Neptunium

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Neptunium is a chemical element with the symbol Np and the atomic number 93. The radioactive actinide metal, neptunium is the first transuranic element. His position on the periodic table just after uranium, named after Uranus planet, led to the name Neptune, the next planet outside of Uranus. The neptunium atom has 93 protons and 93 electrons, seven of which are valence electrons. The Neptunium metal is silver and stains when exposed to air. This element occurs in three allotropic forms and usually exhibits five oxidations, ranging from 3 to 7. It is radioactive, toxic, pyrophoric, and can accumulate in bone, which makes neptunium handling dangerous.

Despite the many false claims of its inventions made over the years, this element was first synthesized by Edwin McMillan and Philip H. Abelson at the Berkeley Radiation Laboratory in 1940. Since then, most of the neptunium has been and is still produced by neutron irradiation from uranium. in a nuclear reactor. Most are produced as byproducts in conventional nuclear power reactors. While neptunium itself has no current commercial use, it is used as a precursor for the formation of plutonium-238, which is used in radioisotope heat generators to provide electricity for the spacecraft. Neptunium has also been used in high-energy neutron detectors.

The most stable neptunium isotope, neptunium-237, is a by-product of nuclear reactors and plutonium production. This, and the neptunium-239 isotope, are also found in trace amounts in uranium ores due to neutron and beta-capture reactions.

Video Neptunium

Characteristics

Physical

Neptunium is a strong, silvery, ductile, radioactive actinic metal. In the periodic table, it is located to the right of the actinide uranium, to the left of the actinide plutonium and below the lanthanide promethium. Neptunium is a hard metal, has a bulk modulus of 118Ã, GPa, proportional to manganese. Neptunium metal is similar to uranium in terms of physical work ability. When exposed to air at normal temperatures, it forms a thin oxide layer. This reaction takes place more quickly as the temperature rises. Neptunium has determined to melt at 639 Ã, Â ± 3 Ã, Â ° C: this low melting point, a property that shares a metal with a neutral element of plutonium (which has a melting point of 639.4 Ã, Â ° C), is due to hybridization of 5f and Orbital 6d and the formation of a directional bond in the metal. The neptunium boiling point is unknown empirically and the value usually given 4174 Â ° C is extrapolated from the vapor pressure of the element. If accurate, this will give neptunium the largest fluid range of any element (3535 K through its melting point and boiling point).

Neptunium is found in at least three allotropes. Several claims from the fourth allotrope have been made, but so far not proven. Allotropic diversity is common among actinides. The crystalline structures of neptunium, protactinium, uranium, and plutonium do not have a clear analogue between lanthanides and are more similar to 3d transition metals.

? -neptunium takes an orthorhombic structure, resembling a highly distorted body-centered cube structure. Each neptunium atom is coordinated to the other four and the length of the Np-Np bond is 260 μm. It is the most dense of all actinides and the fifth most solid of all naturally occurring elements, behind only rhenium, platinum, iridium, and osmium. ? -neptunium has semimetallic properties, such as strong covalent bonds and high electrical resistivity, and the metallic physical properties closer to the metalloid than the actual metal. Some allotropes of other actinides also exhibit similar behavior, albeit to a lesser extent. The density of different neptunium isotopes in the alpha phase is expected to be observed differently:? - ²³⁵ Np should have a density of 20.303 g/cm ³ ; ? - ²³⁶ Np, density 20,389 g/cm ³ ; ? - ²³⁷ Np, density 20,476 g/cm ³ .

? -neptunium takes on distorted distorted tetragonal structure. Four neptunium atoms form a unit cell, and the Np-Np bond length is 276 pm. ? -neptunium has a cubic body-centered structure and has a Np-Np 297 pm bond length. That? the shape becomes less stable with increased pressure, although the neptunium melting point also increases with pressure. The -Np/? - Np/liquid triple point occurs at 725 Â ° C and 3200 MPa.

Alloys

Because of the 5f valence electrons, neptunium and its alloys exhibit very interesting magnetic behavior, like many other actinides. These can range from characteristic characteristics such as bands traveling from transition metals to typical localized behavior of scandium, yttrium, and lanthanides. This comes from the hybridization of the 5f orbitals with metal ligand orbitals, and the fact that the 5f orbitals are relativistically unstable and extends outwards. For example, pure neptunium is paramagnetic, NpAl ₃ is ferromagnetic, NpGe ₃ has no magnetic order, and NpSn ₃ behaves in fermion. Ongoing investigations of neptunium alloys with uranium, americium, plutonium, zirconium and iron, so to recycle long-lived waste isotopes such as neptunium-237 into shorter-lived isotopes are more useful as nuclear fuel.

One neptunium-based superconducting alloy has been found with the formula NpPd ₅ Al ₂ . This incident in the neptunium compounds is somewhat surprising because they often show strong magnets, which usually destroy superconductivity. This alloy has a tetragonal structure with a superconductivity transition temperature of -268.3Ã, Â ° C (4.9Ã, K).

