Bohrium – facts and properties
- Symbol
- Bh
- Atomic number
- 107
- Atomic mass
- 270 u
- Period
- 7
- Group
- 7
- Category
- transition metal
- Electron configuration
- [Rn] 5f¹⁴ 6d⁵ 7s²
- Valence electrons
- 7
- State at standard conditions
- solid
- Discovered
- 1976
Bohrium, atomic number 107, is artificial and radioactive. It was discovered in 1981 at GSI in Darmstadt and carries the name of the Danish physicist Niels Bohr. Bohrium shows well how one proves a new element whose first atoms lasted only a fraction of a second: by recognising it from its decays. As a member of group 7 it stands below rhenium. A single experiment on a few atoms supports the expectation that it behaves similarly, and every other property is predicted.
Position in the periodic table
Occurrence and extraction
Bohrium exists only in the laboratory. The known isotopes are radioactive. For the longest-lived, bohrium-270, half-lives of a few minutes are quoted, though the figure is uncertain. Many other isotopes decay after milliseconds or seconds.
The discovery was made by a team at GSI in Darmstadt, then called Gesellschaft für Schwerionenforschung, led by Peter Armbruster and Gottfried Münzenberg. They bombarded a thin bismuth-209 target with chromium-54 ions. This type of reaction is called cold fusion, because the merged nuclei carry little excess energy and so are likelier to survive. A separator divided the reaction products from the beam particles, and a detector logged every decay by position and time.
The decisive evidence was the decay chain. The newly formed bohrium-262 decays by alpha radiation into a daughter nucleus, that into another, and the chain leads down to isotopes that were already known. That is how a new element is identified: through its descendants. A group at Dubna had reported signs as early as 1976, but an international working group did not find that evidence convincing and recognised GSI as the discoverer.
The element honours Niels Bohr, whose atomic model of 1913 placed electrons on orbits of fixed energy. GSI first proposed nielsbohrium. IUPAC chose the shorter form in 1994, and in 1997 the name bohrium was internationally accepted.
Properties and reactions
Bohrium is in period 7 and group 7, below rhenium, with technetium and manganese above that. Its predicted electron configuration is [Rn] 5f¹⁴ 6d⁵ 7s². The atomic mass is given as 270 u, the mass number of bohrium-270. Melting point, density and most other properties are unknown. A dense metal is expected, but that is a guess, not a measurement.
Its place below rhenium leads to a clear supposition: bohrium should reach the oxidation state plus seven like rhenium and form volatile oxide chlorides. An experiment in 2000 at the Paul Scherrer Institute in Switzerland put this to the test. The researchers made bohrium atoms from berkelium and neon ions, passed them into a mixture of hydrogen chloride and oxygen, and detected six atoms that formed a volatile compound, interpreted as bohrium oxide chloride, BhO₃Cl. It behaved much like the corresponding rhenium compound, though it was slightly less volatile, as calculations had predicted. With only six atoms the conclusion stays cautious, but it agrees with a position in group 7.
Research and synthesis
Bohrium is put to work in research alone. Its importance lies in the method that led to its discovery. Identifying an element through the chain of its decay products was not new, but GSI refined it with a separator and position-sensitive detectors until single atoms sufficed. Soon afterwards it found meitnerium and hassium the same way, and the approach is still the standard.
Bohrium is also a testing ground for theory. The stability of superheavy nuclei depends on shell effects, and for bohrium isotopes one compares measured half-lives with predictions. If a nucleus lives clearly longer than calculated, that says something about how its protons and neutrons are arranged.
Making it is laborious. The reactions occur with very low probability, and days to weeks of irradiation yield single atoms.
Quick question
What is the atomic number of bohrium (Bh)?
FAQ
Who is bohrium named after?
The Danish physicist Niels Bohr, who in 1913 presented an atomic model with fixed electron orbits and later received the Nobel Prize for it. The name was accepted internationally in 1997.
How was bohrium discovered?
At GSI in Darmstadt, bismuth was bombarded with chromium ions and the resulting nucleus was followed through several decay steps. Because the chain led to already known isotopes, the assignment to a new element was unambiguous.
Is bohrium like rhenium?
According to the first chemistry experiment, on a few atoms, yes: like rhenium it forms a volatile compound with oxygen and chlorine. Everything else is predicted.