Rutherfordium (Rf)

For most of the second half of the twentieth century, element 104 had two names, and which one you used depended on which side of the Iron Curtain your textbook was printed on. Soviet and Eastern European tables gave it as Ku, kurchatovium. American and Western European tables gave it as Rf, rutherfordium. Both appeared in peer-reviewed journals simultaneously, describing the same square, for close to thirty years.

Two claims, from two accelerators

Dubna went first. In 1964 Georgy Flyorov's group bombarded plutonium with neon ions and reported spontaneous fission events they attributed to element 104, proposing the name kurchatovium after Igor Kurchatov, who had directed the Soviet atomic programme. The identification rested on fission half-lives, and the half-life they reported was one that nobody else could reproduce; Dubna itself revised the value more than once.

Berkeley published in 1969. Albert Ghiorso's group used a californium target under a carbon beam and identified two isotopes by their alpha decay chains, each chain ending in a nobelium isotope whose properties were already known — a far more convincing form of evidence, because it ties the new nucleus to established ground rather than to a single number. They proposed rutherfordium, after Ernest Rutherford.

Neither side accepted the other's work, and the argument was not only technical. Honouring Kurchatov was read in the United States as honouring the man who built the Soviet bomb, and the objection was made in print.

What IUPAC decided, and what it almost decided

The IUPAC/IUPAP Transfermium Working Group reported in 1992 that the credit for element 104 should be shared. Neither the 1964 nor the 1969 experiment was decisive on its own; together they established the element.

The naming took another five years and produced one genuine surprise. IUPAC's 1994 draft recommendation assigned element 104 the name dubnium — a nod to the Soviet laboratory, attached to the square Berkeley had wanted. That proposal was rejected loudly, principally by the American Chemical Society, and in the 1997 settlement element 104 became rutherfordium after all while dubnium moved down one place.

So the square has, at various points and in various publications, been legitimately called kurchatovium, rutherfordium and dubnium. No other element has had three serious names in circulation.

The experiment that ended the actinide series

Element 104's real scientific importance has nothing to do with its name. It is the first element past the actinides, and until somebody checked, that was a prediction rather than a fact.

Seaborg's actinide concept said the 5f shell fills from thorium to lawrencium and then stops, after which the 6d shell resumes and element 104 becomes a heavier hafnium. The alternative — that the f-block kept going — was not obviously wrong. The test was chemical: if element 104 is a group 4 metal, it should follow zirconium and hafnium through a separation and leave the actinides behind.

In 1970 a team at Berkeley led by Robert Silva did exactly that, with a handful of atoms, and element 104 went with the group 4 metals. The actinide series ends at 103. That single result is why the modern periodic table is drawn the way it is.

A group 4 metal that is not always well behaved

Later work complicated the tidy picture in interesting ways. Rutherfordium is a group 4 element, but it does not simply extrapolate from zirconium and hafnium.

Its chloride and bromide are more volatile than hafnium's, in the opposite direction to the trend down the group. In fluoride solution its complexes are weaker than expected, and in some extraction systems it tracks plutonium rather than its own group. These are the fingerprints of relativistic effects: the inner orbitals of a nucleus with 104 protons are contracted and stabilised enough to change how the outer ones bond, so periodic trends established over five lighter rows begin to bend.

Everything known about rutherfordium's chemistry has been established a few atoms at a time. Its most durable isotope, rutherfordium-267, lasts a little over an hour; the ones used in most experiments survive for seconds. No sample of the element in bulk exists, and the melting point and density sometimes quoted for it are calculated values rather than anything that has been measured.