Chemistry

Neptunium has five ionic oxidations ranging from 3 to 7 when forming a chemical compound, which can be observed simultaneously in solution. This is the toughest actinide that can lose all valence electrons in a stable compound. The most stable state in solution is 5, but valence 4 is preferred in solid neptunium compounds. Neptunium metal is highly reactive. The neptunium ion is susceptible to hydrolysis and the formation of the coordinating compound.

Atom

The neptunium atoms have 93 electrons, arranged in a configuration of 5f ⁴ 6d ¹ 7s ² . This differs from the configuration expected by the Aufbau principle in one electron in the 6d subshell instead of as expected in Section 5f. This is because of the similarity of electron energy from subfields 5f, 6d, and 7s. In forming compounds and ions, all valence electrons can be lost, leaving the inert nucleus of the inner electron with the electron configuration of the noble gas radon; more commonly, only a few valence electrons will be lost. The electron configuration for the tripositive ion Np ³ is [Rn] 5f ⁴ , with the outer electrons 7s and 6d disappearing first: it is exactly the same as the homologous lanthanide of neptunium promethium, trends set by other actinides with their electron configuration [Rn] Ã, 5f ⁿ them in a tripositive state. The first ionization potential of neptunium measured at most 6.19 Â ± 0.12Ã, eV in 1974, based on the assumption that the 7s electrons would be ionized before 5f and 6d; the newer measurements have refined this to 6.2657 eV.

Isotope

20 neptunium radioisotope has been characterized with the most stable is ²³⁷ Np with a half-life of 2.14 million years, ²³⁶ Np with a half-life of 154,000 years, and ^{235 Np with a half-life of 396.1 days. All remaining radioactive isotopes have a half-life of less than 4.5 days, and most have half-lives of less than 50 minutes. This element also has at least four meta states, with the most stable being 236m Np with a half-life of 22.5 hours.}

The isotope of the neptunium range in atomic weight from 225.0339 u ( ²²⁵ Np) to 244.068 u ( ²⁴⁴ Np). Most of the isotopes that are lighter than the most stable, ²³⁷ Np, decay mainly by electron capture even though the amounts are quite large, especially ²²⁹ Np and ²³⁰ Np, also shows different degrees of decay through the emission of alpha into protactinium. ²³⁷ Np itself, being a beta-stable isobar mass number 237, decays almost exclusively by alpha emission to ²³³ Pa, very rarely (only about once in trillions of shedding fission spontaneous and group decay ( ³⁰ Mg emission to form ²⁰⁷ Tl.) All known isotopes except one heavier than this decay exclusively through beta emissions The sole exception , ^240m Np, showing a rare decay (& gt; 0.12%) by an isomer transition other than the beta emission. ²³⁷ Np finally decays to form bismuth-209 and thallium-205 , unlike most other common heavy nuclei that decay into lead isotopes.These decay chains are known as the neptunium series.The chain of decay has long been extinct on Earth because of the short part-time life of all its isotopes above bismuth-209, but is now being raised thanks to artificial neptunium production at s ton.

The isotopes of neptunium-235, -236, and -237 are predicted to be fissile; only the fissionability of neptunium-237 has been experimentally demonstrated, with a critical mass of about 60 kg, only about 10 kg more than the commonly used uranium-235. The calculated values ​​of the critical mass of neptunium-235, -236, and -237 were 66.2 kg, 6.79 kg, and 63.6 kg respectively; the value of neptunium-236 was even lower than that of plutonium-239. Especially ²³⁶ Np also has a low neutron cross section. Nevertheless, the neptunium atom bomb has never been built: uranium and plutonium have a critical mass lower than ²³⁵ Np and ²³⁷ Np, and ²³⁶ Np is difficult to be purified because it is not found in quantities in spent fuel and is virtually impossible to separate in significant quantities from its parent ²³⁷ Np.

Genesis

Since all neptunium isotopes have a half-life that is much shorter than Earth's age, any primordial neptunium should have decomposed now. After only about 80 million years, even the longest living isotope concentration, ²³⁷ Np, will decrease to less than a trillion (10 ^-12 ) from the original amount; and even if the whole Earth was originally made of pure ²³⁷ Np (and neglected that this would be more than its critical mass of 60 kg), 2100 and a half of life would pass since the formation of the Solar System, and thus all that will rot. Thus neptunium is present in nature only in negligible quantities which are produced as decay products between other isotopes.

