Master of Missing Elements

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Henry Moseley’s discoveries sorted out the periodic table and transformed how scientists look for new forms of the most basic substances.

Eric R. Scerri

ust over 100 years ago, a young Henry Moseley seemed destined molecules) might be capable of diffract- English physicist named Henry to leave his mark on science. He was ing x-rays. This prediction was almost Moseley gave new life to the pe- born in the county of Dorset in Eng- immediately confirmed by indepen- riodic table of the elements, and land in 1887, into a family with an ex- dent researchers, who found that the Jhelped resolve a number of prob- traordinary scientific pedigree. Both of diffracted rays produced discrete lines lems in chemistry and physics. There his grandfathers as well as his father, on photographic film. is a good chance you have never heard who died when he was just four years The diffraction of x-rays differed of him. Soon after Moseley published old, were fellows of the Royal Society. based on the type of crystal used; the this work, World War I broke out, and One grandfather was a mathematician; pattern of lines could be precisely mea- the 27-year-old patriot felt compelled to the other and Moseley’s father were sured to obtain detailed information volunteer for service on the war front, zoologists. In 1901 the young Moseley about the distance between the planes where he was killed by a sniper’s bullet. earned a scholarship to study at Eton, of atoms that make up the crystals, un- Despite his abbreviated life, Moseley’s one of England’s most prestigious veiling their structure. Furthermore, a work continues to influence the world schools. There he excelled at academic British physicist, Charles Barkla, found of chemistry. In fact, his research is more work as well as sports; he also became that each element scattered x-rays to a influential than ever today, as new ele- infused with a patriotic spirit that different extent, producing distinctive ments are being synthesized and added eventually cost him his life. lines and thereby indicating the scat- to the periodic table, such as the yet-to- In 1906 Moseley gained another terer’s composition. Two other Brit- be-named elements 115 and 118. scholarship, this time to Trinity Col- ish physicists, Henry Bragg and his Moseley discovered a way to use lege at the University of Oxford. He son William, found that even reflection x-rays in a vacuum to identify the was disappointed with the lack of from crystals could be used to obtain atomic charge of elements (a critical interest in academic work in general useful information on the distances be- measurement of their chemical prop- and physics in particular at the college, tween planes of atoms. erties, now known to correspond to however. He noticed that pursuits At the same time, there was a great the number of electrons in each ele- such as fox hunting were regarded as deal of debate as to whether x-rays ment). His work allowed for the de- more important than academic excel- were waves or particles. The study finitive identification of substances, lence. After failing to gain a first-class of x-rays was one of the first areas of and revealed that several elements honors degree in physics—probably physics in which wave-particle duality— were missing from the periodic table because he was too interested in the the view that some phenomena might of the time, setting up subsequent re- subject matter rather than techniques be neither one nor the other but both searchers to make groundbreaking for passing exams—Moseley trans- together—began to take shape. That discoveries about the composition of ferred his studies to the University of same realization was eventually made the world. His methods also showed Manchester to work with Rutherford. of electrons, thus opening up the pos- the logic of the now standard ordering Rutherford immediately set Moseley sibility of treating the electron as a of the periodic table. Moseley worked to work on experiments with radioac- wave, as Erwin Schrödinger went on with some giants in physics, including tive isotopes, apparently recognizing to do. Modern ideas about quantum the legendary Ernest Rutherford, who his talents. But what really excited physics emerged from there. discovered the structure of the atom. It Moseley was another new area of phys- Henry Moseley followed all these is likely that Moseley would have won ics, one that had begun slightly before developments closely and asked Ruth- the Nobel Prize himself, if not for his the discovery of radioactivity. In 1897 erford if he too could initiate a pro- untimely death. the German physicist Wilhelm Röntgen gram of research on x-rays. Initially had discovered some mysterious rays Rutherford was reluctant to allow that he named x-rays, which found im- Moseley to discontinue his work on Eric R. Scerri has been researching and teaching at mediate applications in medicine as isotopes, but he relented after Bragg the University of California at Los Angeles for the past 15 years, both in chemistry and the philosophy well as basic scientific research. In 1912 the elder invited Moseley in October of science. He is also a book author, a public speaker, another German physicist, Max von 1912 to the University of Leeds to train and an avid blues guitarist. He received his Ph.D. Laue, proposed that crystals of inor- him in working with x-rays. On his in the history and philosophy of science from King’s ganic compounds (substances that are return to Manchester a month later, College London. Internet: www.ericscerri.com not based on carbon–hydrogen bonded Moseley teamed up with the math-

