# neptunium

properties
[ Rn ] 5 f 4 6 d 1 7 s 2
93 Np
General
Name , symbol , atomic number Neptunium, Np, 93
Element category Actinoids
Group , period , block Ac , 7 , f
Appearance silvery
CAS number 7439-99-8
EC number 231-108-8
ECHA InfoCard 100.028.280
Mass fraction of the earth's envelope 4 · 10 −14  ppm
Atomic
Atomic mass 237.0482 u
Electron configuration [ Rn ] 5 f 4 6 d 1 7 s 2
1. Ionization energy 6th.26554 (25) eV604.53 kJ / mol
2. Ionization energy 11.5 (4) eV1 110 kJ / mol
3. Ionization energy 19th.7 (4) eV1 900 kJ / mol
4. Ionization energy 33.8 (4) eV3 260 kJ / mol
5. Ionization energy 48.0 (1.9) eV4 630 kJ / mol
Physically
Physical state firmly
Modifications 3
Crystal structure orthorhombic
density 20.45 g / cm 3
Melting point 912 K (639 ° C)
boiling point 4175 K (3902 ° C)
Molar volume 11.59 · 10 −6 m 3 · mol −1
Heat of evaporation 1420 kJ / mol
Heat of fusion 39.91 kJ mol −1
Electric conductivity 0.82 A V −1 m −1 at 293 K
Thermal conductivity 6.30 W m −1 K −1 at 300 K
Chemically
Oxidation states +3, +4, +5 , +6, +7
Normal potential −1.79  V
(Np 3+ + 3 e - → Np)
Electronegativity 1.36 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
235 Np {syn.} 396.1 d α 5.192 231 Pa
ε 0.124 235 U
236 Np {syn.} 1.54 · 10 5 a ε 0.940 236 U
β - 0.490 236 Pu
α 5.020 232 Pa
237 Np {syn.} 2.144 x 10 6 a α 4,959 233 Pa
238 Np {syn.} 2.117 d β - 238 Pu
239 Np {syn.} 2,355 d β - 239 Pu
For other isotopes see list of isotopes
Hazard and safety information

GHS hazard labeling
no classification available
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Neptunium is a chemical element with the element symbol Np and the atomic number 93. In the periodic table it is in the group of actinides ( 7th period , f-block ). Neptunium is the first of the so-called transuranic elements , which, apart from traces of neptunium and plutonium , no longer occur naturally on earth . Neptunium is a toxic and radioactive heavy metal . It was named after the planet Neptune , which succeeds the planet Uranus . Neptunium follows uranium in the periodic table , followed by plutonium, the naturally occurring element on earth with the highest atomic number.

## history

Neptunium is the first of the so-called transuranic elements , the existence and properties of which have long been controversial among physicists.

### Possibility of transurans

Mendeleev's periodic table from 1871 with a gap for Neptunium at the bottom, behind uranium ( U = 240 )

Already in Mendeleev's periodic table of elements from 1871 there was a gap behind uranium, the heaviest known element at the time.

More than sixty years later, Ida Noddack commented in May 1934 on the gaps in Mendeleev's periodic table that existed at the time and, at the end of her work, considered the possibility of transuranics. A few weeks later, Enrico Fermi published three papers on this topic. In September 1934, Noddack took a critical look at the supposed discovery of element 93 by Fermi. In her remarks she took u. a. the discovery of neutron-induced nuclear fission in advance: “It is conceivable that when heavy nuclei are bombarded with neutrons, these nuclei disintegrate into several larger fragments, which are isotopes of known elements, but not neighbors of the irradiated elements. “Regardless of Noddack's objections, at that time all working groups followed the hypothesis that irradiating uranium with neutrons always produced elements heavier than uranium.

At the Kaiser Wilhelm Institute for Chemistry in Berlin , Otto Hahn , Fritz Straßmann and Lise Meitner also went in search of transuranium elements during this time. Over many years they tried to elucidate the processes observed in Fermi's experiments. While searching for heavier elements, they found some substances that they described as evidence of transuranium elements.

Irène Joliot-Curie and Paul Savitch also devoted themselves to the search for transurans in Paris from 1937 onwards . In 1937/1938 her working group carried out experiments in which an element similar to lanthanum was released, the chemical identification of which turned out to be extremely difficult and which they interpreted as a possible evidence of element 93 based on the assumptions about chemical relationships at the time. They considered it possible that the substance discovered “ has an atomic number of 93 and that the transuranic elements found by Hahn, Meitner and Straßmann to date are elements 94 to 97. "

In fact, the observations made by Hahn, Straßmann, Meitner, Joliot-Curie and Savitch were not evidence of the transuranic elements they were looking for, but rather the nuclear fission of uranium, which was still unrecognized at the time . Their research therefore contributed comparatively little to the knowledge about the chemical element later called "neptunium", since they considered the numerous fission products formed during the nuclear fission of uranium to be evidence of the sought-after transuranic elements and described them as such in their publications.

