actinides (the 5f series)
/ AK-tih-nides /
Directly below the lanthanides sits a second mysterious strip: the actinides, the fifteen elements from actinium and thorium and uranium through to lawrencium, in which a deeper set of orbitals, the 5f shell, is filling. Two of them — uranium and thorium — occur in rock and powered the nuclear age; the rest, the transuranium elements, are mostly human-made, born in reactors and accelerators. Every actinide shares one defining trait: they are all radioactive.
Chemically the actinides are the lanthanides' rougher, more complicated cousins. At first glance they should mirror the 4f series — fill the 5f shell, expect a dominant +3 state, expect near-identical chemistry — and the later actinides do behave that way. But the early actinides break the pattern in a revealing way. In thorium, uranium, neptunium, and plutonium the 5f orbitals are not yet buried as deeply as the 4f orbitals are in the lanthanides; they lie closer to the surface and overlap more with bonding orbitals, so their electrons are more available to chemistry. The upshot is a wide and colorful range of oxidation states for the early actinides (uranium reaching +6, neptunium and plutonium even more flexible) before the series settles, in the later actinides from about americium onward, into the same lanthanide-like +3 behavior as the 5f shell finally sinks out of reach.
Why they matter is impossible to overstate. Uranium and thorium are the fuels of nuclear fission; plutonium, made from uranium in a reactor, is both a fuel and a weapons material; and the slow march to ever-heavier human-made actinides traced out the far end of the periodic table. They also matter as a warning: handling them is hazardous because of radioactivity, and their long-lived waste is one of the central problems of nuclear power. The honest caveat: the actinides are studied far less completely than the lanthanides because most are intensely radioactive, scarce, or available only as a few atoms at a time — much of what we 'know' about the heaviest ones rests on a handful of fleeting measurements.
Uranium in solution can be found as green U4+, but oxidize it and it becomes the pale-yellow uranyl ion UO2-2+, a +6 species with the uranium double-bonded to two oxygens. A lanthanide of similar position, neodymium, refuses to do anything of the sort — it stays stubbornly Nd3+. That difference is the early actinides' more accessible 5f electrons made visible.
Uranium reaches +6 (uranyl); the lookalike lanthanide neodymium stays +3.
Do not assume the actinides simply copy the lanthanides. The early ones are far more like ordinary transition metals in their flexible oxidation states; only from americium onward does the 'lanthanide-like +3' picture become a good description.