ematician Charles Darwin (the grand- A young Henry Moseley, shown during his student days at Trinity College, holds laboratory son of the “right Darwin,” as the Dan- equipment similar to pieces he later used to make breakthrough discoveries about the ordering ish physicist Niels Bohr later described of the periodic table. Moseley’s career was cut short by his death during the Battle of Gallipoli him). Together Moseley and Darwin in 1915, but his work continues to influence elemental studies today. repeated the work of the Braggs and tried to extend it into new directions. when physics was undergoing great nucleus of a helium atom, consisting of After publishing a couple of articles discoveries about the atom. Sev- two protons and two neutrons), which together, Moseley rather mysteriously eral lines of research were begin- also scatter in patterns dependent on decided to return to Oxford, where he ning to illuminate the relationship the element. would work as an independent schol- between the electrical charge of the The next piece in this puzzle to ar, possibly awaiting an appointment nucleus and the atomic weight of any understand the elements came from at the university. Whatever the reason, particular element. Barkla’s experi- a most unlikely source, but proved the move to Oxford proved enormously ments, on the degree to which x-rays highly influential for Moseley and fruitful. It was after returning that Mose- were scattered by samples of differ- other researchers. A Dutch economist ley conducted his epoch-making work, ent elements, suggested that nuclear and amateur scientist, Anton van den which resolved a puzzle about the or- charge was approximately equal to Broek, had a long-standing interest in dering of the periodic table and which half of the atomic weight of any ele- the periodic table, and attempted to continues to be felt up to the present day. ment. Rutherford and his colleagues improve on Dmitri Mendeleev’s clas- came to exactly the same conclusion sic system of listing the chemical ele- Dawn of the Nuclear Era from their work bombarding elements ments. Van den Broek attempted vari- Moseley began his crucial experi- with highly energetic, radioactive al- ous improvements by drawing on the ment in the early 1900s, at a time pha particles (now understood as the results of Barkla and Rutherford. He

www.americanscientist.org © 2014 Sigma Xi, The Scientific Research Society. Reproduction 2014 September–October 359 with permission only. Contact [email protected]. Atomic weights had Moseley’s results were his great also created three plac- breakthrough. He found that van den es in the periodic table Broek had been correct in assuming where a mysterious- that the elements are more properly looking anomaly had ordered using atomic charge, or atomic long bothered chem- number as it became known, than by ists. In the cases of ar- using atomic weight. gon and potassium, In the case of atomic weight, the cobalt and nickel, and variation between successive elements tellurium and iodine, is rather irregular, making it unclear something strange was whether new elements might be lurk- evident. The first ele- ing undiscovered between the already ment in each of the pairs known elements. A good example is had an atomic weight provided by the two first elements hy- that was higher than drogen and helium, which have ap- the subsequent ele- proximate atomic weights of one and ment. But the chemical four units, respectively, a gap of three properties of these ele- units. In other parts of the periodic ments, in keeping with table, the gap in atomic weight values the properties of their between successive elements is usu- surrounding elements, ally closer to two units, leading many made it necessary to chemists and physicists to suppose invert their order and that one or even two elements might violate the principle of lie between hydrogen and helium. The atomic weight order- result of Moseley’s research helped to Wikimedia Commons Wikimedia ing. Such pair reversals, settle many debates about precisely A figure from one of Moseley’s publications shows how he as they became known, how many elements remained to be ordered his x-ray spectra data by wavelength to indicate where implied that all was discovered in the periodic table. additional elemental spectra should be inserted, demonstrat- not well and that there In July 1913, Moseley and Bohr, who ing the presence of elements yet to be discovered at the time. might be a more funda- was visiting Rutherford’s lab in Man- When he made this figure, Moseley had not yet examined mental way of ordering chester, had at least one conversation scandium, which should lie between calcium (Ca) and tita- the elements. about pair reversals. Bohr suggested to nium (Ti) at the top of the spectra. Brass showed a mixture of It was a view that Moseley that in the case of cobalt and lines from copper and zinc. also had begun to fer- nickel, the degree of scattering of x-rays suggested the existence of a new fun- ment in the minds of physicists such might be proportional to the atomic damental particle that he called the al- as Rutherford and Bohr, but it was charge of each of these two elements phon, which was to have a charge of +1 not until the complete outsider van rather than to their atomic weights. The and a mass of two units, or twice the den Broek stepped into the picture thought linked the ideas of the periodic mass of the hydrogen atom. (At this that it became stated explicitly. Per- table, atomic number, and x-rays. Mose- point, the separate positive protons haps physicists were focusing more ley’s response was: “We shall see.” and neutral neutrons in the atomic nu- on individual elements, cleus had not been discovered, hence whereas van den Broek van den Broek’s use of the alphon par- took a wider chemical ticle in their stead.) perspective involving all Most periodic tables at the time or- the elements in the pe- dered the elements by their atomic riodic table. Whatever weights. Van den Broek proceeded to the precise sequence of publish a version of the periodic table events, Moseley used the that included all the known elements, skills in handling x-ray up to uranium, which he took to have experiments that he had an atomic weight of 240. In this table learned from Bragg and each element differed from the follow- in Rutherford’s lab and ing one by two units of atomic weight set out to test van den or by one alphon particle. Moreover, Broek’s hypothesis. because the alphon bore a charge of +1, each element differed from the One of Moseley’s vacuum following one by one unit of positive chambers, along with the small train in the attached charge. Here is the origin of the idea tube that brought different that the elements can be ordered by samples into the path of an means of increasing nuclear charge, x-ray beam, is on display at or units of +1 charge, instead of by the Museum of the History weight, which Moseley proceeded to of Science at the University confirm experimentally. of Oxford.

