Atomic number 97

Berkelium

Bk
+ phase
− phase

Shells Orbitals 5f 7s

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97 electrons in 7 shells: 2, 8, 18, 32, 27, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Atomic mass: 247.07031 Da (longest-lived isotope)
Electron configuration: [Rn] 5f⁹ 7s²
Phase at 25 °C: solid
Melting point: 986 °C
Boiling point: 2627 °C
Density: 14 g/cm³
Electronegativity (Pauling): 1.3
Discovered: December 1949, Stanley G. Thompson, Glenn T. Seaborg, Kenneth Street, Jr. and Albert Ghiorso

Neighbours in the table

Cm Cf

In December 1949 a team at the University of California, Berkeley, fired alpha particles at americium-241 and made element 97. The reaction, in IUPAC’s notation, was ²⁴¹Am(⁴He, 2n)²⁴³Bk, run in the Berkeley 60-inch cyclotron. Credit goes to Stanley G. Thompson, Glenn T. Seaborg, Kenneth Street, Jr. and Albert Ghiorso in Jefferson Lab’s account, and to Thompson, Seaborg and Ghiorso in IUPAC’s. The element took the name of Berkeley, the city of its discovery, Los Alamos says.

The new element was recognised by its chemistry. A fresh electron-capture activity, in which a nucleus absorbs one of its atom’s electrons, washed off a chromatography column just ahead of curium, Los Alamos records. That activity was assigned to mass number 243. Berkelium-243 halves in 4.6 hours for the Atomic Mass Data Center. The centre gives about 100 per cent beta-plus decay and about 0.15 per cent alpha decay.

Los Alamos counts berkelium as the eighth actinide, opening the second half of the series. Early tracer experiments, on amounts too small to weigh, showed Bk(III), berkelium in the +3 oxidation state, stable in water and Bk(IV) within reach. The metal and both states are now known in bulk, with some evidence for Bk(II), Los Alamos records.

Where it is found

No berkelium occurs naturally in the Earth’s crust, IUPAC states, and it has no stable isotope. PubChem gives the mass of its longest-lived isotope, berkelium-247, 247.07031 Da. That isotope halves in 1,380 years, give or take 250, with alpha decay for 100 per cent of decays, in the evaluation of the International Atomic Energy Agency (IAEA). It becomes americium-243, Jefferson Lab records; Haz-Map gives its half-life as 1,400 years. The Atomic Mass Data Center also lists spontaneous fission, a nucleus splitting unprompted, as a possible branch of berkelium-247.

Tracer experiments came first. They could not complete the study of the element on their own, so a campaign began in 1952, Los Alamos explains. It irradiated about 8 grams of plutonium-239 for a long period in a reactor at Arco, Idaho. In 1958 Cunningham and co-workers recovered, separated and purified berkelium-249 from it. The Atomic Mass Data Center gives nuclides from berkelium-233 to berkelium-254.

Why it matters

With that 1958 sample, chemists measured the light dissolved berkelium absorbs and how strongly Bk(III) responds to a magnetic field, Los Alamos records. 1962 brought two firsts. The first visible amount of a berkelium compound was about 3 billionths of a gram of BkCl₃, Jefferson Lab records. The first structure came from 4 nanograms of berkelium-249 dioxide, whose X-ray pattern fitted a face-centred cubic crystal, in Los Alamos’s account. The first metal samples above a microgram followed in 1969, when Haire, Peterson and co-workers reduced BkF₃ with lithium vapour.

Berkelium-249 halves in 327.2 days in the IAEA’s evaluation and 330 days for Los Alamos. In the IAEA’s evaluation, 99.9986 per cent of its decays are beta-minus, the release of an electron from the nucleus. Alpha decay takes 0.00137 per cent and spontaneous fission 4.7 × 10⁻⁸ per cent. The alpha branch is 0.00145 per cent in the Atomic Mass Data Center’s figures. That half-life lets berkelium-249 be isolated and studied in amounts large enough to handle, IUPAC notes.

Its relatively long half-life and microgram supply make berkelium-249 a common target for building heavier elements by charged-particle bombardment, Los Alamos says. It served as the target for tennessine, in IUPAC’s record. Berkelium helps chemists extrapolate to heavier elements, where scarcity, short half-lives and intense radioactivity limit experiments.

Properties

Properties of berkelium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	97
Atomic mass	247.07031 Da	Da	PubChem, longest-lived isotope
Melting point	986 °C	°C	Los Alamos: 986 °C
Boiling point	2627 °C	°C	Los Alamos: 2627 °C
Density	14 g/cm³	g/cm³
Electronegativity (Pauling)	1.3
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁹
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Berkelium’s hazard is ionising radiation from its isotopes, and it has no use outside basic scientific research, Jefferson Lab records. Berkelium-247, made in nuclear reactors or particle accelerators, is the element’s most important radionuclide in Haz-Map’s summary in PubChem, an alpha emitter rated very high for internal toxicity. Like other actinides, berkelium tends to gather in the skeleton, Los Alamos says, and studies reported by IUPAC found it building up in rats’ skeletons. Its radiation is also unfavourable to the formation of red blood cells, IUPAC adds, and PubChem’s Laboratory Chemical Safety Summary for berkelium is the reference sheet.

Questions

Why is berkelium called berkelium?

Berkelium is named for Berkeley, California, where it was discovered, Los Alamos says; IUPAC calls it the town in California where the element was first synthesised. Stanley G. Thompson, Glenn T. Seaborg, Kenneth Street, Jr. and Albert Ghiorso made it there at the University of California in December 1949, in Jefferson Lab's account, by striking americium-241 with alpha particles.

What is berkelium used for?

Berkelium has no commercial use because of its rarity, Los Alamos says. No use for the metal or its compounds is known outside basic scientific research, according to Jefferson Lab. Inside the laboratory, berkelium-249 is bombarded with charged particles to build heavier elements, according to Los Alamos, and IUPAC records that it served as the target when tennessine was produced.

Is berkelium radioactive?

Yes. Berkelium has no stable isotope at all, Jefferson Lab records. Its longest-lived one, berkelium-247, lasts about 1,380 years and turns into americium-243 by alpha decay, in Jefferson Lab's words. Haz-Map treats berkelium-247 as the most important radionuclide of the element, made in nuclear reactors or particle accelerators, and rates its internal toxicity as very high.

What does berkelium metal look like?

Berkelium metal is silvery, according to Los Alamos, and it dissolves easily in dilute mineral acids. Heated in air or oxygen, it quickly forms an oxide. The solid metal has two crystal forms, Los Alamos records: face-centred cubic, which is cubic close packing, and a double hexagonal close-packed form. It melts at 1050 °C in Jefferson Lab's figure and at 986 °C in Los Alamos's.