Bohrium

Bohrium is located below rhenium in group 7. To check whether it follows that group's trends, researchers produced individual atoms and studied how its compounds behave with oxygen and chlorine. The results were compatible with the expected properties of a transition metal of that group. Since only a few atoms are produced, the tests require indirect measurements.

When the German GSI team proposed 'nielsbohrium' to honor the brilliant Danish physicist, IUPAC considered that the full name created an uncomfortable precedent in chemical nomenclature, where traditionally only the scientist's surname is used (as with curium, fermium or einsteinium). Furthermore, the Russians had previously used the name nielsbohrium for element 105. After reconciling positions, in 1997 'bohrium' was formally adopted as the definitive and harmonious international name.

With a nuclear charge of one hundred seven protons, the electrons of the inner orbitals rotate at speeds close to light, increasing their relativistic inertial mass. This generates a radial contraction and enormous energy stabilization of the spherical 7s orbitals. As a counterpart, the 6d electrons remain unscreened and expand, facilitating that bohrium shares its five 6d electrons and two 7s electrons to form covalent bonds of maximum oxidation state of valence seven.

Studying bohrium chemistry required unprecedented international cooperation. Switzerland's Paul Scherrer Institute designed the miniaturized chromatography column; GSI's UNILAC accelerator in Germany supplied the heavy ion beam; and Dubna specialists contributed software and nuclear correlation detectors. This multinational alliance managed in 2000 to isolate six individual atoms of bohrium oxychloride, crowning one of the greatest technical feats of contemporary radiochemistry.

In superheavy element research, 'cold fusion' does not mean using low temperatures. It is a technique in which nuclei are collided with carefully adjusted energy to form a compound nucleus with relatively little excitation energy. This may favor survival long enough to be detected. The method was used in experiments to produce some very heavy elements, including bohrium.

When the bohrium atom is deposited on a silicon semiconductor detector chip, it measures with micrometric precision the arrival position and the exact energy of the alpha particle emitted upon decay. In that same pixel, the detector waits and records the decay of the daughter nucleus (dubnium), the grandchild (lawrencium) and the great-grandchild. If the energies and times match the theoretical sequence, the creation of the bohrium atom is confirmed unequivocally.

Since its initial discovery, the total amount of bohrium produced in all the world's laboratories does not exceed a few tens of atoms. Each atom requires weeks of continuous bombardment in gigantic accelerators at a multi-million cost of energy and equipment. Each time one is detected, its decay within a few seconds is recorded by computers as an extraordinary scientific event.