The trace amounts of neptunium isotopes of neptunium-237 and -239 are found naturally as decay products of transmutation reactions in uranium ores. In particular, ²³⁹ Np and ²³⁷ Np are the most common of these isotopes; they are directly formed from the capture of neutrons by uranium-238 atoms. These neutrons are derived from the spontaneous fission of uranium-238, the fission induced by the natural neutrons of uranium-235, the cosmic ray spacings of the nuclei, and the light elements that absorb alpha particles and emit neutrons. The half-life of ²³⁹ Np is very short, although the detection of its longer-lived daughter ²³⁹ Pu in nature in 1951 definitively establishes its natural event. In 1952, ²³⁷ Np was identified and isolated from the uranium ore concentrate of the Belgian Congo: in this mineral, the ratio of neptunium-237 to uranium is less than or equal to about 10 ^{- 12} number 1.

Most of the neptunium (and plutonium) now found in the environment is due to the atmospheric nuclear explosion that occurred between the first atomic bombing of 1945 and the ratification of the Partial Nuclear Trial Ban Treaty in 1963. The total amount of neptunium released by these explosions and some the atmospheric test that has been conducted since 1963 is estimated to be around 2500 kg. The majority of these consist of long-lived isotopes ²³⁶ Np and ²³⁷ Np because even ²³⁵ Np (half-length) -Life 396 days) will rot to less than a billion (10 ^-9 ) of its original concentration for decades. A very small amount of neptunium, created by the neutron irradiation of natural uranium in the cooling water of a nuclear reactor, is released when water is discharged into rivers or lakes. The concentration of ²³⁷ Np in seawater is about 6.5 Ã,ÃÆ' â € "10 ^-5 Ã, millibecquerels per liter: this concentration is between 0.1% and 1% of plutonium.

Once in the environment, neptunium generally oxidizes rapidly, usually to a state of 4 or 5. Irrespective of the oxidation state, elements exhibit much greater mobility than other actinides, in large part because of their ability to easily form aqueous solutions with various other elements. In one study comparing the diffusion rate of neptunium (V), plutonium (IV), and americium (III) in sandstone and limestone, neptunium penetrated more than ten times as well as other elements. Np (V) will also react efficiently at a pH level greater than 5.5 if no carbonate exists and under these conditions have also been observed to be readily bonded with quartz. It has also been observed to bind to goetite, colloidal iron oxide, and some clays including kaolinite and smectite. Np (V) does not bind easily to soil particles in a slightly acidic condition such as an actinide americium and curium counterpart almost one order of magnitude. This behavior allows him to migrate quickly through the ground while in a solution without being fixed in place, contributing further to his mobility. Np (V) is also easily absorbed by concrete, which due to the radioactivity element is a consideration that must be overcome when building a nuclear waste storage facility. When absorbed in the concrete, it is reduced to Np (IV) in a relatively short time. Np (V) is also reduced by humic acid if present on the surface of goetite, hematite, and magnetite. Np (IV) is efficiently absorbed by tuff, granodiorite, and bentonite; although the uptake by the latter is most evident in slightly acidic conditions. It also shows a strong tendency to bind colloidal particulates, an enhanced effect when in soils with high clay content. Such behavior provides additional assistance in the high mobility observed by the elements.

Maps Neptunium

History

Background and initial claim

When the first periodic table of elements was published by Dmitri Mendeleev in the early 1870s, it showed "Ã, -" in place after uranium similar to some other places for undiscovered elements. Subsequent tables of known elements, including the 1913 publication of the radioactive isotope known by Kasimir Fajans, also show an empty space after uranium, element 92.

Until and after the discovery of the last component of the atomic nucleus, neutrons in 1932, most scientists did not seriously consider the possibility of heavier elements than uranium. While nuclear theories at the time did not explicitly prohibit their existence, there was little evidence to suggest that they did so. However, the discovery of radioactivity induced by IrÃÆ'¨ne and Frà © jÃÆ'  © ric Joliot-Curie in late 1933 opened 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 that involves firing neutrons. Although the Joliot-Curies experiment involves bombarding a sample of ²⁷ Al with alpha particles to produce radioactive ³⁰ P, Fermi realizes that using neutrons, which have no electrical charge, is likely to produce results which is better than positively charged alpha particles. Therefore, in March 1934, he began to systematically subjugate all known elements to the bombing of neutrons to determine whether others could also be induced into radioactivity.

After several months of work, the Fermi group has tentatively determined that lighter elements will dissolve the energy of the captured neutrons by emitting proton or alpha particles and heavier elements will generally achieve the same thing by emitting gamma rays. This latter behavior will result in beta decay from neutrons to protons, thus moving the isotopes generated one place onto the periodic table. When the Fermi team bombarded uranium, they observed this behavior as well, which strongly suggested that the isotope produced had atomic number 93. Fermi was initially reluctant to publish such claims, but after his team observed some unknown beaks in uranium. a bomb product that did not fit into the well-known isotope, he published a paper entitled The Possibility of Production of Element Number of Atoms Higher than 92 in June 1934. In it he proposed the name ausonium (atomic symbol Ao) for element 93, after the Greek name Ausonia (Italian).

