# Neptunium

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Neptunium
93Np
 Pm ↑Np ↓ (Uqp) uranium ← neptunium → plutonium
Appearance
silvery metallic
General properties
Name, symbol, number neptunium, Np, 93
Pronunciation / n ɛ p ˈ tj n i ə m /
nep-TEW-nee-əm
Element category actinide
Group, period, block n/a, 7, f
Standard atomic weight (237)
Electron configuration [Rn] 5f4 6d1 7s2
2, 8, 18, 32, 22, 9, 2
History
Discovery Edwin McMillan and Philip H. Abelson (1940)
Physical properties
Phase solid
Density (near r.t.) 20.45 g·cm−3
Melting point 910 K, 637 °C, 1179 °F
Boiling point 4273 K, 4000 °C, 7232 °F
Heat of fusion 3.20 kJ·mol−1
Heat of vaporization 336 kJ·mol−1
Molar heat capacity 29.46 J·mol−1·K−1
Vapor pressure
 P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2194 2437
Atomic properties
Oxidation states 7, 6, 5, 4, 3
( amphoteric oxide)
Electronegativity 1.36 (Pauling scale)
Ionization energies 1st: 604.5 kJ·mol−1
Miscellanea
Crystal structure orthorhombic
Magnetic ordering paramagnetic
Electrical resistivity (22 °C) 1.220 µΩ·m
Thermal conductivity 6.3 W·m−1·K−1
CAS registry number 7439-99-8
Most stable isotopes
Main article: Isotopes of neptunium
iso NA half-life DM DE ( MeV) DP
235Np syn 396.1 d α 5.192 231Pa
ε 0.124 235U
236Np syn 1.54×105 y ε 0.940 236U
β 0.940 236Pu
α 5.020 232Pa
237Np trace 2.144×106 y SF & α 4.959 233Pa
239Np trace 2.356 d β 0.218 239Pu

Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element, and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production, and it can be used as a component in neutron detection equipment. Neptunium is also found in trace amounts in uranium ores due to transmutation reactions.

## History

The periodic table of Dmitri Mendeleev published in the 1870s showed a " — " in place after uranium similar to several other places for at that point undiscovered elements. Also, a 1913 publication of the known radioactive isotopes by Kasimir Fajans shows the empty place after uranium.

### False reports of discovery

In 1934, Odolen Koblic extracted a small amount of material from the wash water of roasted pitchblende. He assumed the sample was element 93, and called it bohemium, but after being analyzed, it turned out that the sample was a mixture of tungsten and vanadium. Also in 1934, Enrico Fermi attempted to bombard uranium with neutrons to produce elements 93 and 94. He was also unsuccessful, but had unknowingly discovered nuclear fission. In 1938, Horia Hulubei, a Romanian physicist; and Yvette Cauchois, a French chemist; claimed to have discovered element 93 via spectroscopy in minerals. They named their element sequanium, but the claim was opposed at the time because neptunium was thought to occur exclusively artificially. However, as neptunium does occur in nature, it is possible that Hulubei and Cauchois did in fact discover neptunium.

### Actual discovery

The search for element 93 in minerals was encumbered by the fact that the predictions on the chemical properties of element 93 were based on a periodic table which lacked the actinide series, and therefore placed thorium below hafnium, protactinium below tantalum, and uranium below tungsten. This periodic table suggested that element 93, at that point often named eka-rhenium, should be similar to manganese or rhenium. With this misconception it was impossible to isolate element 93 from minerals, although neptunium was later found in uranium ore, in 1952.

Enrico Fermi believed that bombarding uranium with neutrons and subsequent beta decay would lead to the formation of element 93. Chemical separation of the new formed elements from the uranium yielded material with low half-life, and, therefore, Fermi announced the discovery of a new element in 1934, though this was soon found to be mistaken. Soon it was speculated and later proven that most of the material is created by nuclear fission of uranium by neutrons. Small quantities of neptunium had to be produced in Otto Hahn's experiments in late 1930s as a result of decay of 239U. Hahn and his colleagues experimentally confirmed production and chemical properties of 239U, but were unsuccessful at isolating and detecting neptunium.

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was discovered by Edwin McMillan and Philip H. Abelson in 1940 at the Berkeley Radiation Laboratory of the University of California, Berkeley. The team produced the neptunium isotope 239Np (2.4 day half-life) by bombarding uranium with slow moving neutrons. It was the first transuranium element produced synthetically and the first actinide series transuranium element discovered.

$\mathrm{^{238}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{239}_{\ 92}U\ \xrightarrow[23 \ min]{\beta^-} \ ^{239}_{\ 93}Np\ \xrightarrow[2.355 \ d]{\beta^-} \ ^{239}_{\ 94}Pu}$

## Occurrence

The most stable isotope of neptunium is 237Np, with a half-life of two million years. Thus, all primordial neptunium should have decayed by now. However, trace amounts of the neptunium isotopes neptunium-237 through neptunium-240, are found naturally as decay products from transmutation reactions in uranium ores.

Artificial 237Np is produced 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.

2 NpF3 + 3 Ba → 2 Np + 3 BaF2

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

## Characteristics

Silvery in appearance, neptunium metal is chemically fairly reactive and is found in at least three allotropes:

• α-neptunium, orthorhombic, density 20.45 g/cm3
• β-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 melting point and boiling point. It is the densest of all the actinides and the fifth-densest of all naturally occurring elements. Neptunium has no biological role. It is not absorbed by the digestive tract. When injected into the body, it accumulates in bones, which it is slowly released from.

