Actinides

Seaborg’s new row

Until the 1940s, thorium, protactinium and uranium were placed in the main body of the periodic table, below hafnium, tantalum and tungsten. When Glenn Seaborg’s team made americium and curium and found they behaved like rare earths, he proposed that the heaviest elements form a second series below the lanthanides. Colleagues warned that the idea would damage his career. It proved correct and guided the discovery of every actinide that followed.

Made, not found

Only actinium, thorium, protactinium and uranium occur in meaningful amounts in nature, with traces of neptunium and plutonium. Every element from americium onwards was first made artificially, in nuclear reactors, cyclotrons or, in the case of einsteinium and fermium, in the debris of a hydrogen bomb test.

The further along the series, the harder they become to produce and the faster they decay. Plutonium is made by the tonne; californium in grams per year worldwide; and the heaviest actinides, from fermium to lawrencium, only ever a few atoms at a time.

Every actinide is radioactive

None of the actinides has a stable isotope. Thorium-232 and uranium-238 decay so slowly, over billions of years, that much of what formed with the Earth is still here. Their decay heats the Earth’s interior and produces most of the helium we extract from the ground.

Others are so short-lived that their properties are known only from theory. Handling the more radioactive actinides requires glove boxes, remote manipulators and shielding.

Power, weapons and smoke detectors

The same nuclear properties that make actinides useful make their waste hard to manage. Spent fuel stays dangerous for thousands of years, and countries such as Finland are building deep geological repositories to isolate it.

• Uranium: fuel for nuclear power stations, which supply about a tenth of the world’s electricity
• Plutonium: nuclear weapons, reactor fuel, and plutonium-238 power sources for space probes such as Voyager
• Americium: the ionisation source inside most household smoke detectors
• Californium: portable neutron sources for starting reactors and analysing materials
• Thorium: a possible future reactor fuel, more abundant than uranium