360 American Scientist, Volume 102 © 2014 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. Moseley’s Experiment root of the frequency of the x-rays provided with a physical foundation Moseley made good on his comment to reflected from an element was pro- (and as it was later discovered, atomic Bohr and set up equipment to conduct portional to Z – 1, where Z is a whole number relates to the number of elec- the appropriate experiments. His appa- number representing the charge on the trons in an element, therefore provid- ratus consisted of an evacuated glass nucleus of the atoms of any particular ing the basis for placing the elements bulb, which allowed the passage of a element. The symbol Z (from the Ger- in ascending order of charge, which af- beam of x-rays to strike a target sample, man Zahl, meaning number) became fects the element’s chemical properties). and a photographic plate to record the known as the atomic number for an ele- Moseley had confirmed van den Broek’s resulting position of the reflected x-ray ment and is of fundamental importance hypothesis that atomic number is a bet- beam when it arrived at the detector in chemistry and physics. (See the fig- ter ordering principle for the elements in screen. From knowledge of the position ure on page 362 for more on what happens the periodic table than is atomic weight. of the beam, Moseley was able to calcu- late the frequency of the rays. He also devised a method of varying the sample without opening the bulb, because he Bohr suggested the degree of scattering of wanted to examine the effect of x-rays on a range of elements under precisely x-rays might be proportionate to the the same pressure. At this time reduc- ing pressure was not a well-developed atomic charge of each of these elements. technology and starting again each time for separate element samples would Moseley’s response was “We shall see.” have complicated the experiments too much. Moseley’s experimental setup contained a small train onto which the various samples were mounted, and a within an atom when it is struck by x-rays.) But Moseley’s method went far simple device allowed the movement This result solved the long-standing beyond merely confirming van den of the train to bring successive samples issue of pair reversals. It now became Broek’s idea and providing a physical into the line of fire of the x-ray beam. clear that the reversal of elements such foundation. It also provided a simple After conducting his experiments, as iodine and tellurium was perfectly experimental method to identify any Moseley found an unexpectedly sim- justified on the basis of tellurium having element by measuring the wavelength ple relationship between the reflected a lower atomic number than iodine. The of the x-ray lines that it produced, and x-rays and the elements, which is now reversals like this one that chemists had checking whether the observed value known as Moseley’s law. The square made on chemical grounds was now corresponded with the value calculat- ed using Moseley’s law. For example, Moseley was able to put his new meth- Moseley’s experimental apparatus consisted of an evacuated glass bulb, which allowed the passage of a beam of x-rays to strike a sample, connected to a spectrometer. Inside the spec- od to good use in showing that certain trometer was a crystal that deflected the x-rays onto a photographic plate, which recorded the reported new elements did not in fact resulting spectrum. Moseley did not want to reestablish a vacuum in the bulb each time he exist. Such was the case with an ele- switched samples, so he devised a small train onto which he mounted the various samples, ment that the French chemist Georges and he created a simple device to move the train into the x-ray beam. Urbain had claimed to find.