Some of the theoretical objections to the claims of the Fermi paper are quickly put forward; in particular, the exact process that occurs when an atom captures a neutron is not understood at that time. The inadvertent discovery of this and Fermi three months later that nuclear reactions could be caused by slow neutrons raises further doubts in the minds of many scientists, notably Aristid von Grosse and Ida Noddack, that the experiment created the 93 element. While von Grosse claims that Fermi is actually producing protactinium (element 91) was quickly tested and refuted, Noddack's proposal that uranium has been destroyed into two or more smaller fragments is ignored by most because the existing nuclear theory does not include a way for this to be possible. Fermi and his team stated that they were actually synthesizing new elements, but the problem remained unresolved for several years.

Although many different and unknown radioactive lives in the experimental results show that some nuclear reactions occur, Fermi groups can not prove that element 93 is being made unless they can chemically isolate. They and many other scientists sought to achieve this, including Otto Hahn and Lise Meitner who were one of the world's best radiochemists at the time and supporters of the Fermi claim, but they all failed. Much later, it was determined that the main reason for this failure was that the prediction of the chemical properties of element 93 was based on a periodic table that had no actinide series. This arrangement places protactinium below tantalum, uranium below tungsten, and further suggests that element 93, at a point referred to as eka-rhenium, should be similar to elements of group 7, including manganese and rhenium. Thorium, protactinium, and uranium, with the dominant oxidation of 4, 5, and 6 each, fooled scientists into thinking they belonged under hafnium, tantalum, and tungsten, rather than under the lanthanide series, which at the time were seen as accidental , and whose members all have the dominant status 3; neptunium, on the other hand, has a much rarer, more unstable state, with 4 and 5 being the most stable. Having discovered that plutonium and other transuranic elements also have dominant 3 and 4 states, along with the discovery of f-block, the actinide series has been formed steadily.

While the question of whether the Fermi experiment has produced the 93 element is deadlocked, two additional claims about the discovery of that element appear, though unlike Fermi, they both claim to have observed in nature. The first of these claims was by Czech engineer Odolen Koblic in 1934 when he extracted a small amount of material from heated pitchblende washing water. He proposed the bohemium name for the element, but after being analyzed it was a mixture of tungsten and vanadium. Another claim, in 1938 by Romanian physicist Horia Hulubei and French chemist Yvette Cauchois, claimed to have discovered a new element through spectroscopy in minerals. They named the element sequences, but the claim was discounted because the theory that prevailed at the time was that if anything at all, element 93 would not exist naturally. However, since neptunium actually occurs in nature in very small quantities, as shown when it was discovered in uranium ore in 1952, it is possible that Hulubei and Cauchois actually observed neptunium.

Although in 1938 some scientists, including Niels Bohr, were still reluctant to accept that Fermi had actually produced a new element, he was still awarded the Nobel Prize in Physics in November 1938. "For a demonstration of the existence of new radioactive elements produced by neutron irradiation , and for the invention related to nuclear reactions posed by slow neutrons ". A month later, the almost unexpected discovery of nuclear fission by Hahn, Meitner, and Otto Frisch put an end to the possibility that Fermi had discovered element 93 because most of the unknown part-time lives that the Fermi team had observed were quickly identified. as a fission product.

Perhaps the closest of all attempts to produce the missing element 93 is that of Japanese physicist Yoshio Nishina working with chemist Kenjiro Kimura in 1940, just before the outbreak of the Pacific War in 1941: they bombarded ²³⁸ U with fast neutrons. However, while slow neutrons tend to induce neutron capture via reaction (n,?), Neutrons rapidly tend to induce "knock-out" (n2nn) reactions, in which one neutron is added and two are removed, resulting in neutron neutron loss. Nishina and Kimura, after testing this technique on ²³² Th and succeeded in producing ²³¹ Th known and long-lived beta decay ²³¹ Pa (both occur in natural decay chain ²³⁵ U), therefore precisely assigns a new 6.75 day part-time activity they observe to the new isotope ²³⁷ U. They confirm that this isotope also is a beta transmitter and must thus decompose to an unknown nucoid ²³⁷ 93. They try to isolate this nuclide by carrying it with a milder crushing renerium, but no beta or alpha decay is observed from the fraction which contains rhenium: Nishina and Kimura rightly speculate that the half-life of ²³⁷ 93, as well as ²³¹ Pa, is very long and therefore the activity will be so weak that it can not be measured with their equipment , so that m summarize the last and most unsuccessful search of the transuranic element.

