[21][29] This decay chain had long been extinct on Earth due to the short half-lives of all of its isotopes above bismuth-209, but is now being resurrected thanks to artificial production of neptunium on the tonne scale.

However, the discovery of induced radioactivity by Irène and Frédéric Joliot-Curie in late 1933 opened up an entirely new method of researching the elements and inspired a small group of Italian scientists led by Enrico Fermi to begin a series of experiments involving neutron bombardment.

[45][46] Several theoretical objections to the claims of Fermi's paper were quickly raised; in particular, the exact process that took place when an atom captured a neutron was not well understood at the time.

This and Fermi's accidental discovery three months later that nuclear reactions could be induced by slow neutrons cast further doubt in the minds of many scientists, notably Aristid von Grosse and Ida Noddack, that the experiment was creating element 93.

While von Grosse's claim that Fermi was actually producing protactinium (element 91) was quickly tested and disproved, Noddack's proposal that the uranium had been shattered into two or more much smaller fragments was simply ignored by most because existing nuclear theory did not include a way for this to be possible.

[47][48][49] Although the many different and unknown radioactive half-lives in the experiment's results showed that several nuclear reactions were occurring, Fermi's group could not prove that element 93 was being produced unless they could isolate it chemically.

They and many other scientists attempted to accomplish this, including Otto Hahn and Lise Meitner who were among the best radiochemists in the world at the time and supporters of Fermi's claim, but they all failed.

Much later, it was determined that the main reason for this failure was because the predictions of element 93's chemical properties were based on a periodic table which lacked the actinide series.

[58][59][60][61][62] Perhaps the closest of all attempts to produce the missing element 93 was that conducted by the Japanese physicist Yoshio Nishina working with chemist Kenjiro Kimura in 1940, just before the outbreak of the Pacific War in 1941: they bombarded 238U with fast neutrons.

They attempted to isolate this nuclide by carrying it with its supposed lighter congener rhenium, but no beta or alpha decay was observed from the rhenium-containing fraction: Nishina and Kimura thus correctly speculated that the half-life of 23793, like that of 231Pa, was very long and hence its activity would be so weak as to be unmeasurable by their equipment, thus concluding the last and closest unsuccessful search for transuranic elements.

Although he did not discover anything of note from this, McMillan did observe two new beta decay half-lives in the uranium trioxide target itself, which meant that whatever was producing the radioactivity had not violently repelled each other like normal fission products.

As a final step, McMillan and Abelson prepared a much larger sample of bombarded uranium that had a prominent 23-minute half-life from 239U and demonstrated conclusively that the unknown 2.3-day half-life increased in strength in concert with a decrease in the 23-minute activity through the following reaction:[67] This proved that the unknown radioactive source originated from the decay of uranium and, coupled with the previous observation that the source was different chemically from all known elements, proved beyond all doubt that a new element had been discovered.

[74] Neptunium's unique radioactive characteristics allowed it to be traced as it moved through various compounds in chemical reactions, at first this was the only method available to prove that its chemistry was different from other elements.

As the first isotope of neptunium to be discovered has such a short half-life, McMillan and Abelson were unable to prepare a sample that was large enough to perform chemical analysis of the new element using the technology that was then available.

Neptunium-237 is the most commonly synthesized isotope due to it being the only one that both can be produced via neutron capture and also has a half-life long enough to allow weighable quantities to be easily isolated.

[28] Artificial 237Np metal is usually isolated through a reaction of 237NpF3 with liquid barium or lithium at around 1200 °C and is most often extracted from spent nuclear fuel rods in kilogram amounts as a by-product in plutonium production.

As it has a long half-life of just over 2 million years, the alpha emitter 237Np is one of the major isotopes of the minor actinides separated from spent nuclear fuel.

Unlike its neighboring homologues UO+2 and PuO+2, NpO+2 does not spontaneously disproportionate except at very low pH and high concentration:[85] It hydrolyzes in basic solutions to form NpO2OH and NpO2(OH)−2.

[84][87] Neptunium(III) hydroxide is quite stable in acidic solutions and in environments that lack oxygen, but it will rapidly oxidize to the IV state in the presence of air.

The first two are fairly stable and were first prepared in 1947[contradictory] through the following reactions:[citation needed] Later, NpF4 was obtained directly by heating NpO2 to various temperatures in mixtures of either hydrogen fluoride or pure fluorine gas.

This volatility has attracted a large amount of interest to the compound in an attempt to devise a simple method for extracting neptunium from spent nuclear power station fuel rods.

[120][121][122] Neptunium chalcogen and pnictogen compounds have been well studied primarily as part of research into their electronic and magnetic properties and their interactions in the natural environment.

It is isomorphous to uranocene and plutonocene, and they behave chemically identically: all three compounds are insensitive to water or dilute bases but are sensitive to air, reacting quickly to form oxides, and are only slightly soluble in benzene and toluene.

[146] Complex nitrate compounds are also known: the experimenters who produced them in 1986 and 1987 obtained single crystals by slow evaporation of the Np(IV) solution at ambient temperature in concentrated nitric acid and excess 2,2′-pyrimidine.

[147] A great many complexes for the other neptunium oxidation states are known: the inorganic ligands involved are the halides, iodate, azide, nitride, nitrate, thiocyanate, sulfate, carbonate, chromate, and phosphate.

Many organic ligands are known to be able to be used in neptunium coordination complexes: they include acetate, propionate, glycolate, lactate, oxalate, malonate, phthalate, mellitate, and citrate.

[147] An important use of 237Np is as a precursor in plutonium-238 production, where it is irradiated with neutrons to form 238Pu, an alpha emitter for radioisotope thermal generators for spacecraft and military applications.

As of 2009, the world production of neptunium-237 by commercial power reactors was over 1000 critical masses a year, but to extract the isotope from irradiated fuel elements would be a major industrial undertaking.

"[31] The United States Federal government made plans in March 2004 to move America's supply of separated neptunium to a nuclear-waste disposal site in Nevada.

[153] Neptunium accumulates in commercial household ionization-chamber smoke detectors from decay of the (typically) 0.2 microgram of americium-241 initially present as a source of ionizing radiation.