Atomic number 94

Plutonium

Pu
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Shells Orbitals 5f 7s

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94 electrons in 7 shells: 2, 8, 18, 32, 24, 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: 244.06420 Da (longest-lived isotope)
Electron configuration: [Rn] 5f⁶ 7s²
Phase at 25 °C: solid
Melting point: 640 °C
Boiling point: 3228 °C
Density: 19.84 g/cm³
Electronegativity (Pauling): 1.28
Discovered: 1940/41, Glenn T. Seaborg, J. W. Kennedy, Edwin M. McMillan and A. C. Wahl

Neighbours in the table

Np Am

A sizeable lump of plutonium feels warm, heated by the energy of its own alpha decay, and Los Alamos notes that larger lumps give off enough heat to boil water. As for plutonium’s electrons, the configuration is [Rn] 5f⁶ 7s², with 2, 8, 18, 32, 24, 8 and 2 in successive shells. Los Alamos places the 5f electrons on a border between delocalised behaviour, spread across many atoms, and localised behaviour, held by one atom. That transitional position, in its account, gives plutonium physical and chemical complexities no other element shares.

The metal normally takes six allotropes, distinct crystal forms of one element, labelled with the Greek letters alpha to epsilon plus a delta prime. A seventh, zeta, appears at high temperature within a narrow range of pressure. Their densities run from 16.00 to 19.86 g/cm³. So many forms leave the metal sensitive to temperature, pressure and chemistry, Los Alamos explains, and allow dramatic changes in volume when one phase gives way to another.

At room temperature plutonium sits in the alpha form, as hard and brittle as cast iron. Alloying with gallium, aluminium, americium, scandium or cerium holds it in the delta phase instead, which makes it soft and ductile. Unlike most metals, it conducts heat and electricity poorly.

Where it is found

The US Environmental Protection Agency (EPA) treats plutonium as a man-made element, though natural traces have formed under highly unusual geological circumstances. PubChem gives the mass of its longest-lived isotope, plutonium-244, 244.06420 Da. That isotope’s half-life is 81.3 million years for the International Atomic Energy Agency (IAEA), 80.8 million according to Los Alamos and about 82 million in Jefferson Lab’s figure. IAEA assigns 99.877 per cent of decays to alpha emission and 0.123 per cent to spontaneous fission, the nucleus splitting unprompted.

Such a half-life is just long enough for traces to survive in nature, Los Alamos says. It calls plutonium the heaviest primordial element, one still present from the Earth’s formation. In 1971 Hoffman and co-workers measured plutonium-244 by mass spectrometry in plutonium separated from bastnasite of Precambrian age, a chloride-fluoride mineral of the lanthanides. The isotope had earlier been proposed to account for xenon isotope ratios in meteorites.

Uranium ores hold traces too. Most plutonium, though, is a reactor by-product: some fission neutrons are captured by uranium-238, which passes through uranium-239 and neptunium-239 to plutonium-239 by two beta decays.

Why it matters

Glenn Seaborg’s group made element 94 by striking uranium-238 with deuterons, heavy hydrogen nuclei, forming neptunium-238 that beta-decayed into the new element. Los Alamos dates the work 1940/41 and names J. W. Kennedy, Edwin M. McMillan and A. C. Wahl with him; Jefferson Lab gives 1941. A paper of March 1941 was withdrawn once plutonium-239 proved able to undergo fission, and wartime security, Jefferson Lab says, hid the discovery until 1946. Reducing a few milligrams of PuF₃ in November 1943 gave the first metal sample, according to Los Alamos.

Plutonium-238 halves in 87.7 years for IAEA, EPA and Los Alamos, and IAEA gives alpha emission 100 per cent of decays, spontaneous fission 0.00000019 per cent. Rarer cluster decays emit silicon-32 or magnesium nuclei, about 1.4 × 10⁻¹⁴ and 6 × 10⁻¹⁵ per cent respectively, in the Atomic Mass Data Center’s figures. Plutonium-238’s relatively high heat output makes it a long-lived power source, Los Alamos explains, for Apollo’s lunar instruments, Cassini and the Mars rovers. Heart pacemakers used it until lithium cells replaced it.

Plutonium-239 splits under neutrons and generates energy in nuclear reactors, IUPAC notes, and EPA adds that it is used to make nuclear weapons. Blended with uranium as a mixed oxide, MOX, it returns to power reactors as fuel.

Properties

Properties of plutonium 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	94
Atomic mass	244.06420 Da	Da	PubChem, longest-lived isotope
Melting point	640 °C	°C	Los Alamos: 640 °C
Boiling point	3228 °C	°C	Los Alamos: 3228 °C
Density	19.84 g/cm³	g/cm³
Electronegativity (Pauling)	1.28
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁶
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

Plutonium’s hazard is ionising radiation, chiefly alpha particles emitted inside the body. Inhaled particles lodge in the lungs, EPA says, and plutonium can pass to the kidneys and gather in the bones, liver and spleen. In Los Alamos’s account, plutonium that reaches the bones, liver or other organs stays for decades. Haz-Map, in the PubChem Laboratory Chemical Safety Summary, gives bone as the critical organ and rates internal toxicity very high; that summary is the reference sheet.

Questions

Why is plutonium named after Pluto?

Uranium had taken its name from the planet Uranus and neptunium, element 93, from Neptune, so Los Alamos calls it fitting that element 94 followed with Pluto, the next planetoid out. Glenn T. Seaborg first weighed the name 'plutium', then judged that it sounded worse than plutonium. Seaborg was later called from Berkeley to lead the Met Lab, the Plutonium Production Lab at the University of Chicago, Los Alamos records.

What colour is plutonium?

The metal starts out bright and silvery and tarnishes to dull grey, yellow or olive green as it oxidises in air, according to Los Alamos. In water the colour depends on the oxidation state and on the ligands, the ions or molecules bound to each plutonium ion: Pu³⁺ is blue lavender, uncomplexed Pu⁴⁺ salmon, the two oxygen-bearing cations lavender and orange-brown, and a basic solution dark green.

What is the half-life of plutonium-239?

Plutonium-239 takes 24,110 years to halve for the International Atomic Energy Agency (IAEA), Los Alamos and EPA, and 24,000 years for Haz-Map. IAEA lists alpha emission for 100 per cent of decays and spontaneous fission for 3.1 × 10⁻¹⁰ per cent. Plutonium-241 lasts 14.329 years for the Atomic Mass Data Center and 14.4 for HSDB (Hazardous Substances Data Bank); it turns into americium-241 by beta decay, with alpha decay taking 0.00245 per cent in the Atomic Mass Data Center's figures.

How does plutonium harm the body?

Breathing it in is the main danger, EPA says; swallowed plutonium is poorly absorbed by the stomach. Once inhaled, alpha particles from particles lodged in the lungs kill cells and scar the tissue, leading to further disease and cancer. The International Agency for Research on Cancer (IARC), as Haz-Map reports it, found sufficient human evidence that inhaled plutonium-239 aerosols cause lung cancer, liver cancer and bone sarcoma.