An artificially produced, highly unstable, transuranic rare earth element of the actinide series with no detectable natural occurrence on Earth.

Berkelium is made in minute amounts within nuclear reactors by the neutron bombardment of plutonium.

Berkelium exhibits two valences, +4, +2, and the commonly expressed +3. Amongst the compounds prepared are berkelium monoxide, BkO, the sesquioxide, Bk₂O₃, and the dioxide, BkO₂. The halides known include berkelium trifluoride, BkF₃, trichloride, BkCl₃, and tetrafluoride, BkF₄. Berkelium has not been prepared in its elemental form, but like most actinides, it is expected to be a silvery metal.

Berkelium accumulates in bones, and due to its radioactivity, the maximum permissible burden of berkelium in human bones is 0.4 nanograms.

The longest lived isotope of berkelium is berkelium-247 with a halflife of 1400 years decaying by alpha decay into the alpha decaying americium-243 which has a halflife of 7370 years.

Altogether, 11 isotopes of berkelium are known, all radioactive, and ranging from the inverse beta decaying berkelium-240 which has a halflife of 5 minutes to the beta decaying berkelium-251 with a halflife of 56 minutes.

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