to power source

cathode target elements on train

bobbin and line bobbin and line for moving train for moving train Wikimedia Commons photographic plateholder

spectrometer with potassium cover removed ferrocyanide crystal

www.americanscientist.org © 2014 Sigma Xi, The Scientific Research Society. Reproduction 2014 September–October 361 with permission only. Contact [email protected]. predictions for the existence of such incident intermediate elements that had been radiation made by the likes of Mendeleev, influ- from primary ential Swiss inorganic chemist Alfred x-ray Werner, the Swedish spectroscopist source Johannes Rydberg, and several others. ejected L-shell M-shell It is a tragedy that Moseley’s death electron lls ended his work just as it was about to K-shell electron lls ejected vacancy electron vacancy L-shell have huge scientific repercussions. electron Fights for First Knowledge of the complete periodic table was a necessary precursor for full understanding of the natural world, L x-ray K x-ray emitted and the discovery of an element would emitted guarantee the finder a place in history. One might think that the knowledge that precisely seven elements remained to be discovered, as well as Moseley’s x-ray method that allowed K their experimental identification, L would have made the race to discover these elements a relatively straightfor- M ward matter. But this was not the case, and in fact the attempts to discover the missing elements were fraught with As Moseley discovered, the pattern, or spectrum, of wavelengths produced when an x-ray difficulties and led to many bitter pri- beam strikes an atom is unique to each element, so it can be used to identify the composition ority disputes among the participating of samples. Electrons in an atom fall into shells, the innermost denoted K and subsequent scientists. However, in each discovery, ones labeled with the next alphabetical letter. An x-ray beam knocks out certain electrons in Moseley’s x-ray methods were the key each shell. Electrons from the next higher shell move down to fill the vacancy. But inhabiting for proving or disproving the finding. a lower shell takes less energy, so the electrons emit the extra energy as x-rays, each of a char- The first to be identified was the ele- acteristic wavelength that depend on the shell it now inhabits. ment with the highest atomic number among the missing seven, element 91, Mendeleev, a main early creator of element for which he had made mea- which was eventually given the name comprehensive periodic tables, had surements, and so there was no guaran- protactinium (Pa). It was discovered by predicted an element would lie directly tee that his law would hold for higher several physicists and chemists and it is below zirconium in the periodic table. atomic numbers. In addition, he lacked rather difficult to state categorically who Urbain called it celtium and gave it the samples of some of the elements, so he the first discoverers might have been. symbol Ct. This symbol even appeared missed a few gaps.) The three missing First the Polish-born chemist Kasimir on published periodic tables in several elements that he identified had atomic Fajans discovered a short-lived radioac- parts of the world. But not everybody numbers of 43, 61, and 75. Some time tive isotope (which has the same atomic accepted the claim by Urbain, which later, other scientists used Moseley’s number but a different number of neu- is why he seized the opportunity after method to determine that there were trons, thus a changed atomic weight) of hearing that Moseley had developed in fact four more elements yet remain- the element. Fajans named it brevium a unique method for identifying and ing to be discovered between the old on account of its short half-life (the time validating new elements. Urbain trav- boundaries of the periodic table, which it takes for half of its radioactive atoms eled to Oxford and brought with him spanned atomic numbers between 1 to decay, or transform, into something some samples that he believed con- (hydrogen) and 92 (uranium). The ad- more stable) of just 1.2 minutes. tained some celtium. It took Moseley ditional missing elements, beyond the A little later, a far longer-lived isotope just a few hours to conclude that it did three that Moseley himself identified, of element 91 was discovered at about not give any spectral lines expected were elements 72, 85, 87, and 91. That the same time by two independent of an element that had not previously meant a total of seven gaps in the list of teams. In 1917 in Berlin, Lise Meitner been observed but was instead a mix the constituent atoms that make up all and Otto Hahn, who later discovered of already known rare earth metals. of the natural world. nuclear fission, discovered an isotope