Discovery

When research on nuclear cleavage developed in early 1939, Edwin McMillan at the Berkeley Radiation Laboratory of the University of California, Berkeley decided to run an uranium-bombarded experiment using a powerful 60-inch (1.52 m) cyclotron recently built at the university. The goal is to separate the various fission products produced by bombing by exploiting the great power gained by fragments from mutual electrical repulsion after fission. Though he found nothing of note from this, McMillan observed two new beta decay halves in the uranium trioxide target itself, which meant that whatever produced the radioactivity did not strongly reject one another like normal fission products. He quickly realized that one half of his life fits perfectly with the known 23-minute uranium-23 decay period, but another half-day of 2.3 days is unknown. McMillan took his experimental results to chemist and associate professor Berkeley Emilio SegrÃÆ'¨ to try to isolate the source of radioactivity. Both scientists began their work using the prevailing theory that element 93 would have chemistry similar to rhenium, but SegrÃÆ'¨ quickly determined that the McMillan sample was not at all similar to rhenium. Conversely, when it reacts with hydrogen fluoride (HF) with a strong oxidizing agent, it behaves like a member of a rare earth. Since these elements consisted of most fission products, SegrÃÆ'¨ and McMillan decided that half-life would be just another fission product, titled on the paper "Unsuccessful Search for Transuranium Elements".

This proves that an unknown source of radioactivity comes from uranium decay and, coupled with a previous observation that the source is chemically different from all known elements, proves beyond doubt that a new element has been discovered. McMillan and Abelson published their results in a paper titled Radioactive Element 93 in the Physical Review on May 27, 1940. They did not propose names for the elements in the paper, but they immediately decided on the name < i> neptunium because Neptune is the next planet outside of Uranus in our solar system. The success of McMillan and Abelson compared with Nishina and Kimura's close range can be attributed to a favorable half-life of ²³⁹ Np for rapid radiochemical analysis and decay ²³⁹ U, in contrast to slower decay than ²³⁷ U and very long half-life ²³⁷ Np.

Next development

It was also realized that beta decay <239 Np had to produce 94 element isotopes (now called plutonium), but the numbers involved in the original McMillan and Abelson experiments were too small to isolate and identify plutonium together. with neptunium. The discovery of plutonium had to wait until the end of 1940, when Glenn T. Seaborg and his team identified the plutonium-238 isotope.

The unique radioactive characteristics of Neptunium allow it to be traced as it travels through various compounds in chemical reactions, initially these are the only methods available to prove that chemistry differs from other elements. Since the first isotopes of neptunium were found to have short half-lives, McMillan and Abelson were unable to prepare samples large enough to perform chemical analysis of new elements using the then available technology. However, after the discovery of the long-lived Nook isotope in 1942 by Glenn Seaborg and Arthur Wahl, forming an amount of neptunium that can be weighed into a realistic effort. Its beak was originally set at about 3 million years (later revised to 2,144 million years), confirming Nishina and Kimura's predictions about a very long half-life.

Initial research into the element is somewhat limited because most nuclear physicists and chemists in the United States at that time focused on a massive effort to examine the properties of plutonium as part of the Manhattan Project. Research into that element continues as a small part of the project and the first bulk sample of neptunium was isolated in 1944.

Much of the research on the properties of neptunium has since been focused on understanding how to reduce it as part of nuclear waste. Because it has an isotope with a very long half-life, it is of particular concern in the context of designing a confinement facility that can last for thousands of years. It has found some limited uses as a radioactive tracer and precursor for various nuclear reactions to produce useful plutonium isotopes. However, most of the neptunium produced as a by-product of reaction in a nuclear power plant is considered a waste product.

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Production

Synthesis

The heavier isotopes of neptunium decay quickly, and the lighter isotopes of neptunium can not be produced by neutron capture, so the chemical separation of neptunium from the refrigerated nuclear fuel consumes almost pure ²³⁷ Np. The heavy, short-lived isotope ²³⁸ Np and ²³⁹ Np, useful as a radioactive tracer, is produced by irradiation of ²³⁷ Np and ^{238 U respectively, whereas mild isotics longer 235 Np and 236 Np are produced by irradiation 235 U with protons and deuterons in cyclotron.}

Artificial ²³⁷ Np metal is usually isolated by a ²³⁷ NpF ₃ reaction with liquid or lithium barium at about 1200 ° C and most often extracted from the material bar spent nuclear fuel in kilogram as a by-product in plutonium production.

2 NpF ₃ 3 Ba -> 2 Np 3 BaF ₂

By weight, neptunium-237 discharges about 5% and plutonium discharge and about 0.05% of spent nuclear fuel expenditure. However, even this fraction still amounts to more than fifty tons per year globally.

Purification method

Recovering uranium and plutonium from spent nuclear fuel for reuse is one of the main processes of the nuclear fuel cycle. Because it has a long half-life of more than 2 million years, the alpha ^<237 Np transmitter is one of the main isotopes of minor actinide separate from spent nuclear fuel. Many separation methods have been used to separate neptunium, operating on a small and large scale. Small scale refining operations have the goal of preparing pure neptunium as a metallic neptunium precursor and its compounds, and also to isolate and precede neptunium in the sample for analysis.

