The Actinides: Uranium, Plutonium and the Radioactive f-Block

The actinides are the 15 elements from actinium (89) to lawrencium (103), usually shown as the bottom of the two separate rows beneath the main periodic table. Every one of them is radioactive. Only a few occur naturally in meaningful amounts; the rest were made by scientists, mostly in the two decades after 1940.

The list

Number	Element	Symbol	Origin
89	Actinium	Ac	Natural (trace)
90	Thorium	Th	Natural
91	Protactinium	Pa	Natural (trace)
92	Uranium	U	Natural
93	Neptunium	Np	Synthetic (traces in uranium ore)
94	Plutonium	Pu	Synthetic (traces in uranium ore)
95	Americium	Am	Synthetic
96	Curium	Cm	Synthetic
97	Berkelium	Bk	Synthetic
98	Californium	Cf	Synthetic
99	Einsteinium	Es	Synthetic
100	Fermium	Fm	Synthetic
101	Mendelevium	Md	Synthetic
102	Nobelium	No	Synthetic
103	Lawrencium	Lr	Synthetic

Filling the 5f orbitals

Across the actinides, electrons are added to the 5f subshell, just as the lanthanides above them fill 4f. That’s why they’re placed in the f-block. But the 5f electrons are less deeply buried than 4f electrons, so the early actinides (thorium to plutonium) use them in bonding and show many oxidation states — uranium from +3 to +6, plutonium from +3 to +7. The later actinides settle into mostly +3 chemistry, much like the lanthanides.

The Seaborg rearrangement

Until the 1940s, thorium, protactinium and uranium were placed in the main table under hafnium, tantalum and tungsten. Glenn Seaborg, working on the Manhattan Project, realized that the new elements he was making (americium and curium) behaved nothing like the transition metals they’d be under. In 1944 he proposed moving the whole series into a separate row below the lanthanides — the “actinide concept”. It was controversial at the time, but it worked, and it’s the layout on every periodic table today. Seaborg shared the 1951 Nobel Prize in Chemistry, and element 106, seaborgium, is named after him.

The natural actinides

Thorium and uranium have isotopes with half-lives of billions of years (thorium-232: about 14 billion years; uranium-238: about 4.5 billion), so they’ve survived since before the Earth formed. They’re found in rocks worldwide, and their slow decay is one of the main sources of heat inside the Earth. Actinium and protactinium exist only as short-lived members of their decay chains.

How the synthetic actinides were made

Most were produced by bombarding lighter elements with neutrons or light nuclei:

• Neptunium and plutonium (1940) were made by bombarding uranium in cyclotrons at Berkeley.
• Americium and curium (1944) came from bombarding plutonium.
• Einsteinium and fermium were first found in 1952 in the debris of the first hydrogen bomb test, “Ivy Mike” — the intense burst of neutrons had pushed uranium nuclei up many steps at once. The discovery was classified for a few years.
• Mendelevium, nobelium and lawrencium were made atom by atom in accelerators.

Many are named after places and scientists: americium (the Americas, paralleling europium above it), berkelium and californium (where they were made), einsteinium and fermium (Einstein and Fermi), mendelevium (Mendeleev), nobelium (Alfred Nobel), lawrencium (Ernest Lawrence, inventor of the cyclotron). See elements named after scientists.

Uses

• Uranium: fuel for nuclear power reactors (uranium-235 splits when struck by a neutron — see nuclear fission vs. fusion); depleted uranium in armour and counterweights.
• Plutonium: made in reactors from uranium-238; used as reactor fuel and in nuclear weapons. Plutonium-238, which gives off steady heat, powers deep-space probes such as Voyager and the Curiosity and Perseverance Mars rovers.
• Thorium: studied as an alternative nuclear fuel; formerly in gas lantern mantles.
• Americium: americium-241 in household smoke detectors.
• Californium: californium-252 is a strong neutron source, used to start reactors and in materials analysis.
• Actinium: actinium-225 is being developed for targeted cancer therapy.
• Beyond californium, the actinides are made in such tiny amounts that they’re used only for research.

Quick answers

Are all actinides radioactive? Yes. None has a stable isotope.

Which actinides occur naturally? Thorium and uranium in significant amounts; actinium, protactinium, and traces of neptunium and plutonium in uranium ores.

Why are the actinides separate from the main table? Because the f-block would make the table very wide. They’re really part of period 7 — see s, p, d and f blocks.