Actinides (actinium to lawrencium)

The 5f series

In the actinides, electrons are added to the 5f subshell, just as the lanthanides fill 4f. The pattern is less regular because the 5f and 6d orbitals are close in energy: actinium is [Rn] 6d¹ 7s², thorium [Rn] 6d² 7s², uranium [Rn] 5f³ 6d¹ 7s² and plutonium [Rn] 5f⁶ 7s². The 5f subshell is full by nobelium and lawrencium.

Glenn Seaborg proposed the separate actinide row in 1944. Before that, thorium, protactinium and uranium were placed under hafnium, tantalum and tungsten.

Radioactivity and fission

Every actinide is radioactive; none has a stable isotope. Thorium-232 and uranium-238 have half-lives of about 14 billion and 4.5 billion years, so they have survived since Earth formed, but the heavier actinides decay much faster. Einsteinium and fermium were first found in debris from the first hydrogen bomb test in 1952, and the elements after fermium can only be made a few atoms at a time in particle accelerators.

The key actinide reaction is nuclear fission. A uranium-235 nucleus that absorbs a neutron splits into two smaller nuclei, releasing more neutrons and a large amount of energy, for example uranium-235 + neutron → barium-141 + krypton-92 + 3 neutrons. Those neutrons split further nuclei in the chain reaction that runs a nuclear reactor.

Chemical properties and trends

The actinides are dense, silvery metals that tarnish in air. The early actinides have many oxidation states because their 5f electrons are easy to remove: uranium and plutonium both range from +3 to +6. Uranium reacts with fluorine to form uranium hexafluoride (U + 3F₂ → UF₆), a volatile compound used to enrich uranium. From americium onward the actinides behave more like the lanthanides, with +3 as the main oxidation state.

Uses of the actinides

Uranium enriched in uranium-235 fuels nuclear power plants, and plutonium-239 is used in nuclear weapons and some reactor fuel. Plutonium-238 gives off steady heat as it decays and powers spacecraft such as the Voyager probes and the Curiosity and Perseverance Mars rovers. Americium-241 is the radiation source in many household smoke detectors.

Californium-252 is a neutron source for starting reactors, and actinium-225 is being tested against some cancers. Because they are radioactive and toxic, all actinides must be handled with great care.