Plutonium (Pu)

Plutonium has six solid crystal forms between room temperature and its melting point, more than any other element, and they differ in density by as much as 25% — a plutonium casting can crack itself simply by cooling through a phase change, which is why weapons plutonium is alloyed with a little gallium to hold it in one phase. It is a poor conductor of heat and electricity, unusually for a metal, and a lump of it is warm to the touch from its own decay. Voyager 1 and 2 have been running on plutonium-238 since 1977, and both are still transmitting.

Properties

Atomic number

94

Symbol

Pu

Mass number of most stable isotope

[244]

Category

Actinide

Period

7

Block

f-block

Electron configuration

[Rn] 7s² 5f⁶

Electrons per shell

2, 8, 18, 32, 24, 8, 2

Melting point

913 K — 639.9 °C — 1183.7 °F

Boiling point

3,501 K — 3227.8 °C — 5842.1 °F

Density

19.84 g/cm³

Electronegativity

1.28 (Pauling)

Oxidation states

+4, +6, +5, +3

Discovered

1940 by Glenn Seaborg, Edwin McMillan, Joseph Kennedy and Arthur Wahl

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁶

Noble gas shorthand

[Rn] 7s² 5f⁶

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 24 · P: 8 · Q: 2

What it is used for

• Nuclear weapon pits
• Plutonium-238 radioisotope generators powering Voyager, Cassini and Curiosity
• MOX reactor fuel, blending plutonium with uranium
• Historically, plutonium-238 pacemaker batteries

Hazards

• Alpha emitter that is dangerous mainly when inhaled as fine oxide dust
• Fissile and subject to criticality accidents in the wrong geometry
• Finely divided plutonium is pyrophoric and ignites in air

No stable isotope; the mass given is the mass number of plutonium-244, the longest-lived at 81 million years. The weapon and reactor isotope is plutonium-239, half-life 24,100 years, and the spacecraft isotope is plutonium-238, half-life 87.7 years.

Neighbours in the table