Most methods that separate neptunium ions exploit different chemical behaviors from different oxidizing states of neptunium (from 3 to 6 or sometimes even 7) in solution. Among the methods that have been or have been used are: solvent extraction (using various extractants, usually multidentate? -the derivatives, organophosphorus compounds, and amine compounds), chromatography using various ion exchange or chelating resin, coprecipitation (matrix which may include LaF ₃ , BaSO ₄ , Fe (OH) ₃ , and MnO ₂ ), electrodeposition, and biotechnology methods. Currently, commercial reprocessing plants use the Purex process, involving the extraction of uranium and plutonium solvents with tributyl phosphate.

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Chemicals and compounds

Chemical solutions

When in an aqueous solution, neptunium may be present in one of its five oxidation states (3 to 7) and each of them exhibits a distinctive color. The stability of each oxidation state depends on many factors, such as the presence of an oxidizer or reducing agent, the pH of the solution, the presence of complex forming ligand coordination, and even the concentration of neptunium in the solution.

In acidic solutions, neptunium (III) to neptunium (VII) ions exists as Np ³ , Np ⁴ , NpO
₂ , NpO ²
₂ , and NpO
₃ . In the basic solution, they exist as oxides and hydroxides Np (OH) ₃ , NpO ₂ , NpO ₂ OH, NpO ₂ (OH) ₂ , and NpO ^3- ₅ . Not much has been done to characterize neptunium in the basic solution. Np ³ and Np ⁴ can be easily reduced and oxidized to each other, as can NpO
₂ and NpO ² ₂ .

Neptunium (III)

Np (III) or Np ³ exists as a hydrated complex in an acid solution, Np (H
₂ O) ³
_n . It is a dark blue-violet and analogous to a lighter congener, a pink rare-earth ion Pm ³ . In the presence of oxygen, it rapidly oxidizes into Np (IV) unless a strong reducing agent is also present. However, it is the second most easily hydrolyzed ion in water, forming a NpOH ² ion. Np ³ is the dominant neptunium ion in pH 4-5 solution.

Neptunium (IV)

Np (IV) or Np ⁴ is pale yellow green in acidic solution, where it exists as a hydrated complex ( Np (H
₂ O) ⁴
_n ). It is highly unstable for hydrolysis in acidic aqueous solutions at pH 1 upwards, forming NpOH ³ . In the bottom solution, Np ⁴ tends to hydrolyze to form neutral neptunium (IV) hydroxide (Np (OH) ₄ ) and neptunium (IV) oxide (NpO ₂ ).

Neptunium (V)

Np (V) or NpO ₂ blue-green in aqueous solution, where it behaves as a strong Lewis acid. It is a stable ion and is the most common form of neptunium in aqueous solutions. Unlike homologous neighbors UO ₂ and PuO
₂ , NpO
₂ is not spontaneously disproportionate except at very low pH and high concentrations:

2 NpO ₂ 4 H ? Np ⁴ NpO ²
₂ 2 H ₂ O

Hydrolyses in basic solution to form NpO ₂ OH and NpO
(OH) ^-
₂ .

Neptunium (VI)

Np (VI) or NpO ²
₂ , neptunyl ions, showing pink or reddish in acidic and yellow-green solutions instead. It is a strong Lewis acid and is the main neptunium ion found in a 3-4 pH solution. Although stable in acidic solutions, it is quite easily reduced to Np (V), and unstable ions such as homologous esterium hexavalent uranium ions and plutonium (uranil and plutonil ions). Hydrolyses the basic solution to form oxo and hydroxo ions NpO ₂ OH , (NpO
₂ )
₂ (OH) ²
< sub style = "font-size: inherit; line-height: inherit; vertical-align: baseline"> 2 , and (NpO sub style = "font-size: inherit; line-height: inherit; vertical-align: baseline"> 2 )
₃ (OH)
₅ .

Neptunium (VII)

Np (VII) is dark green in a very basic solution. Although the chemical formula in the base solution is often referred to as NpO ^{3 -}
< sub style = "font-size: inherit; line-height: inherit; vertical-align: baseline"> 5 , this is a simplification and the actual structure may be closer to hydroso species such as [NpO
₄ (OH) < br> ₂ ^{3 -} . Np (VII) was first prepared in a base solution in 1967. In a strongly acidic solution, Np (VII) was found as NpO
₃ ; water quickly reduces this to Np (VI). The hydrolysis product is not as usual.

Hydroxide

Oxides and neptunium hydroxides are closely related to their ions. In general, the Np hydroxide at various levels of oxidation is less stable than actinide prior to the periodic table such as thorium and uranium and more stable than those afterwards such as plutonium and americium. This phenomenon is due to increased ionic stability when the ratio of atomic numbers to ionic radius increases. Thus higher actinides on the periodic table will more easily experience hydrolysis.

Neptunium (III) hydroxide is quite stable in acidic solutions and in environments lacking oxygen, but will rapidly oxidize to a state IV in the presence of air. It does not dissolve in water. Np (IV) hydroxide exists primarily as an electrically neutral Np (OH) ₄ and minor solubility in water is not affected at all by the pH of the solution. This shows that other Np (IV) hydroxides, Np (OH) ^-
₅ , has no significant presence.

