms. Horia Hulubei, a Romanian physicist fact that it has just one outer-shell electron, developed what he called a magneto- working with Perrin, claimed to use highly has made it the object of considerable optical method for detecting elements and accurate X-ray measurements and that this attention among researchers wanting to compounds based on a supposed time-lag enabled him to record several spectral lines probe the finer details of current theories in the development of the Faraday effect, with precisely the frequencies expected of atomic physics. In 2002, a group in the whereby the application of a magnetic field of element 87, which he promptly named US succeeded in trapping 300,000 atoms of causes a beam of polarized light passing moldavium. But alas, these lines also turned francium on which they performed several through a liquid solution to be rotated. out to be spurious. key experiments of this kind3. ❐ He mistakenly thought that every The eventual discovery of element element gave a particular time lag, which 87 was made in 1939 by a remarkable ERIC SCERRI teaches chemistry at UCLA incidentally was observed with the naked Frenchwoman, Marguerite Perey, who and is the author of The Periodic Table, eye, and that this effect could be used to began life as a laboratory assistant to none Its Story and Its Significance, Oxford Univ. identify each substance. He boldly claimed other than Marie Curie in Paris. Perey Press, 2007. e‑mail: [email protected] in a number of journal articles and even quickly became skilful in purifying and in a special feature in Time Magazine that manipulating radioactive substances and References he had observed elements 87 and also 85, was asked to examine the radioactivity of 1. Langmuir, I. Phys. Today 42, 36–48 (1989). 2. Kauffman, G. B. & Adloff, J. P. Educ. Chem. 135–137 actinium, which is element number 89 in the (September 1989). periodic table. She was the first to observe 3. Orozco, L. A. Chem. Eng. News (2003); available at http://pubs. the α and β radiation produced by actinium acs.org/cen/80th/francium.html. Po At Rn Fr Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf 670 NATURE CHEMISTRY | VOL 1 | NOVEMBER 2009 | www.nature.com/naturechemistry © 2009 Macmillan Publishers Limited. All rights reserved.