Berkelium (Bk)

• Element Symbol: Bk
• Atomic Number: 97
• Atomic Mass: 247
• Group # in Periodic Table: n/a
• Group Name: Actinides
• Period in Periodic Table: 7
• Block of Periodic Table: f-block
• Discovered by: Stanley G. Thompson, Glenn T. Seaborg, Kenneth Street Jr., Albert Ghiorso (1949)

Berkelium is a silvery radioactive element that is part of the actinide series in the periodic table. The actinides, which are the metals with atomic numbers between 89 and 103, also include actinium, thorium, uranium, plutonium, americium, curium, einsteinium, nobelium, and lawrencium. Some actinide elements, such as uranium and thorium, have isotopes that are naturally abundant within minerals in Earth’s crust. Radioactive decay of these natural isotopes produces other actinides, such as actinium. Not all actinides, however, are produced in nature. Heavier actinides must be produced synthetically. Berkelium is a synthetic actinide that is produced within particle accelerators or nuclear reactors. It can also be produced during the explosion of a thermonuclear bomb.

Berkelium is a transuranium element. This category of elements includes all those with atomic numbers greater than 92, the atomic number of uranium. Most actinides are also transuranium elements, and they are all radioactive and unstable. The transuranium actinides were discovered between 1940 and 1961.

Berkelium was discovered in 1949 by a team of American physicists at the University of California, Berkeley. This team included Stanley G. Thompson, Albert Ghiorso, and Glenn T. Seaborg. These scientists had spent many years producing a sample of americium-241 that was large enough to use as a target within a cyclotron particle accelerator. After setting up this americium-241 target, they bombarded it with helium nuclei. The target was then dissolved in acid and subjected to ion exchange. A sample of a new element with the atomic number 97 was obtained through this process. This sample was so small that it was invisible to the naked eye. However, it could be detected using chemical analysis. The team named this element berkelium, after Berkeley, its place of origin. Berkelium’s associated lanthanide element, terbium (the element directly above berkelium in the periodic table), was also named after the place where it was discovered. In the case of terbium, this was Ytterby, Sweden.

Berkelium was one of many transuranium actinide elements to be discovered by researchers at Berkeley. In fact, Seaborg, who was part of the team to discover berkelium, was also involved in the discovery of the actinides plutonium, curium, americium, californium, einsteinium, fermium, mendelevium, and nobelium. Early funding for work on radioactive elements in the United States was linked to the Manhattan Project, which had resulted in the development of the atomic bomb during World War II.

Physical Properties

Berkelium is a silvery metal that is solid in its standard state at 298 kelvins (K). Many properties of berkelium are unknown because only tiny samples of this element have been produced. Berkelium has a density of 14.78 grams per cubic centimeter (g/cm³) at standard state. Its melting point is 1050 degrees Celsius (°C). Its boiling point is unknown. Also unknown are its specific heat, electrical conductivity, resistivity, and magnetic type. The thermal conductivity of berkelium is 10 watts per meter-kelvin (W/m·K).

Chemical Properties

Berkelium has a simple, close-packed, hexagonal crystal structure. Its Bk⁴⁺ ion has been shown to fluoresce. This element is chemically reactive. When exposed to air, it produces an oxide layer on its surface. Berkelium forms compounds with many nonmetallic elements. It also dissolves in aqueous mineral acids.

The electron affinity of berkelium is unknown. This element has four valence electrons. Its electronegativity is 1.3. Its ionization energy is 601 kilojoules per mole (kJ/mol). Berkelium has two oxidation states when in aqueous solution: +3 and +4. Its trivalent (+3) oxidation state is the more common of the two. The solubility properties of berkelium are analogous to those of cerium and other lanthanide and actinide elements in these oxidation states.

Berkelium has no stable isotopes. It has twenty known radioactive isotopes, with mass numbers ranging from 235 to 254. These isotopes decay through alpha emission. Half-lives of these isotopes range from a few seconds to almost 1,400 years. The first berkelium isotope discovered, berkelium-243, has a half-life of around 4.5 hours. The isotope produced in the largest quantities, berkelium-249, has a half-life of 314 days. The longest-lived isotope, berkelium-247, is the one whose half-life is around 1,400 years. Berkelium has an electron configuration of [Rn]5f⁹7s².

Applications

Berkelium is a very rare element. It is not found in nature and is only produced in small quantities within particle accelerators or nuclear reactors. Within these devices other lanthanide and actinide elements are also generally present. Berkelium can be separated from most of these other elements using liquid-liquid extraction, extraction chromatography, precipitation of iodate, ion exchange, or a combination of these procedures. Separation of berkelium from cerium, however, involves an extra step. This is due to the similarity in redox behavior in these two elements.

After the discovery of berkelium in 1949, some effort was made to produce enough of the element for tracer analysis. This effort allowed scientists to study the stability of berkelium and its ability to combine with other elements in aqueous solution. However, it was not until 1958 that scientists produced a sample of berkelium metal large enough to see. This sample was also used to determine the absorption spectrum and the magnetic susceptibility of the element. The compound berkelium dioxide was then studied in 1962.

Neutron irradiation of plutonium, americium, and curium within nuclear reactors in the United States produced enough berkelium-249 to create a sample of 0.73 grams between 1967 and 1985. Another 0.28-gram sample was produced in the United States between 1986 and 2001. This material has been used as a target within particle accelerators to produce heavier elements such as lawrencium, rutherfordium, and bohrium.

Berkelium-249 decays rapidly to form californium-249. A berkelium-californium alloy forms as the berkelium isotope decays. This process has made berkelium-249 useful to chemists studying the properties of californium. Researchers working with this actinide alloy must take safety precautions. Radioactive berkelium-249 is not generally dangerous to humans, as it emits low-energy beta particles; however, its decay product, californium-249, emits high-energy neutrons and alpha particles that can cause devastating damage to health.

Bibliography

Aldersey-Williams, Hugh. Periodic Tales: A Cultural History of the Elements, from Arsenic to Zinc. New York: Viking, 2011. Print.

"Berkelium." Periodic Table. Royal Soc. of Chemistry, 2015. Web. 18 Sept. 2015.

"Berkelium (Bk)." Encyclopædia Britannica. Encyclopædia Britannica, 12 Nov. 2013. Web. 18 Sept. 2015.

Gray, Theodore. The Elements: A Visual Exploration of Every Known Atom in the Universe. New York: Black Dog, 2009. Print.

Hobart, David E., and Joseph R. Peterson. "Berkelium." The Chemistry of the Actinide and Transactinide Elements. Ed. Lester R. Morss, Norman M. Edelstein, and Jean Fuger. 4th ed. Vol. 3. Dordrecht: Springer, 2010. 1444–98. Print.

Parsons, Paul, and Gail Dixon. The Periodic Table: A Visual Guide to the Elements. New York: Quercus, 2014. Print.

"Technical Data for Berkelium." The Photographic Periodic Table of the Elements. Element Collection, n.d. Web. 18 Sept. 2015.