Fortunately Urbain reacted with grace Moreover, Moseley was able to cat- of element 91 with a half-life of 32,500 Commons Commons, images-of-elements.com, Wikimedia Wikimedia images-of-elements.com, Scott Camazine/Science Source, despite his undoubted disappointment. egorically rule out the possible exis- years. The same isotope was observed In addition, Moseley recognized that tence of any elements lying between by John Cranston and Frederick Soddy at least three undiscovered elements hydrogen and helium. In spite of the in Glasgow, but they failed to character- existed between hydrogen with atom- large atomic weight gap that exists be- ize it chemically. To their credit, both Fa- ic number 1 and gold whose atomic tween these two elements, there is no jans and the team from Glasgow ceded number is 79. (He could not extrapolate gap between their atomic numbers. At priority to Hahn and Meitner. Fajans fol- beyond gold because that was the last a stroke, Moseley was able to refute lowed a rule that if isotopes of any new

rhenium hafnium Discovered in 1925 by Named after Hafnia, Walter Noddack, Ida hydrogen the Latin name for the Tacke, and Otto Berg helium 1 city of Copenhagen. in Germany. 2 H He 1.0079 4.0026 lithium beryllium boron carbon nitrogen oxygen uorine neon 3 4 technetium 5 6 7 8 9 10 Li Be Discovery conrmed B C N O F Ne 6.941 9.0122 by Carlo Perrier and 10.811 12.011 14.007 15.999 18.998 20.180 sodium magnesium Emilio Segrè in 1937. aluminium silicon phosphorus sulfur chlorine argon 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar 22.990 24.305 26.982 28.086 30.974 32.065 35.453 39.948 potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 39.098 40.078 44.956 47.867 50.942 51.996 54.938 55.845 58.933 58.693 63.546 65.38 69.723 72.64 74.922 78.96 79.904 83.798 rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 85.468 87.62 88.906 91.224 92.906 95.96 [98] 101.07 102.91 106.42 107.87 112.41 114.82 118.71 121.76 127.60 126.90 131.29 caesium barium lutetium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon 55 56 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 132.91 137.33 174.97 178.49 180.95 183.84 186.21 190.23 192.22 195.08 196.97 200.59 204.38 207.2 208.98 [209] [210] [222] francium radium lawrencium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicum ununtrium ununquadium ununpentium livermorium ununseptium ununoctium 87 88 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 Fr Ra Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Uuq Uup Uuh Uus Uuo [223] [226] [262] [261] [262] [266] [264] [277] [268] [271] [272] [285] [286] [289] [288] [292] [294] [294]

lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium 57 58 59 60 61 62 63 64 65 66 67 68 69 70 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb 138.91 140.12 140.91 144.24 [145] 150.36 151.96 157.25 158.93 162.50 164.93 167.26 168.93 173.05 actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium 89 90 91 92 93 94 95 96 97 98 99 100 101 102 Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No [227] 232.04 231.04 238.03 [237] [244] [243] [247] [247] [251] [252] [257] [258] [259] Fr 87 At 85 Pm 61 Pa 91

francium astatine Discovered by First produced by Dale R. Corson, Kenneth Marguerite Perey in promethium France in 1939. Ross MacKenzie, and protactinium First produced and Emilio Segrè at the Discovery credited to characterized at Oak University of Lise Meitner and Otto Ridge National California, Berkeley Hahn in 1913. Laboratory in 1945. in 1940.

