The Race to Fill the Last Gaps

By 1914 the search for new elements had stopped being open-ended. Henry Moseley's X-ray work had shown that each element occupies a whole-numbered slot set by its nuclear charge, which converted "how many elements are there?" from a philosophical question into a counting exercise. Between hydrogen at 1 and uranium at 92 there were exactly seven numbers with nothing assigned to them: 43, 61, 72, 75, 85, 87 and 91.

Seven holes, all of them findable in principle, and a technique — X-ray spectroscopy — that could prove a claim rather than argue for one. It still took twenty-six years, and four of the seven turned out not to be sitting in the ground at all.

The first one out was the easy one

Protactinium at 91 fell first, in 1917 and 1918, and it fell twice: to Otto Hahn and Lise Meitner in Berlin, and independently to Frederick Soddy and John Cranston in Glasgow. It was an easier target than the rest because it is a member of the uranium decay chain and therefore present wherever uranium ore is, in quantities that radiochemistry could work with.

That left six, and they turned out to be six of the hardest entries in the whole periodic table.

Element 72 was in the wrong place because the theory was wrong

Nobody could find element 72 for years, largely because everyone was looking in rare-earth minerals. Georges Urbain in Paris had announced it there in 1911 and called it celtium, and the prevailing assumption was that the lanthanides ran on far enough to include it.

Niels Bohr's account of electron shells said otherwise. If the 4f subshell closes where his model said it closes, element 72 is not a rare earth at all — it is a transition metal, a near-twin of zirconium, and the reason nobody had spotted it was that everybody was sieving the wrong rocks. Dirk Coster and George de Hevesy, working in Bohr's institute in Copenhagen, went straight to zirconium minerals and read the X-ray spectrum in December 1922. The lines were there. Nature carried the result within weeks, under a name that credited the institute rather than the mineral: hafnium, for the city Bohr's model had been argued out in.

It is one of the cleanest cases in chemistry of a theoretical prediction telling experimentalists where to point the instrument. Urbain's celtium sample, re-examined, contained no element 72.

Element 75 was found by the people who also got element 43 wrong

Walter Noddack, Ida Tacke and Otto Berg announced two elements in Berlin in 1925: number 75 from platinum ores and columbite, and number 43. The first became rhenium, named after the Rhine, and it stands as the last element with a stable isotope ever to be discovered. Everything found since has been radioactive.

The second claim, which they called masurium, was never reproduced by anyone and was withdrawn from the record. The judgement has softened somewhat over the decades — some later analyses have argued the Berlin group's X-ray evidence may have been genuine and simply beyond what their contemporaries could confirm — but the credit went elsewhere. Ida Noddack has a second and larger claim on posterity: in 1934 she published the suggestion that a bombarded uranium nucleus might split into several large fragments, which is nuclear fission, four years early. It was dismissed and largely forgotten until after the fact.

Four of the six had to be manufactured

The reason the remaining gaps resisted so long is that three of them contain no stable isotope and the fourth is vanishingly scarce. There was nothing in the rocks to find.

Element 43 was settled in 1937 by Carlo Perrier and Emilio Segrè in Palermo, working on a discarded molybdenum foil that Segrè had asked Ernest Lawrence to send him from the Berkeley cyclotron. Deuteron bombardment had converted a trace of the molybdenum into the missing element. They named it technetium a decade later, from the Greek for artificial, because it was the first element to be made rather than found.

Element 87 went the other way: it was found, in nature, in 1939, by Marguerite Perey at the Curie Institute. Purifying actinium-227, she noticed radiation that did not belong to any known decay product, and traced it to a minor alpha-decay branch of actinium — about one decay in a hundred — that produces an isotope of the missing alkali metal. She named it francium. No element found since has been spotted in nature before it was built in a laboratory. Perey had joined the institute as Marie Curie's technician at nineteen, with no degree, and was later the first woman elected to the French Académie des Sciences.

Element 85 was made in 1940 at Berkeley by Dale Corson, Kenneth MacKenzie and Segrè again, by firing alpha particles at bismuth-209. They called it astatine, from the Greek astatos, unstable — an honest name for a substance whose longest-lived isotope survives about eight hours. It does turn up in nature, as a fleeting step in decay chains, but it never accumulates, and how little of it stands in the crust at any instant is still unsettled: published estimates span more than an order of magnitude, all of them calculated from branching ratios rather than weighed. Astatine and francium have been swapping the title of rarest natural element on the strength of numbers of that kind ever since.

Element 61 was the last to fall. Jacob Marinsky, Lawrence Glendenin and Charles Coryell identified it at Oak Ridge in 1945 among the fission products of uranium, separated by the then-new technique of ion-exchange chromatography, and announced it in 1947. Coryell's wife suggested the name promethium, for the Titan who stole fire — a deliberately double-edged choice from people who had spent the war on the Manhattan Project.

Why those two middle-of-the-table elements had no stable isotope to find is a separate and rather elegant argument about their neighbours, covered in what makes an element radioactive.

The wrong answers matter as much as the right ones

Four false discoveries were announced for these gaps, and the pattern in them is instructive.

• Masurium for 43, Berlin 1925 — X-ray evidence, unreproducible.
• Illinium for 61, University of Illinois 1926 — followed almost immediately by a competing Italian claim, florentium, for the same element. Neither survived scrutiny.
• Alabamine for 85 and virginium for 87, both announced by Fred Allison at the Alabama Polytechnic Institute around 1930–31, using a "magneto-optic" method of his own devising. No other laboratory could reproduce a single result with it, and the technique was eventually discredited entirely.

Each of these was published in good faith by a working scientist, and each rested on an instrumental signal at the edge of what the instrument could resolve. That is exactly the regime in which a new element gets claimed, which is why replication rather than announcement is what actually decides these things. Where the discovery record for an element on this site is genuinely contested, its page names both parties rather than picking a winner quietly.

Three found in rock, four made in machines

Moseley turned the periodic table into a checklist with seven blanks. Protactinium came out of uranium ore; hafnium came out of zirconium ore once Bohr's model said to look there; rhenium came out of platinum residues and closed the roster of stable elements forever.

The last four could not come out of anything, because three of them have no stable isotope and the fourth barely exists. Technetium, astatine and promethium had to be built — in a cyclotron, in a cyclotron and in a reactor's waste — and francium was caught as a rare branch of a decay that had been studied for forty years without anyone noticing. The natural table was completed in 1945. Everything after that has been construction rather than discovery.