## Isotopes

19 neptunium radioisotopes have been characterized, with the most stable being 237Np with a half-life of 2.14 million years, 236Np with a half-life of 154,000 years, and 235Np with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has 4 meta states, with the most stable being 236mNp (t½ 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (225Np) to 244.068 u (244Np). The primary decay mode before the most stable isotope, 237Np, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before 237Np are element 92 (uranium) isotopes (alpha emission produces element 91, protactinium, however) and the primary products after are element 94 (plutonium) isotopes.

237Np is fissionable. 237Np 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.

## Synthesis

Chemically, neptunium is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200 °C. Most Np is produced in nuclear reactions:

• When an 235U atom captures a neutron, it is converted to an excited state of 236U. About 81% of the excited 236U nuclei undergo fission, but the remainder decay to the ground state of 236U by emitting gamma radiation. Further neutron capture creates 237U which has a half-life of 7 days and thus quickly decays to 237Np through beta decay. During beta decay, the excited 237U emits an electron, while the atomic weak interaction converts a neutron to a proton, thus creating 237Np.
$\mathrm{^{235}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{236}_{\ 92}U_m\ \xrightarrow[120 \ ns]{} \ ^{236}_{\ 92}U\ +\ \gamma}$
$\mathrm{^{236}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{237}_{\ 92}U\ \xrightarrow[6.75 \ d]{\beta^-} \ ^{237}_{\ 93}Np}$
• 237U is also produced via an (n,2n) reaction with 238U. This only happens with very energetic neutrons.
• 237Np is the product of alpha decay of 241Am.

Heavier isotopes of neptunium decay quickly, and lighter isotopes of neptunium cannot be produced by neutron capture, so chemical separation of neptunium from cooled spent nuclear fuel gives nearly pure 237Np.

## Chemistry

Neptunium ions in solution.

This element has four ionic oxidation states while in solution:

• Np3+ (pale purple), analogous to the rare earth ion Pm3+
• Np4+ (yellow-green)
• NpO+
2
(green-blue)
• NpO2+
2
(pale pink)

Neptunium(III) hydroxide is not soluble in water and does not dissolve in excess alkali. Neptunium(III) is susceptible to oxidation in contact to air forming neptunium(IV).

Neptunium forms tri- and tetra halides such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides of the various compositions such as are found in the uranium-oxygen system, including Np3O8 and NpO2.

Neptunium hexafluoride, NpF6, is volatile like uranium hexafluoride.

Neptunium, like protactinium, uranium, plutonium, and americium readily forms a linear dioxo neptunyl core (NpO2n+), in its 5+ and 6+ oxidation states, which readily complexes with hard O-donor ligands such as OH, NO2, NO3, and SO42– to form soluble anionic complexes which tend to be readily mobile with low affinities to soil.

• NpO2(OH)2
• NpO2(CO3)
• NpO2(CO3)23–
• NpO2(CO3)35–

## Applications

### Precursor in plutonium-238 production

237Np is irradiated with neutrons to create 238Pu, an alpha emitter for radioisotope thermal generators for spacecraft and military applications. 237Np will capture a neutron to form 238Np and beta decay with a half-life of two days to 238Pu.

$\mathrm{^{237}_{\ 93}Np\ +\ ^{1}_{0}n\ \longrightarrow \ ^{238}_{\ 93}Np\ \xrightarrow[2.117 \ d]{\beta^-} \ ^{238}_{\ 94}Pu}$

238Pu also exists in sizable quantities in spent nuclear fuel but would have to be separated from other isotopes of plutonium.

### Weapons applications

Neptunium is fissionable, and could theoretically be used as fuel in a fast neutron reactor or a nuclear weapon, with a critical mass of around 60 kilograms. In 1992, the U.S. Department of Energy declassified the statement that neptunium-237 "can be used for a nuclear explosive device". It is not believed that an actual weapon has ever been constructed using neptunium. 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.

In September 2002, researchers at the Los Alamos National Laboratory briefly created the first known nuclear critical mass using neptunium in combination with shells of enriched uranium ( U-235), discovering that the critical mass of a bare sphere of neptunium-237 "ranges from kilogram weights in the high fifties to low sixties," showing that it "is about as good a bomb material as U-235." 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.

### Physics applications

237Np is used in devices for detecting high-energy (MeV) neutrons.

## Role in nuclear waste

Neptunium-237 is the most mobile actinide in the deep geological repository environment. This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation. 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. With a half-life of 432 years, the americium-241 in a smoke detector includes about 3% neptunium after 20 years, and about 15% after 100 years.

Due to its long half-life, neptunium becomes the major contributor of the total radiation in 10,000 years. As it is unclear what happens to the containment in that long time span, an extraction of the neptunium would minimize the contamination of the environment if the nuclear waste could be mobilized after several thousand years.

## Literature

• Guide to the Elements – Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1
• Lester R. Morss, Norman M. Edelstein, Jean Fuger (Hrsg.): The Chemistry of the Actinide and Transactinide Elements, Springer-Verlag, Dordrecht 2006, ISBN 1-4020-3555-1.
• Ida Noddack (1934). "Über das Element 93". Zeitschrift für Angewandte Chemie 47 (37): 653. doi: 10.1002/ange.19340473707. http://www.chemteam.info/Chem-History/Noddack-1934.html.