### Exploration of Neptunium

Edwin M. McMillan and Philip H. Abelson synthesized the radioactive element neptunium for the first time in 1940 by bombarding uranium with neutrons.

${\ displaystyle \ mathrm {^ {238} _ {\ 92} U \ + \ _ {0} ^ {1} n \ \ longrightarrow \ _ {\ 92} ^ {239} U \ {\ xrightarrow [{23 \ min}] {\ beta ^ {-}}} \ _ {\ 93} ^ {239} Np \ {\ xrightarrow [{2,355 \ d}] {\ beta ^ {-}}} \ _ {\ 94} ^ {239} Pooh}}$
The times given are half-lives .

Arthur C. Wahl and Glenn T. Seaborg discovered the neptunium isotope 237 Np in 1942 . It arises from 237 U, which is a β-emitter with a half-life of around 7 days, or through an (n, 2n) process from 238 U. 237 Np is an α-emitter with a half-life of 2.144 · 10 6 years.

${\ displaystyle \ mathrm {^ {238} _ {\ 92} U \ {\ xrightarrow [{}] {(n, \ 2n)}} \ _ {\ 92} ^ {237} U \ {\ xrightarrow [{ 7 \ d}] {\ beta ^ {-}}} \ _ {\ 93} ^ {237} Np \ {\ xrightarrow [{2,144 \ x \ 10 ^ {6} \ a}] {\ alpha}} \ _ {\ 91} ^ {233} Pa}}$

In 1950 the Neptunium isotopes 231 Np, 232 Np and 233 Np were generated from 233 U, 235 U and 238 U by bombardment with deuterons . In 1958, the Neptunium isotopes 234 Np, 235 Np and 236 Np were generated from highly enriched 235 U by bombardment with deuterons . The 1-hour Neptunium activity, which was previously assigned to the 241 Np, on the other hand, belongs to the isotope 240 Np.

## Extraction and presentation

### Extraction of Neptunium isotopes

Neptunium is a by-product of energy generation in nuclear reactors . One tonne of spent nuclear fuel can contain around 500 grams of neptunium. The resulting neptunium consists almost exclusively of the isotope 237 Np. It arises from the uranium isotope 235 U through double neutron capture and subsequent β-decay .

${\ displaystyle \ mathrm {^ {235} _ {\ 92} U \ + \ _ {0} ^ {1} n \ \ longrightarrow \ _ {\ 92} ^ {236} U_ {m} \ {\ xrightarrow [ {120 \ ns}] {}} \ _ {\ 92} ^ {236} U \ + \ \ gamma}}$
${\ displaystyle \ mathrm {^ {236} _ {\ 92} U \ + \ _ {0} ^ {1} n \ \ longrightarrow \ _ {\ 92} ^ {237} U \ {\ xrightarrow [{6, 75 \ d}] {\ beta ^ {-}}} \ _ {\ 93} ^ {237} Np}}$

### Depiction of elemental Neptunium

Metallic neptunium can be obtained from its compounds by reduction . First, neptunium (III) fluoride was reacted with elemental barium or lithium at 1200 ° C.

${\ displaystyle {\ ce {2 NpF3 + 3 Ba -> 2 Np + 3 BaF2}}}$

## properties

### Physical Properties

Neptunium metal has a silver appearance, is chemically reactive and exists in at least three different modifications:

Modifications at atmospheric pressure
Phase designation stable temperature range Density (temperature) Crystal system
α-Np 20.25 g / cm 3 (20 ° C) orthorhombic
β-Np above 280 ° C 19.36 g / cm 3 (313 ° C) tetragonal
γ-Np above 577 ° C 18.0 g / cm 3 (600 ° C) cubic

Neptunium has one of the highest densities of all elements. Along with rhenium , osmium , iridium and platinum , it is one of the few elements that have a density higher than 20 g / cm 3 .

### Chemical properties

Neptunium forms a number of compounds in which it can exist in the oxidation states +3 to +7. Thus, together with plutonium, neptunium has the highest possible oxidation level of all actinides. In aqueous solution, the neptunium ions have characteristic colors, the Np 3+ ion is purple-violet, Np 4+ yellow-green, Np V O 2 + green, Np VI O 2 2+ pink-red and Np VII O 2 3+ deep green.

### Biological aspects

A biological function of the neptunium is not known. Anaerobic microorganisms can reduce Np (V) to Np (IV) using Mn (II / III) and Fe (II) species. The factors that influence the biosorption and bioaccumulation of neptunium by bacteria were also examined .

## Isotopes

A total of 20 isotopes and 5 nuclear isomers are known of neptunium . The longest-lived isotopes are 237 Np with 2.144 million years, 236 Np with 154,000 years and 235 Np with 396.1 days half-life. The remaining isotopes and core isomers have half-lives between 45 nanoseconds ( 237 m 1 Np) and 4.4 days ( 234 Np).

