Berkelium Facts – Element Bk Atomic Number 97

Berkelium (Bk) is a synthetic, radioactive element in the actinide series of the periodic table. With the atomic number 97, it is the fifth transuranium element. Berkelium was first synthesized in 1949 at the University of California, Berkeley, after which it is named. Berkelium holds scientific importance due to its use in the synthesis of other heavy elements.

Discovery

Berkelium was discovered in December 1949 by a team of scientists led by Glenn T. Seaborg, Albert Ghiorso, Stanley G. Thompson, and Kenneth Street, Jr., at the University of California, Berkeley. The element was synthesized by bombarding americium-241 (²⁴¹Am) with alpha particles (helium nuclei) in a cyclotron:

₉₅²⁴¹Am + ₂⁴He → ₉₇²⁴³Bk + 2n

This reaction yields berkelium-243, an isotope with a half-life of about 4.5 hours.

The discovery of berkelium marked the seventh actinide synthesized and contributed to understanding the chemical properties of transuranium elements.

Naming

The element takes its name after Berkeley, California, the home of the University of California, Berkeley, where it was discovered. The name pays homage to the institution’s significant contributions to nuclear chemistry and the study of heavy elements.

Synthesis and Isolation

Berkelium synthesis typically involves bombarding lighter actinides, such as curium or americium, with neutrons or charged particles in nuclear reactors or particle accelerators. For example, berkelium-249, the most stable isotope, results from irradiating curium-244 with neutrons:

₉₆²⁴⁴Cm + n → ₉₇²⁴⁹Bk

Isolating berkelium is challenging due to its radioactivity, low production yields, and the need for sophisticated chemical separation techniques. Solvent extraction and ion-exchange chromatography separate berkelium from other actinides and fission products.

The element also results from the decay schemes of heavier isotopes, but this is not a practical route of synthesis.

Location on the Periodic Table

• Group: Actinides
• Period: 7
• Block: f-block
• Atomic Number: 97

Berkelium is located between curium (Cm) and californium (Cf) in the actinide series and below terbium (Tb). This puts it near the middle of the bottom row of a standard periodic table. Its chemical behavior is closely related to these elements due to similarities in electron configuration.

Appearance and Allotropes

Berkelium is a silvery-white metal at room temperature. It readily oxidizes when exposed to air, forming a thin oxide layer. Self-heating and ionization of air make this element faintly glow in the dark.

The most stable allotrope is the α form, with a double-hexagonal close packed (dhcp) crystal structure, similar to the alpha form of lanthanum. Berkelium transitions to the β form at room temperature under a pressure of 7 GPa. This allotrope exhibits a face-centered cubic (fcc) crystal structure. Further compression results in an orthorhombic  γ-berkelium form.

Physical Properties

Berkelium’s physical properties align with trends observed in the actinide series, displaying high density and melting points typical of heavy metals.

Chemical Properties

Berkelium behaves chemically like other actinides, exhibiting multiple oxidation states in aqueous solutions, primarily +3 and +4. The +3 state is the most stable, while the +4 state is typically observed in oxidizing conditions. Berkelium ions form complexes with various ligands, and its chemistry is similar to that of curium and californium.

Nuclear Properties and Isotopes

Berkelium has no stable isotopes. There are several radioactive isotopes, with mass numbers ranging from 235 to 254. The most notable isotopes are:

• Berkelium-249: Half-life of 330 days, used as a precursor for californium-249 and studies of heavier elements.
• Berkelium-247: Half-life of 1,380 years, the longest-lived isotope.

Berkelium’s isotopes primarily undergo alpha decay.

Origin, Abundance, and Sources

Any berkelium present when the Earth formed is now gone due to its short half-lives compared to geological time scales. Trace amounts exist in nature due to rare neutron capture events in uranium ores, but these quantities are negligible. It is a synthetic element, made in nuclear reactors or particle accelerators. The element remains at the site of nuclear weapons tests and nuclear accidents, such as Three Mile Island and Chernobyl.

Nucleosynthesis likely makes berkelium in certain stars. For example, it has potentially been detected in Przybylski’s Star, a rapidly oscillating Ap star.

Uses

Berkelium has limited practical applications, primarily due to its scarcity and radioactivity. Some uses include:

1. Scientific Research: Berkelium-249 is a target for synthesizing heavier elements like tennessine (Z=117).
2. Nuclear Studies: It aids in understanding the chemical and nuclear properties of actinides.

Oxidation States

Berkelium exhibits oxidation states of +2, +3 +4, and +5. The +3 state dominates in aqueous and solid-state chemistry, similar to other actinides.

Compounds

Because of the relatively large amount of berkelium that has been made, scientists know quite a bit about its chemistry. Berkelium forms compounds such as:

• Berkelium dioxide (BkO₂​): A black oxide where berkelium is in the +4 state.
• Berkelium chloride (BkCl₃​): A pale yellow salt with berkelium in the +3 state.
• Berkelium fluoride (BkF₃​): A stable compound with berkelium in the +3 state.

In addition to oxides, chlorides, and fluorides, the element also forms hydroxides, hydrides, several salts, and also organoberkelium compounds.

Biological Role, Health Effects, and Toxicity

Berkelium has no known biological role. Berkelium primarily emits alpha particles, which are easily shielded but cause severe damage if internalized. Its health risks are less than those of elements like plutonium or californium, as it produces lower gamma radiation and fission neutrons. However, its heat generation and radiotoxicity demand careful handling.

The primary risks arise from inhalation, ingestion, or accidental absorption through wounds. Based on animal studies, berkelium accumulates in the bones and lungs. Around 10% of the element gets excreted, with the remainder staying in the body anywhere from 20 to 50 years. The main risk comes from cancer.

Key Facts Table for Scientists

Interesting Facts About Berkelium

1. Berkelium was the first element synthesized in weighable quantities after plutonium.
2. It was instrumental in the discovery of heavier elements, such as tennessine.
3. Because it is an alpha particle emitter, handling it is safer than many actinides. However, this also means self-heating occurs.
4. Berkelium compounds glow faintly due to self-radiation.
5. Cooling berkelium below 34 K alters its magnetism from paramagnetic to antiferromagnetic.

References

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
• Kovács, Attila; Dau, Phuong D.; Marçalo, Joaquim; Gibson, John K. (2018). “Pentavalent Curium, Berkelium, and Californium in Nitrate Complexes: Extending Actinide Chemistry and Oxidation States”. Inorg. Chem. 57 (15). American Chemical Society: 9453–9467. doi:10.1021/acs.inorgchem.8b01450
• Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). “The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248”. Nuclear Physics. 71 (2): 299. doi:10.1016/0029-5582(65)90719-4
• Peterson, J. R.; Hobart, D. E. (1984). “The Chemistry of Berkelium”. In Emeléus, Harry Julius (ed.). Advances in Inorganic Chemistry and Radiochemistry. Vol. 28. Academic Press. doi:10.1016/S0898-8838(08)60204-4. ISBN 978-0-12-023628-2.
• Thompson, S.; Ghiorso, A.; Seaborg, G. (1950). “The New Element Berkelium (Atomic Number 97)”. Physical Review. 80 (5): 781. doi:10.1103/PhysRev.80.781