Berkelium – facts and properties
- Symbol
- Bk
- Atomic number
- 97
- Atomic mass
- 247 u
- Period
- 7
- Category
- actinide
- Electron configuration
- [Rn] 5f⁹ 7s²
- Electronegativity
- 1.3
- Valence electrons
- 11
- State at standard conditions
- solid
- Melting point
- 1050 °C
- Density
- 14 g/cm³
- Discovered
- 1949
Berkelium is a silvery, radioactive metal with atomic number 97. It does not exist in nature. Glenn Seaborg, Albert Ghiorso and Stanley Thompson first made it in 1949 at the University of California in Berkeley, and the name recalls the city. Today a research reactor in Oak Ridge, Tennessee, produces it in quantities of milligrams, and one such sample supplied the target material with which tennessine was discovered in 2010.
Position in the periodic table
Occurrence and extraction
The longest-lived isotope, berkelium-247, has a half-life of about 1400 years. That is tiny next to the age of the Earth, so any original berkelium has long since decayed. For research, berkelium-249 matters more, with a half-life of just under a year.
In 1949 Seaborg, Ghiorso and Thompson bombarded americium-241 with alpha particles in a cyclotron and obtained berkelium-243. The amount was far too small to see. They recognised the new element by its radiation and by how it behaved in an ion-exchange column, a separation tube that releases chemically similar substances one after another. The name had a model: terbium, which sits above berkelium in the table, is named after the Swedish village of Ytterby, and berkelium was likewise named after the place where it was found.
Today it is made in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Curium is exposed to an intense stream of neutrons for months. The nuclei capture neutrons one after another and turn into heavier elements by beta decay, and berkelium-249 is among the end products. Chemists then separate it in shielded chambers. One production campaign yields only a few milligrams, and only a handful of facilities worldwide can do it.
Properties and reactions
Berkelium is in period 7 and is the ninth actinoid, in the actinoid row between curium on its left and californium on its right. Terbium sits above it in the lanthanoid row. The electron configuration of the atom is [Rn] 5f⁹ 7s². The electronegativity of 1.3 is an estimate, and the density is 14 g/cm³. The melting point of 1050 °C is also only an approximation, because only tiny samples of the metal were ever available. Its atomic mass is given as 247 u, the mass number of berkelium-247.
Berkelium metal is silvery and reacts with oxygen and with acids. In compounds the oxidation state plus three is the rule. A special feature is that tetravalent berkelium is unusually stable, for example in the dioxide BkO₂. The ion Bk⁴⁺ has a half-filled 5f shell of seven electrons, and such an arrangement is especially stable. Terbium above it behaves similarly, since its ion Tb⁴⁺ has a half-filled 4f shell.
Berkelium-249 gives off only low-energy beta radiation and is therefore a weak emitter. Its decay product californium-249, however, emits alpha and gamma radiation, so a sample becomes more strongly radioactive as time passes. If berkelium enters the body it settles mainly in the skeleton. It is therefore handled only in closed, shielded facilities.
Research and synthesis
Berkelium has no everyday applications. Berkelium-249 became famous as target material for a new element. In 2009 Oak Ridge sent a sample of a little over twenty milligrams to the Joint Institute for Nuclear Research in Dubna, Russia. The material had to be used quickly, because with its half-life part of it turned into californium-249 on the way.
Researchers from Russia and the United States bombarded the sample with calcium-48 ions. When the nuclei fused, single atoms of element 117 formed from mid-2009, later named tennessine after the state where Oak Ridge lies. The proton numbers of berkelium and calcium add up exactly to that of the new element.
Beyond that, berkelium serves basic research. Its tetravalent chemistry provides reference values for testing how bonding changes along the actinoid series.
Quick question
What is the atomic number of berkelium (Bk)?
FAQ
Who or what is berkelium named after?
It is named after the city of Berkeley in California, where it was discovered. The model was terbium above it, which is named after the Swedish village of Ytterby.
What is berkelium good for?
It plays no role in daily life. Its most important role was as the target material with which Russian and American researchers made tennessine. Otherwise it serves basic research.
How dangerous is berkelium?
It is radioactive and harmful inside the body, especially when it lodges in the skeleton. From outside, the low-energy beta radiation of berkelium-249 is easy to shield. Yet it exists only in milligram quantities in specialised facilities.