in your element Neglected neptunium Jim Ibers talks about neptunium, an element that has remained largely unnoticed despite the flurry of activity devoted to its neighbours in the periodic table, uranium and plutonium. he actinides, the radioactive chemical quantity of primordial 237Np to remain plutonium from nuclear spent fuels, in elements beginning with element 89 today, 4.5 × 109 years after the Earth’s which neptunium and small amounts of T(actinium) and extending to element formation. The mass of 237Np being formed the heavier actinides are rejected into a 103 (lawrencium), are a neglected part of from various nuclear-decay processes in the high-level radioactive liquid waste. New chemical pedagogy, at least in the USA. As Earth’s crust is estimated to be only about advanced commercial processes are needed a result, some chemists may not know the 10−12 times that of uranium-containing to remove 237Np, which will otherwise names of most of these elements, or where minerals. Thus, neptunium could not be become the main contributor to the total they lie in the periodic table, but most would ‘discovered’; it had to be synthesized. Today, radiation from these wastes in about 104 guess correctly from knowledge of the Solar the production of plutonium in nuclear years — a time being discussed at present System that somewhere in the periodic table reactors is the main source of neptunium. for their containment. uranium, neptunium and plutonium would As opposed to their solution chemistry, be found in that order. Some might even knowledge of the solid-state chemistry of guess correctly that this order reflects their the lighter actinides is far less extensive. year of discovery. Even here, neptunium is the ‘stepchild’. However, it does not reflect the relative Pure metals and compounds of uranium attention these elements have received. and plutonium have been produced in far Although neptunium is barely talked about, greater quantities than those of neptunium. uranium and plutonium are famous — some In the solid state one finds some surprising would say infamous — because in the early differences in structures and properties. For 1940s isotopes of uranium and plutonium example, the crystal structure of neptunium were found to be fissile, meaning they can under ambient conditions differs from sustain a nuclear chain reaction, and hence those of uranium and plutonium, although had weapons applications. Neptunium was the structural complexity of plutonium also known to be fissile, but the quantity metal is another subject altogether. The needed made it unattractive for such intermetallic compounds UCoGa5 and applications. NpCoGa5 do not superconduct, whereas Yet neptunium ought to be famous PuCoGa5 is a non-conventional high- because it was the first actinide element Extended view of the crystal structure of neptunium. temperature superconductor. Unlike to be synthesized. In a classic study in isotopes of uranium or plutonium, 237Np 1940, McMillan and Abelson bombarded The need to characterize new is one of the best Mössbauer nuclei in the a thin layer of UO3 with neutrons from a actinide elements, and to separate their periodic table — Mössbauer spectroscopy cyclotron1. Investigation of the chemistry compounds from each other and those is a valuable tool for probing valence of the product that possessed a 2.3-day of the lanthanides, has led to extensive electrons, and hence oxidation states and half-life led them to conclude that it was knowledge of actinide solution chemistry. chemical bonding. element 93 with a mass of 239, now written Here, oxidation states are critical: these Many surprises await the development of 239 as 93Np. They also speculated on the nature dictate acid–base properties, coordination more extensive knowledge of the chemistry of the daughter product obtained from the chemistry and solubilities in aqueous and of neptunium compounds, particularly radioactive decay of their new element, soon non-aqueous solvents. In solution the the solid-state chemistry. The renaissance characterized2 by Seaborg and co-workers elements americium to lawrencium closely of nuclear energy may well accelerate as an isotope of element 94 (plutonium). resemble one another and often exhibit this development. ❐ Much of this work was classified for the next oxidation states of +3, as do the lanthanides. five years. In contrast, uranium has formal oxidation JIM IBERS is in the Department of Chemistry Now, about 22 isotopes of neptunium states of +3 to +6, and neptunium and at Northwestern University, USA. 237 have been characterized. Of these, Np is plutonium have those oxidation states as e-mail: [email protected] the most stable, with a half-life of 2.14 × 106 well as +7. years — too short for any significant These differences are the basis for References sophisticated procedures, such as the 1. McMillan, E. & Abelson, P. H. Phys. Rev. 57, 1185–1186 (1940). PUREX (plutonium–uranium extraction) 2. Seaborg, G. T., McMillan, E. M., Kennedy, J. W. & Wahl, A. C. process for removing uranium and Phys. Rev. 69, 366–367 (1946). Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf Db Sg Bh Hs Mt Ds 996 NATURE CHEMISTRY | VOL 2 | NOVEMBER 2010 | www.nature.com/naturechemistry © 2010 Macmillan Publishers Limited. All rights reserved nchem_.888_NOV10.indd 996 7/10/10 16:34:31.