Bohrium (Bh)

In 2000 a group of chemists made a compound out of six atoms. Not six moles, not six milligrams — six individual atoms of bohrium-267, arriving one at a time over weeks, each of which survived long enough to react with a gas mixture, travel down a column, stick to a surface at a characteristic temperature, and then decay where it had landed. From those six events, element 107 was placed in group 7 of the periodic table.

Chemistry with a sample size of six

The experiment, led by Robert Eichler with Heinz Gäggeler's group and published in Nature, took the obvious question about element 107 and made it answerable. Technetium and rhenium, the group 7 elements above bohrium, both form volatile oxychlorides of the form MO₃Cl. If bohrium is really a group 7 metal, it should do the same, and the temperature at which its oxychloride sticks to a cold surface should extend the trend those two set.

It did. Bohrium formed a volatile compound consistent with BhO₃Cl, and it proved less volatile than the technetium and rhenium analogues — which is the direction the group trend predicts, and which was also what relativistic calculations had said in advance.

The statistical point is worth pausing on. Six atoms sounds like nothing, but the measurement is not a bulk property that needs averaging over many particles. Each atom independently either reaches the detector at a given temperature or does not, and the distribution of six such outcomes across a temperature gradient constrains the adsorption enthalpy to a usable range. This is what heavy-element chemistry actually is: not weighing things, but asking single atoms yes-or-no questions and counting the answers.

A claim, and then a chain

The reaction was never the difficulty. Dubna fired chromium-54 at bismuth-209 in 1976 and announced element 107 five years before anyone else managed it. What the group could show was spontaneous fission attributed to descendants rather than a decay sequence traced atom by atom, and on that evidence the claim went unaccepted.

In 1981 Peter Armbruster and Gottfried Münzenberg's group at the heavy-ion laboratory in Darmstadt made five atoms of bohrium-262 from the same combination of target and projectile, and this time followed each one down a full chain of alpha decays into nuclides that were already characterised. That is the standard of proof this field settled on, and it is why the Transfermium Working Group awarded element 107 to Darmstadt without splitting the credit.

Why it is not nielsbohrium

The discoverers proposed nielsbohrium, with the symbol Ns, and had a specific reason for including the first name: bohrium alone risked being confused with boron, particularly in languages where the two words sit closer together than they do in English.

IUPAC declined, on a general principle rather than a judgement about this case. Elements named after people use the surname only — rutherfordium, not ernestrutherfordium. The single apparent exception, curium, is not one: it honours two people who shared a surname, which is exactly why the plain form works there.

So element 107 is bohrium, and the name commemorates Niels Bohr, whose 1913 model of the atom introduced the quantised electron shells that the entire periodic table is now understood to be a map of. There is a certain fitness in attaching that name to a square whose occupant only makes sense once you allow that those shells are distorted by relativity.

Half-lives, and how few atoms exist

Roughly a dozen isotopes of bohrium are known. Bohrium-270 and bohrium-274 last on the order of a minute; most of the rest are gone in seconds or fractions of a second. The longer-lived ones are not made directly — they turn up partway down the decay chains of heavier elements, which is a recurring pattern up here: the most durable isotope of an element is often produced as somebody else's by-product.

Nothing else is known about element 107 from measurement. There has never been a piece of bohrium to put an instrument against, so every bulk property a data table prints for it arrived by calculation. What exists is a confirmed position in group 7, one compound inferred from six atoms, and a handful of decay chains.