uranium and thorium chemistry

Uranium and thorium are the two actinides you can actually dig out of the ground in quantity — both are more common in the crust than silver or mercury — and so they are the two whose chemistry is best understood and most used. Long before anyone split an atom, uranium salts colored glass a glowing yellow-green and thorium oxide made the brilliant white mantle of a gas lamp. Their chemistry is the practical heart of the actinides.

Thorium is the simpler of the two. Its accessible electrons give it one dominant state, Th(IV): in solution it is the large, hard Th4+ ion, and its signature compound is thorium dioxide ThO2, an extremely refractory white solid that melts above 3000 degrees Celsius. In that single-valent simplicity thorium behaves a bit like a very large group-4 metal. Uranium is richer. Its 5f and 6d electrons are accessible enough to give the +3 through +6 range, but the chemistry is dominated by two species: uranium(IV), the green U4+ ion, and uranium(VI) as the linear uranyl ion UO2-2+, a remarkably stable cation in which uranium is double-bonded to two axial oxygens and binds further ligands around its waist. Uranyl salts are yellow and famously fluorescent, and uranium hexafluoride UF6 — a solid that sublimes to a gas near room temperature — is the volatile compound on which uranium enrichment depends.

This is where inorganic chemistry meets the nuclear fuel cycle. Mined uranium is purified through uranyl nitrate, converted to UF6 for isotope enrichment, then to uranium dioxide UO2 fuel pellets; thorium dioxide is the basis of proposed thorium reactor fuels. The honest caveats are two. First, uranium and thorium are weakly radioactive and chemically toxic, so their compounds demand care even setting aside fission. Second, although uranium reaches up to +6, do not picture it as just another flexible transition metal — its high-valent chemistry is dominated by that one peculiar uranyl unit, a structural motif with few parallels elsewhere in the periodic table.