Because Np (V) ion NpO ₂ very stable, can only form hydroxides in high acidity level. When placed in 0.1 M sodium perchlorate solution, it does not react significantly for several months, although a molar concentration higher than 3.0 M would result in it reacting to the solid hydroxide NpO ₂ OH immediately. Np (VI) hydroxide is more reactive but still stable enough in acid solution. This will form NpO ₃ Ã, Â · H ₂ O compounds in the presence of ozone under various pressures of carbon dioxide. Np (VII) has not been well studied and no neutral hydroxide is reported. Maybe there's mostly as [NpO
₄ (OH )
₂ ] ^{3 -} .

Oxide

Three anhydrous neptunium oxides have been reported, NpO ₂ , Np ₂ O ₅ , and Np ₅ O ₈ , although some studies suggest that only the first two exist, indicating that the claim Np ₅ O ₈ is actually the result of a false analysis of Np ₂ O ₅ . However, as far as the reaction between neptunium and oxygen has not been investigated, it is not clear which claims are accurate. Although neptunium oxide has not been produced with neptunium in the highest possible oxidation state with adjacent uranium actinide, neptunium oxide is more stable at lower oxidation states. This behavior is illustrated by the fact that NpO ₂ can be produced only by burning the neptunium salts of violet acid in the air.

Greenish green NpO ₂ is highly stable at large pressure and temperature and does not undergo phase transition at low temperature. This shows the phase transition from a face-to-orthorhombic cube at about 33-37GPa, although it returns to the initial phase when pressure is released. Stay stable under oxygen pressure up to 2.84 MPa and temperatures up to 400 Â ° C. Np ₂ O ₅ black-brown and monoclinic with lattice size 418ÃÆ' â € " 658ÃÆ' â € "409 pixometer. Relatively unstable and decomposes into NpO ₂ and O ₂ at 420-695Ã, Â ° C. Although Np ₂ O ₅ was initially subject to several studies claimed to produce it by contradictory methods, finally successfully prepared by heating the neptunium peroxide to 300-350Ã, Â °. C for 2-3 hours or by heating it below the water layer in the ampoule at a temperature of 180 ° C.

Neptunium also forms large amounts of oxide compounds with various elements, although neptunate oxides formed with alkali metals and alkaline earth metals have become the most studied. Neptunium binary oxides are generally formed by reacting NpO ₂ with other elemental oxides or by precipitating from an alkaline solution. Li ₅ NpO ₆ has been prepared by reacting Li ₂ O and NpO ₂ at 400 Ã, Â ° C during 16 hours or by reacting Li ₂ O ₂ with NpO ₃ Ã, Â · H ₂ O at 400 Ã , Â ° C for 16 hours in quartz tube and oxygen flow. Compounds of alkali neptunate K ₃ NpO ₅ , Cs ₃ NpO ₅ , and Rb ₃ NpO ₅ are all made by a similar reaction: <3> ₂ -> M ₃ NpO ₅ (M = K, Cs, Rb)

The oxide compound KNpO ₄ , CsNpO ₄ , and RbNpO ₄ are formed by reacting Np (VII) ( [NpO > ₄ (OH)
₂ ] ^{3 -}
) with alkali metal and ozone nitrate compounds. Additional compounds have been produced by reacting NpO ₃ and water with alkaline peroxide and a solid base at a temperature of 400 - 600 Â ° C for 15-30 hours. Some of them include Ba ₃ (NpO ₅ ) ₂ , Ba ₂ NaNpO ₆ , and Ba

sub> 2 LiNpO ₆ . Also, large amounts of hexavelant neptunium oxide are formed by reacting NpO under solids ₂ with various alkaline or alkaline earth oxides in a flowing oxygen environment. Many of the resulting compounds also have equivalent compounds that replace uranium for neptunium. Some of the compounds that have been characterized include Na ₂ Np ₂ O ₇ , Na ₄ NpO ₅ , Na ₆ NpO ₆ , and Na ₂ NpO ₄ . This can be obtained by heating the various combinations of NpO ₂ and Na ₂ O to various threshold temperatures and further heating will also cause these compounds to show different allotropes of neptunium. The lithium neptunate oxides Li ₆ NpO ₆ and Li ₄ NpO ₅ can be obtained by the same NpO reactions ₂ and Li ₂ O.

A large number of additional alkali and alkaline neptunium oxide compounds such as Cs ₄ Np ₅ 17 and Cs ₂ Np ₃ O ₁₀ have been characterized by various production methods. Neptunium has also been observed to form ternary oxides with many additional elements in groups of 3 to 7, although these compounds are poorly studied.

