Plutonium
Chemical identification of plutonium-238 at Berkeley
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Original prompt & settings (JSON) ↗The plutonium identified during the night of 23–24 February was never seen. It was a radioactive trace, pinned down by watching how it refused to behave like thorium.
The oxidation test in Room 307
During the night of 23–24 February 1941, the graduate student Arthur C. Wahl performed the decisive oxidation experiment in Room 307 of Gilman Hall at the University of California, Berkeley. It showed that the radioactive material behaved chemically unlike every known element, supporting its identification as element 94.
The team had already detected an alpha-emitting daughter product from neptunium-238, but needed chemical evidence separating it from known elements, particularly thorium. Wahl's experiment supplied that evidence.
Where the material came from
The substance was plutonium-238. It was produced after uranium was bombarded with deuterons in Berkeley's 60-inch cyclotron, creating neptunium-238 that beta-decayed into plutonium-238.
That bombardment had taken place on 14 December 1940. The February date commemorates the conclusive chemical identification, not the first nuclear production of the material.
What “isolated” overstates
This was a trace-scale chemical separation rather than the isolation of a visible piece of plutonium metal. The first visible amount was obtained in August 1942, and plutonium was first weighed that September.
The experiment also crossed midnight, which is why the record wobbles: Glenn T. Seaborg dated it to the night of 23–24 February, some timelines therefore use 24 February, and one chemical-history account describes the isolation work as spanning 23–25 February.
Seaborg, who directed the element-94 investigation, later credited the discovery to Wahl, Joseph W. Kennedy, Edwin M. McMillan and himself.
Sources
Researched 23 Aug 2026 6 sources 2 audit passes Traditional date
How this was checked
Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →
Traditional date. This date is the conventional one, not one fixed by a contemporary record.
What the sources leave uncertain
- The discovery experiment crossed midnight. Seaborg dated it to the night of February 23–24; some timelines therefore use February 24, while a chemical-history account describes the isolation work as February 23–25.
- Plutonium-238 had first been produced in a December 14, 1940 bombardment. The February date commemorates its conclusive chemical identification, not its initial nuclear production.
- The word “isolated” can imply too much: this was a trace chemical separation. Visible and weighable plutonium was not obtained until 1942.