Plutonium

The element of the Nagasaki bomb, and of the power sources that run spacecraft too far from the sun for solar panels.

At a glance

Symbol: Pu
Atomic number: 94
Mass number of its longest-lived isotope: [244]
Group: none; it sits in the actinide row below the table
Period: 7
Family: actinide
Electron configuration: [Rn] 5f⁶ 7s²
Electronegativity: 1.28
Common ions: +3, +4, +5 and +6 are all possible in water
Isotopes: none are stable; ²⁴⁴Pu lasts longest, about 80 million years; ²³⁹Pu is used in weapons and reactors, and ²³⁸Pu in power sources

On the table

Period 7, in the actinide row set below the main table. The other actinides are tinted, and every cell opens its own article.

Electrons and bonding

Plutonium has 94 electrons, one for every proton, so the atom carries no charge. Below are the same 94 electrons written two ways.

Simple form: electrons per shell

shell 12shell 28shell 318shell 432shell 524shell 68shell 72

Counting outward from the nucleus: 2, 8, 18, 32, 24, 8, 2. The outer shell is picked out.

Orbital form: electrons per subshell

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

Short form: [Rn] 5f⁶ 7s², where [Rn] stands for the 86 electrons radon already has.

That list runs by shell. In the order the subshells fill it reads [Rn] 7s² 5f⁶, because a subshell fills by its energy, not by its shell number.

5f⁶↑↑↑↑↑↑

7s²↑↓

The boxes hold the electrons beyond [Rn]. Each box is one orbital and takes two electrons at most, spinning opposite ways. They spread out singly before they pair up.

How to read the two

The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 4 here is 4s² 4p⁶ 4d¹⁰ 4f¹⁴, and 2 + 6 + 10 + 14 is 32.

Plutonium sits in period 7 and has 7 shells. The period number counts the shells.

What that means for bonding

The outer shell holds 2 electrons. Those are the ones plutonium can lose, gain or share, so they are the ones that decide what it bonds with.

Radioactive, like every actinide. It gives up electrons to form positive ions.

Properties

State at 25 °C: solid
What it looks like: silvery metal that tarnishes yellow and then olive green, and feels warm from its own radioactivity
Melting point: 640 °C
Boiling point: 3228 °C
Density: 19.8 g/cm³
Odd fact: the metal has six different solid forms between room temperature and its melting point, more than any other element

Where it is found

Only faint traces exist in nature. It is made in nuclear reactors when uranium-238 absorbs neutrons.

Discovery and name

Glenn Seaborg, Arthur Wahl, Joseph Kennedy and Edwin McMillan made it at Berkeley in 1940 and 1941 by bombarding uranium with deuterons, and it was kept secret during the war. It was named for Pluto, then counted as the planet after Neptune. Seaborg said he picked the symbol Pu as a joke and expected it to be rejected; it was not.

What it is used for

Plutonium-239 powered the bomb dropped on Nagasaki in 1945, and mixed with uranium it fuels some power reactors. Plutonium-238 gives off steady heat that generators turn into electricity for spacecraft such as Voyager, Cassini and New Horizons and for the Curiosity and Perseverance rovers on Mars.

In compounds

PuO₂plutonium dioxide, the fuel of space power sourcesPuF₄plutonium tetrafluoridePuCl₃plutonium trichloride

Handle with care

Highly toxic and radioactive if inhaled or swallowed, and a few kilograms of plutonium-239 can reach critical mass.