Fifteen radioactive metals, four of them natural and eleven made by hand

Actinides

Actinium through lawrencium. Every one is radioactive. Only thorium and uranium exist in usable quantity in nature; the rest are either fleeting decay products or entirely synthetic. Between them they power spacecraft, date rocks, treat cancer and split the twentieth century in two.

Members
15
Group
f-block, period 7
Configuration
[Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²
Radioactive
15

Highlighted in the periodic table

Every member

The 15 actinides

89 Ac Actinium 227 u Intensely radioactive and 150 times more so than radium — actinium glows pale blue in the dark. radioactive solid 90 Th Thorium 232.038 u Three times more abundant than uranium and a potential reactor fuel that cannot easily be weaponised. radioactive solid 91 Pa Protactinium 231.036 u One of the rarest and most expensive natural elements — extracting 125 g once cost half a million dollars. radioactive solid 92 U Uranium 238.029 u The heaviest primordial element, and the one that split the twentieth century in two. radioactive solid 93 Np Neptunium 237 u The first transuranic element, made by bombarding uranium with neutrons in 1940. radioactive solid 94 Pu Plutonium 244 u The element that made the atomic age possible and powers spacecraft beyond the Sun's reach. radioactive solid 95 Am Americium 243 u The element quietly sitting in millions of smoke detectors worldwide. radioactive synthetic solid 96 Cm Curium 247 u Named for the Curies, and the isotope that has analysed the soil of Mars. radioactive synthetic solid 97 Bk Berkelium 247 u Made in nanogram quantities and used mainly as a target for creating even heavier elements. radioactive synthetic solid 98 Cf Californium 251 u A neutron source strong enough to be used in oil-well logging and cancer treatment. radioactive synthetic solid 99 Es Einsteinium 252 u Discovered in the fallout of the first hydrogen bomb test, in samples flown through the cloud. radioactive synthetic solid 100 Fm Fermium 257 u The heaviest element that can be made in weighable amounts by neutron bombardment. radioactive synthetic solid 101 Md Mendelevium 258 u The first element produced and identified one atom at a time. radioactive synthetic solid 102 No Nobelium 259 u The subject of a decade-long discovery dispute between Swedish, American and Soviet teams. radioactive synthetic solid 103 Lr Lawrencium 262 u The last actinide, with an electron configuration that broke the expected pattern. radioactive synthetic solid

What defines them

Three things that make an actinide

01

All radioactive, without exception

Beyond bismuth no nucleus is truly stable. Half-lives range from thorium-232's 14 billion years — older than the Earth and still here — down to fractions of a second for the heaviest.

02

More variable than the lanthanides

Early actinides do not bury their 5f electrons the way lanthanides bury their 4f ones, so those electrons take part in bonding. Uranium alone runs from +3 to +6. Beyond americium the 5f electrons localise and the behaviour becomes lanthanide-like, settling on +3.

03

Fissile and fertile

Uranium-235 and plutonium-239 sustain chain reactions. Uranium-238 and thorium-232 do not, but capture neutrons to become nuclides that do. That distinction is the whole of nuclear engineering.

Periodic trends

How they change across the family

Ionic radius contracts across the series, as with the lanthanides. Stability collapses with atomic number: everything past fermium (100) exists only in accelerator experiments, made and detected a handful of atoms at a time.

In the world

What they are used for

Uranium

Reactor fuel and weapons. Natural uranium is 99.3% U-238; only the 0.7% that is U-235 will sustain a chain reaction, which is what enrichment is for.

Plutonium

Weapons, and — as Pu-238 — the radioisotope thermoelectric generators that still power Voyager 1 more than 24 billion km from Earth.

Thorium

Three times more abundant than uranium and a possible reactor fuel that is far harder to weaponise. Gas mantles lit streets with it for a century.

Americium

The ionisation source in millions of domestic smoke detectors — about 0.3 micrograms each.

Californium

A neutron source strong enough for oil-well logging and cancer treatment: one microgram emits 2.3 million neutrons a second.

Actinium & curium

Ac-225 is used in targeted alpha therapy against otherwise untreatable cancers; curium sources ran the X-ray spectrometers on the Mars rovers.

Handling

Safety

The most hazardous family in the table. All are radiotoxic; several are also chemically toxic heavy metals. Alpha emitters like plutonium and polonium are relatively harmless outside the body and extremely dangerous inside it, since alpha particles deposit all their energy in a very short distance. Every actinide beyond trace natural thorium and uranium requires licensing, shielding and containment.

Worth knowing

  • Voyager 1 has been running on plutonium-238 heat since 1977 and is expected to keep transmitting into the mid-2020s.
  • A natural nuclear reactor ran for hundreds of thousands of years at Oklo in Gabon about 1.7 billion years ago, when natural uranium was still enriched enough to go critical on its own.
  • Einsteinium was discovered in the fallout of the first hydrogen bomb test, in debris collected by aircraft flown through the cloud.

Common questions

About the actinides

Why is uranium-235 fissile but uranium-238 not?

Nuclear pairing. U-235 has an odd neutron count, so adding one releases enough binding energy to push the nucleus over its fission barrier immediately, even from a slow neutron. U-238 has an even count; adding a neutron releases less, and only a fast neutron carrying its own kinetic energy can make it fission. That is why reactors moderate their neutrons and why enrichment is necessary.

What is the difference between fissile and fertile?

A fissile nuclide will sustain a chain reaction on its own — U-235, Pu-239, U-233. A fertile one will not, but captures a neutron and transmutes into something that will: U-238 becomes Pu-239, and Th-232 becomes U-233. Breeder reactors exist to run that conversion deliberately.

Are any actinides safe to be near?

Depleted uranium and thorium are weak alpha emitters, and intact metal is stopped by skin — people handled thorium gas mantles for a century. The danger is inhalation or ingestion, where alpha particles deposit their entire energy inside living tissue. Everything past americium is dangerous under any circumstances and exists only inside shielded facilities.

Keep going

The other nine families

6 Alkali metals Li Na K Rb Cs Fr 6 Alkaline earth metals Be Mg Ca Sr Ba Ra 38 Transition metals Sc Ti V Cr Mn Fe Co 12 Post-transition metals Al Ga In Sn Tl Pb Bi 7 Metalloids B Si Ge As Sb Te At 7 Nonmetals H C N O P S Se 5 Halogens F Cl Br I Ts 7 Noble gases He Ne Ar Kr Xe Rn Og 15 Lanthanides La Ce Pr Nd Pm Sm Eu