Halide

Although the neptunium halide compounds have not been nearly as studied as oxides, large numbers have been successfully characterized. Of these, neptunium fluoride has been widely investigated, primarily because of their potential use in separating elements from nuclear waste products. Four neptunium fluoride binary compounds, NpF ₃ , NpF ₄ , NpF ₅ , and NpF ₆ , have been reported. The first two are quite stable and were first prepared in 1947 through the following reactions:

NpO ₂ ¹ / ₂ H ₂ 3 HF -> NpF ₃ 2 H ₂ OÃ,Ã, (400Ã, Â ° C)

NpF ₃ ¹ / ₂ O ₂ HF -> NpF < sub> 4 ¹ / ₂ H ₂ O Â ° (400 Â ° C)

Then, NpF ₄ is obtained directly by heating NpO ₂ to various temperatures in a mixture of hydrogen fluoride or pure fluorine gas. NpF ₅ is much harder to make and the best known preparation method involves reacting NpF ₄ or NpF ₆ compounds with various other fluoride compounds. NpF ₅ will break down into NpF ₄ and NpF ₆ when heated to about 320 ° C.

NpF ₆ or neptunium hexafluoride is highly volatile, as does the adjacent actinide compound uranium hexafluoride (UF ₆ ) and plutonium hexafluoride (PuF ₆ ). This volatility has attracted a considerable amount of interest to the compound in an effort to devise a simple method for extracting neptunium from fuel rods of spent nuclear power plants. NpF ₆ was first prepared in 1943 by reacting NpF ₃ and fluor gas at very high temperatures and the first bulk quantity was obtained in 1958 by heating NpF ₄ and drips pure fluorine on it in a specially prepared apparatus. Successful methods of producing neptunium hexafluoride include reacting Brf ₃ and BrF ₅ with NpF ₄ and by reacting several different neptunium oxide and fluoride compounds with hydrogen fluoride anhydrous.

Four compounds of neptunium oxyfluoride, NpO ₂ F, NpOF ₃ , NpO ₂ F ₂ , and NpOF ₄ , have been reported, although none of them have been studied extensively. NpO ₂ F ₂ is a pink solid and can be prepared by reacting NpO ₃ Ã, Â · H ₂ O and Np ₂ F ₅ with pure fluor around 330Ã, Â ° C. NpOF ₃ and NpOF ₄ can be produced by reacting neptunium oxide with anhydrous hydrogen fluoride at various temperatures. Neptunium also forms various fluoride compounds with various elements. Some of which have been characterized include CsNpF ₆ , Rb ₂ NpF ₇ , Na ₃ NpF ₈ , and K ₃ NpO ₂ F ₅ .

Two neptunium chlorides, NpCl ₃ and NpCl ₄ , have been characterized. Despite some attempts to make NpCl ₅ have been created, they have not succeeded. NpCl ₃ is prepared by reducing neptunium dioxide with hydrogen and carbon tetrachloride (CCl ₄ ) and NpCl ₄ by reacting neptunium oxide with CCl ₄ about 500 Â ° C. Other neptunium chloride compounds have also been reported, including NpOCl ₂ , Cs ₂ NpCl ₆ , Cs < sub> 3 NpO ₂ Cl ₄ , and Cs ₂ NaNpCl ₆ . Neptunium bromides NpBr ₃ and NpBr ₄ have also been created; the latter by reacting aluminum bromide with NpO ₂ at 350 Â ° C and the first in an almost identical procedure but with the presence of zinc. Neptunium iodide NpI ₃ has also been prepared with the same method as NpBr ₃ .

Chalcogenides, pnictides, and carbides

Neptunium chalcogen and pnictogen compounds have been well studied, especially as part of their research on their electronic and magnetic properties and their interactions in the natural environment. The pnictide and carbide compounds also attract interest because of their presence in the fuel of several advanced nuclear reactor designs, although this latter group lacks nearly as much research as it used to be.

Chalcogenides

Various kinds of neptunium sulphide compounds have been characterized, including pure sulfide compounds NpS, NpS ₃ , Np ₂ S ₅ , Np ₃ sub , Np ₂ S ₃ , and Np ₃ S ₄ . Of this amount, Np ₂ S ₃ , prepared by reacting NpO ₂ with hydrogen sulfide and carbon disulfide at about 1000 ° C, most studied well and allotropic forms are known. That? shape there is up to about 1230 Â ° C, is it? up to 1530 Â ° C, and? shape, which can also exist as Np ₃ S ₄ , at higher temperatures. NpS can be made by reacting Np ₂ S ₃ and neptunium metal at 1600 Â ° C and Np ₃ S ₅ can be prepared by decomposing Np ₂ S ₃ at 500 Ã, Â ° C or by reacting sulfur and neptunium hydride at 650 Ã, Â ° C. Np ₂ S ₅ made by heating the mixture of Np ₃ S ₅ and pure sulfur up to 500 Â ° C. All neptunium sulfide except for? and? form Np

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