A modern periodic table is ordered by atomic number (related to methods to show additional gaps (atomic numbers 72, 85, 87, and 91), the charge of an atom), rather than atomic weight, in part because bringing the total number of missing elements to seven. The race to of Moseley’s discoveries. All of the elements above uranium (atom- discover these seven elements proved to be contentious and dramatic, ic number 92) were unknown in Moseley’s time. Moseley’s work with protracted priority disputes arising among the participating proved that three elements were missing from the periodic table scientists. (Image of the periodic table of the elements adapted from at the time (atomic numbers 43, 61, and 75). Others soon used his Brian D’Alessandro, www.briandalessandro.com.) www.americanscientist.org © 2014 Sigma Xi, The Scientific Research Society. Reproduction 2014 September–October 363 with permission only. Contact [email protected]. The next element among the seven husband-and-wife team of Walter and to be found was element 72, which was Ida Noddack, as well as Otto Berg, all discovered by Dirk Coster and George German chemists, after a great deal of Hevesy, working at the Niels Bohr In- painstaking extraction work. Although stitute for Physics in Copenhagen in the discovery of this element did not 1923. It was given the name hafnium cause any controversy, the Noddacks (Hf) after the Latin name for that city, and Berg also claimed to have discov- which is Hafnia. But before the discovered element 43, which they named Uup ery of this element was settled, a rather masurium. This second claim failed to acrimonious priority dispute broke out stand up to the experimental evidence among several participants and their obtained in other labs, yet the Nod- followers. Urbain, whose sample of cel- dacks refused to withdraw their claim. tium had been dismissed by Moseley in Unlike those others, element 43 was 1914, was now claiming his discovery not discovered in nature due to its very was the missing element 72. short half-life, but was artificially syn- The dispute over element 72 took on thesized in 1937, and was eventually almost comical nationalistic overtones given the name technetium (Tc) because The most recently confirmed element on the when the British newspapers sided of its artificial or “technical” origins. periodic table has atomic number 115 and is with Urbain on the basis that France The Italian physicist Emilio Segrè had unofficially known as Unupentium (Uup). It was first synthesized in 2003 but was not had been an ally of England during the spent a period of time working at the officially confirmed until 2013. recently completed World War I. Mean- University of California at Berkeley. while the French press declared in one After returning to his home institution headline “Ça pue le Boche” or “It stinks in Palermo, Sicily, he received a plate element were discovered, the priority of the Hun” to describe the discovery made of molybdenum that had been should be accorded to the discoverer of in Copenhagen. Ironically, Denmark irradiated. On analyzing this substance the longest-lived isotope. Clearly this had been neutral during the war, and with the help of a chemist, Carlo Per- case presented no contest in view of the neither Coster nor Hevesy were either rier, they discovered that a completely vast difference in the half-lives of the German or Danish. Nevertheless the new element had been created. It was two isotopes in question. men from Copenhagen had the last to be the first of what now amounts to As for the British team, they accept- word, because they observed several almost 30 elements that have been arti- ed that they had failed to provide the x-ray spectral lines at precisely the fre- ficially produced and have taken their chemical support for the claim of an el- quencies predicted by Moseley’s law places in the periodic table. ement one place before uranium. Inci- for an element of atomic number 72. Element 87 was falsely claimed by dentally, the name protactinium comes Element 75, rhenium (Re), was next. several people who believed that they from the fact that element 91 is a pre- It is a metal discovered in 1925 by the had isolated it. It was finally discovered cursor to the formation of element 89 or actinium. If protactinium loses an Several prominent women scientists featured highly in the discovery of new elements. Ida alpha particle it undergoes a decrease Noddack (left) helped identify rhenium, element 75, in 1925. Marguerite Perey (middle) in atomic number of two units, result- discovered francium, element 87, in 1939. And Lise Meiner (right) helped definitively find ing in the formation of actinium. protinactium, element 91, in 1917. Science Source (3)

