Plutonium (Pu)
“Nagasaki’s bomb metal is also Voyager’s battery metal — a few kilograms of Pu-238 aboard the probes still whisper power home from beyond the Solar System.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
The notes
Characteristics
- Electron configuration [Rn] 5f⁶ 7s² — period 7, f-block actinide with eight outer electrons; unusually for this stretch of the row it obeys the plain aufbau filling, while compensating with six allotropic crystal forms.
- Pu-239 (half-life 24,100 years) is fissile: slow neutrons split it readily, sustaining the chain reactions of atomic bombs and of MOX-fuelled power reactors.
- Pu-238 (half-life 87.7 years) is a self-warming isotope yielding about 0.57 watts per gram from pure alpha decay — no turbine, no moving parts, just steadily hot metal for decades.
- Fiendishly hazardous: inhaled dust lodges in lung and bone for a lifetime, and solid pieces can suddenly catch blue fire because fine plutonium powder is pyrophoric.
A silvery metal that dulls under a yellowish film, so a freshly cut piece soon looks like old brass. It is the strangest metal in the table: six crystal forms at ordinary pressure, and — against the habit of nearly all metals — it conducts electricity better as it gets hotter.
Where you meet it
✦ Uses
- Space batteries (RTGs): plutonium-238’s tireless heat turns into electricity aboard Voyager 1 and 2 — still calling home after more than 45 years — and powered New Horizons, which flew past Pluto in 2015 on plutonium’s own namesake.
- Mars rovers: Curiosity and Perseverance carry Multi-Mission RTGs, so plutonium keeps their cameras and drills alive through the dust storms that starve solar panels.
- MOX fuel: mixed-oxide pellets of PuO₂ and UO₂ burn surplus weapons plutonium in power reactors — swords melted into kilowatt-hours in France, Belgium and Japan.
- Nuclear weapons: Pu-239 cores triggered by implosion — the Nagasaki bomb and most of the world’s arsenals ever since.
- A medical curiosity: a few score plutonium-238 pacemakers fitted in the 1970s still tick inside elderly patients, never once needing a new battery.
◍ Everyday examples
- When the news reports that Voyager still transmits from interstellar space, the signal’s electricity is Pu-238 heat — the element’s gentlest job.
- New Horizons’ 2015 Pluto flyby handed physics teachers a perfect line: a plutonium-powered craft visiting Pluto.
- Every 9 August the world remembers Nagasaki — history’s second atomic bomb and its first plutonium one — and museum exhibits retell how the metal was born in secret.
History & name
In late 1940 at Berkeley, a team led by Glenn Seaborg — with Edwin McMillan, Joseph Kennedy and Arthur Wahl — bombarded uranium with deuterons, and by February 1941 they had made and chemically identified a new element: plutonium, first as the isotope Pu-238. Months later the same group prepared Pu-239 and found it fissile like U-235 — news so dangerous that the discovery stayed classified until 1948. The Manhattan Project scaled production from micrograms to kilograms at Hanford; in July 1945 the Trinity test proved the implosion design, and that August a plutonium bomb destroyed Nagasaki. In Los Alamos paperwork the secret metal masqueraded under the innocent codename “copper”. Seaborg’s boldest stroke was conceptual: his 1944 actinide concept carved out a whole second f-block row where plutonium and its kin finally belonged.