• 235 Np decays with a half-life of 396.1 days in 99.99740% of cases by electron capture to uranium 235 U and in 0.00260% of cases by alpha decay to protactinium 231 Pa, which is one step behind 235 U on the uranium actinium Row is located.
• With a half-life of 154,000 years, 236 Np decays in 87.3% of cases by electron capture to uranium 236 U, in 12.5% ​​of cases by beta decay to plutonium 236 Pu and in 0.16% of cases by alpha decay to protactinium 232 Pa. The uranium 236 U is part of the thorium series and decays at 23.42 million years to its official initial nuclide thorium 232 Th. The 236 Pu decays with a half-life of 2.858 years through α-decay to the intermediate 232 U, which with a Half-life of 68.9 yearsdecaysto 228 Th, which is on the main strand of the series.
• 237 Np decays with a half-life of 2.144 million years through alpha decay to protactinium 233 Pa. 237 Np is the official starting point of the Neptunium series , a decay chain thatendsat the isotope thallium 205 Tl.

## Cleavage

A sample of neptunium metal ( 237 Np), encased by a thick layer of tungsten and nickel (shiny) in shells made of enriched uranium (tarnished black).

As with all transurane nuclides, neutron-induced nuclear fission is also possible with the Np isotopes. The isotopes with an odd number of neutrons in the nucleus - 236 Np of the long-lived ones - have large effective cross- sections for fission by thermal neutrons ; at 236 Np it is 2600  barns , so it is "easy to split".

With the 237 Np occurring in the nuclear reactor fuel, this cross section is only 20 millibarns. However, due to other nuclear physical properties, this isotope is suitable for maintaining a chain reaction with the fission by fast neutrons in the pure material . At the Los Alamos National Laboratory , its critical mass was experimentally determined to be around 60 kg. Therefore, 237 Np is a possible material for nuclear weapons.

## use

The 237 Np produced in nuclear reactors from 235 U can be used to obtain 238 Pu for use in radionuclide batteries . To do this, it is separated from the spent reactor fuel (along with insignificant amounts of other neptunium isotopes) and put into fuel rods that contain only neptunium. These are put back into the nuclear reactor, where they are again irradiated with neutrons; 238 Pu is incubated from the 237 Np .

${\ displaystyle \ mathrm {^ {237} _ {\ 93} Np \ + \ _ {0} ^ {1} n \ \ longrightarrow \ _ {\ 93} ^ {238} Np \ {\ xrightarrow [{2,117 \ d}] {\ beta ^ {-}}} \ _ {\ 94} ^ {238} Pu}}$
The times given are half-lives .

Neptunium in the oxidation states +3 to +7 in aqueous solution.

→ Category: Neptunium compound

### Oxides

Oxides are known in the stages +4 to +6: Neptunium (IV) oxide (NpO 2 ), Neptunium (V) oxide (Np 2 O 5 ) and Neptunium (VI) oxide (NpO 3  · H 2 O) . Neptunium dioxide (NpO 2 ) is the most chemically stable oxide of neptunium and is used in nuclear fuel rods .

### Halides

For neptunium, halides in the oxidation states +3 to +6 are known.

For level +3 all compounds of the four halogens fluorine , chlorine , bromine and iodine are known. In addition, it forms halides in the levels +4 to +6.

In the +6 oxidation state, neptunium hexafluoride (NpF 6 ) is of particular importance. It is an orange-colored solid with very high volatility that changes to a gaseous state at 56 ° C. In this property it is very similar to uranium hexafluoride and plutonium hexafluoride , so it can also be used in enrichment and isotope separation .

 Oxidation number F. Cl Br I. +6 Neptunium (VI) fluoride NpF 6 orange +5 Neptunium (V) fluoride NpF 5 light blue +4 Neptunium (IV) fluoride NpF 4 green Neptunium (IV) chloride NpCl 4 red-brown Neptunium (IV) bromide NpBr 4 dark red +3 Neptunium (III) fluoride NpF 3 violet Neptunium (III) chloride NpCl 3 green Neptunium (III) bromide NpBr 3 green Neptunium (III) iodide NpI 3 violet

### Organometallic compounds

Analogous to uranocene , an organometallic compound in which uranium is complexed by two cyclooctatetraene ligands, the corresponding complexes of thorium , protactinium , plutonium, americium and also of neptunium, (η 8 -C 8 H 8 ) 2 Np, were prepared.

## safety instructions

Classifications according to the CLP regulation are not available because they only include chemical hazard and play a completely subordinate role compared to the hazards based on radioactivity.

## literature

Commons : Neptunium  - collection of images, videos and audio files
Wiktionary: Neptunium  - explanations of meanings, word origins, synonyms, translations

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6. The hazards emanating from radioactivity do not belong to the properties to be classified according to the GHS labeling. With regard to other hazards, this element has either not yet been classified or a reliable and citable source has not yet been found.
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