364 American Scientist, Volume 102 © 2014 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. in 1939 by a French laboratory techni- element beyond uranium, and so be- prevent scientists from being sent into cian, Marguerite Perey, who had been gan the extension of the periodic table front-line combat in a time of war. trained by Marie Curie, one of the early from atomic numbers 93 to, so far, 118. Would it not be a fitting tribute to pioneers of the study of radioactivity Although many of these elements are Moseley’s legacy if a new element who was also responsible for coining too unstable to be of commercial im- were to be named after him? Unfor- the name for this field. Eventually Perey portance, their synthesis provides new tunately there is currently a regulation earned a Ph.D. and rose to the rank of understanding of nuclear stability and by the International Union of Pure and professor of nuclear chemistry. Her radioactivity, especially under extreme Applied Chemistry that requires that work involved carrying out the care- conditions of very high charge, and also any name given to an element, which ful and rapid handling of radioactive can be used to test relativistic quantum later turns out to be spurious, can nev- isotopes. As it turns out, element 87, theories of atoms. (However, several of er be used again. In 1924 two chemists, C. H. Bosanquet and T. C. Keeley, believed they had extracted element 43 and proposed to name it mosleyum. It American and Russian facilities that was soon shown that their element did not in fact exist. competed to find elements during the Nevertheless regulations can be changed. For example when it was Cold War now collaborate on synthesis. first suggested that element 106 should be named seaborgium (Sg) after Amer- ican chemist Glenn Seaborg, there was a good deal of resistance because this which she named francium (Fr), was the these elements have found industrial too would mean breaking an official last natural element to be discovered. use. For instance, californium is used rule concerning how elements are Estimates of the abundance of franci- in medical imaging and americium is named. This rule was that an element um suggest that there is only about 30 used in home smoke detectors.) cannot be named after a person who grams of it in the whole of the Earth’s This work too has had its share of was still living, and Seaborg was still crust, and its longest-lived isotope has a controversy and priority disputes. For very much alive at the time. However half-life of just 21 minutes. example, during the height of the Cold he had been involved in the discov- The last two of the seven elements War, two of the few facilities that are ery of ten transuranium elements, and were also claimed by several chemists capable of creating such elements— that may have contributed to the inter- and physicists but were only definitely one at Berkeley and the other at Dub- national naming commission’s finally identified after they had been artifi- na in Russia—began a long-running relenting and officially naming one of cially synthesized. Element 85 was dispute as to which site had first pro- these elements after Seaborg. synthesized in 1940 by Dale Corson, duced a sequence of transuranium ele- The periodic table was given a new Kenneth Ross MacKenzie, and Segrè ments. Even more controversial per- lease of life by Moseley’s work, which in the United States. They called it haps was the case of the yet unnamed enabled it to become a far more exact astatine (At) after astatos, the Greek element 118 that was first claimed by and complete system than it had been word for unstable. Indeed the element the American team but later retracted, when it was based solely on macro- has no stable isotopes, but a few years before finally being genuinely discov- scopic chemical and physical properties. later, other researchers discovered that ered in 2006 in Dubna. It is unofficially It would be only appropriate that his it was a natural product of several ra- known as Ununoctium (Uuo). name be used for one of the elements dioactive decay processes. However, in 2003, Russian scientists that has still not been officially named, The final element of the seven was from Dubna worked together with including 113, 115, 117, or 118—or even atomic number 61, promethium (Pm), Americans at Lawrence Livermore Na- one that has yet to be synthesized. which was synthesized in 1945 by tional Laboratory to put forth a claim of Americans Jacob A. Marinsky, Lawrence discovery for element 115, unofficially References Glendenin, and Charles D. Coryell. Like known at Ununpentium (Uup). The Heilbron, J. 1974. H. G. J. Moseley, The Life and astatine and technetium, promethium claim was ruled to be unsupported until Letters of an English Physicist, 1887–1915. Los has no stable isotopes. But promethium two other groups confirmed synthesis in Angeles: University of California Press. has found some specialized applica- 2013. The element’s longest measured Scerri, E. 2007. The Periodic Table, Its Story and Its Significance. New York: Oxford Univer- tions, notably the manufacture of atomic half-life is about 200 milliseconds. sity Press. batteries that have life times of five years In the current age where high-tech Scerri, E. 2013. A Tale of Seven Elements. New or more and thus lend themselves to synthesis seems to be the only means York: Oxford University Press. powering devices where battery chang- to discover new elements, it is ap- es are dangerous to impossible, such as propriate to remember that Henry in pacemakers and spacecraft. Moseley’s atomic number criterion still serves to identify any element. For relevant Web links, consult this Going Beyond Uranium His unfortunate death, so soon after issue of American Scientist Online: Even before the last of these elements his crucial discovery, was lamented http://www.americanscientist.org/ was discovered, in 1940 Edwin McMil- by scientists on both sides in World issues/id.110/past.aspx lan at the University of California at War I. Among other things, it led to Berkeley succeeded in synthesizing an the implementation of regulations to www.americanscientist.org © 2014 Sigma Xi, The Scientific Research Society. Reproduction 2014 September–October 365 with permission only. Contact [email protected].

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