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Neptunium metal buttons (photo courtesy Lawrence Berkeley National Laboratory)

Neptunium

History

Named for the planet Neptune (named after the Roman god of the sea), the next planet out from the Sun after Uranus. There were many early false reports of the discovery of neptunium. The most significant was by Enrico Fermi who believed that bombarding uranium with neutrons followed by beta decay would lead to the formation of element 93. In 1934, he bombarded uranium atoms with neutrons and reported that he had produced elements 93 and 94. As it turned out, Fermi had actually fissioned or split uranium atoms into many fragment radioisotopes. The explanation and announcement of the discovery of fission was later published by Hahn and Strassman, although it was their co-worker Lisa Meitner who had correctly interpreted the results of the experiments. In 1940, with excitement about fission reaching the University of California at Berkeley, Professor Edwin McMillan and graduate student Philip Abelson bombarded uranium with cyclotron-produced moderated (slow) neutrons, resulting not in “fission” but "fusion" of the reactants forming the new element 93, which they named "neptunium":

²³⁸₉₂U + ¹₀n → ²³⁹₉₂U → ²³⁹₉₃Np + β^-

Neptunium-239 was the first transuranium element produced synthetically and the first actinide series transuranium element discovered. This isotope has a beta-decay half-life of 2.3565 days, which forms daughter product plutonium-239 with a half-life of 24,000 years.

Isotopes

There are 25 known radioactive isotopes of neptunium ranging in atomic weights from 225 to 244 with 5 of those as metastable isotopes. The most stable are Np-237 with a half-life of 2.14 million years; Np-236 with a half-life of 154,000 years; and Np-235 with a half-life of 396 days. All of the remaining isotopes have half-lives less than 4.5 days, with most less than 50 minutes. The primary decay mode for isotopes lighter than ²³⁷Np is by electron capture with a great deal of alpha emission. The products are mostly isotopes of uranium. The primary decay mode for Np-237 is by alpha-decay forming protactinium. The primary decay mode for the isotopes heavier than Np-237 is by beta-decay, forming plutonium. Neptunium-237, after decaying to protactinium then to uranium, eventually decays to form bismuth-209 and thallium-205. Unlike most other common heavy nuclei which decay to make isotopes of lead this decay chain is known as the neptunium series.

Properties

Neptunium metal is produced by reaction of NpF₃ with liquid or gaseous barium or lithium at around 1200°C and is often extracted from spent nuclear fuel rods in kilogram quantities. Neptunium metal is silver in appearance, chemically reactive and is found in at least three allotropes:

• α-neptunium, (room temperature) orthorhombic, density 20.45 g/cm³
• ß-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 g/cm³
• γ-neptunium (above 577 °C), cubic, density (600 °C) 18 g/cm³

Neptunium has the largest liquid range of any element, 3363 K, between the metal melting point and boiling point. It is the most dense of all the actinides and the fifth most dense of all naturally occurring elements. Recently a neptunium-based superconductor alloy was discovered with formula NpPd₅Al₂. The occurrence of superconductivity in neptunium compounds is surprising because they often exhibit strong magnetism, which usually destroys superconductivity. Neptunium forms a variety of compounds, including the tri- and tetra-halides such as NpF₃, NpF₄, NpCl₄, NpBr₃, and NpI₃. Neptunium oxides such as Np₃O₈ and NpO₂ as are also found in the uranium-oxygen system.  Neptunium hexafluoride, NpF₆, is volatile like uranium hexafluoride.

In solution, neptunium exhibits five oxidation states, III, IV, V, VI, and VII with the V state being the most stable. The solution ions of III and IV are the simple ions, Np³⁺ and Np⁴⁺. Similar to its uranium counterpart, as the charge on the neptunium ions increases, it is distributed over a larger oxy-cation. Thus Np(V) exist in solution as NpO₂⁺, Np(VI) exits as NpO₂², and Np(VII) is an oxy-cation with a structure probably including hydroxide ions since it is only stable in strongly basic solutions. These latter oxygenated species are in contrast to the rare earths which exhibit only simple ions of the (II), (III), and (IV) oxidation states in aqueous solution. In solution, Np(III) is easily oxidized in air to form Np(IV). Np(VII), stable in basic solutions quickly reduces to Np(VI) if the pH is made more acidic. In acid solutions, Np⁺³ is dark blue-purple; Np⁺⁴ is grass green; NpO₂⁺ is emerald green; NpO₂⁺⁺ is light burgundy and Np(VII) is dark green in strongly basic solutions .