neptunium

Np. An element with atomic number 93. One of the transuranium elements in group ³ of the periodic table with the electron configuration [Rn] ^5f46d7s2 . One of the actinide elements. In addition to the longest-lived nuclide with mass number 237 and half-life of 2.14× ^106y that undergoes alpha decay, radioactive nuclides with mass numbers 225-244 are known. In 1940, E. McMillan and P. Abelson synthesized a beta-decaying nuclide with mass number 239 (half-life 2.4 d) by bombarding natural uranium with neutrons generated by irradiating Be with deuterons in a cyclotron at the University of California, Berkeley. As it is the element next to uranium, it was named neptunium after Neptune, the planet next to Uranus in the solar system.
²³⁷ Np is produced in a nuclear reactor by the ( n , 2n ) reaction of ²³⁸ U or by the two-step neutron capture of ²³⁵ U. It is an artificial radioactive element, but exists in very small amounts in uranium ores. When the trifluoride is reduced with metallic barium, silvery-white neptunium metal is obtained. It has three transformations, α, β, and γ, and the α phase (orthorhombic system) transforms to the β phase (tetragonal system) at 278 °C and to the γ phase (cubic system) at 577 °C. Its melting point is 640 °C and its boiling point is 3900 °C. Density (g cm ^-3 ): α 20.25 (20 °C), β 19.36 (313 °C), γ 18.0 (600 °C). First ionization energy is 6.266 eV. It is stable in air at room temperature. It is quite reactive, reacting with hydrogen to form the hydride NpH _3. Its oxidation numbers range from 3 to 7, with the most stable oxidation number being 5 (uranium has 6). Of the transuranium elements, its chemical properties are most similar to those of uranium, but its lower oxidation states are more stable than uranium's. It is soluble in hydrochloric acid, but sparingly soluble in sulfuric acid, nitric acid, and acetic acid. The oxide Np ^Ⅳ O ₂ (light green) can be obtained by burning the metal in an oxygen atmosphere. When burned at even higher temperatures, it becomes Np ^Ⅴ ₂ O ₅ (dark brown). Np ^Ⅵ O ₃ ･ x H ₂ O (brown) requires strong oxidizing conditions, such as reacting Np ^Ⅳ O ₂ with O ₃ in a high-temperature molten salt. Halides with oxidation states 3 to 6 are known: Np ^III has NpF ₃ (purple) to NpI ₃ (brown), Np ^IV has NpF ₄ (green) to NpBr ₄ (dark red), and Np ^V and Np ^VI have only the fluorides NpF ₅ and NpF ₆ , with the higher oxidation states becoming less stable. NpF ₆ (orange) is an extremely corrosive compound that decomposes at a melting point of 53 °C. In acidic aqueous solutions, they show characteristic colors according to their oxidation states: Np ^{3 +} blue to purple, Np ^{4 +} yellow-green, Np ^Ⅴ O ₂ ⁺ green, Np ^Ⅵ O ₂ ^{2 +} pale red to red, Np ^Ⅶ O ₅ ^{3 -} dark green (alkaline solution). The diversity of colors of compounds and aqueous solutions is characteristic of actinides. [CAS 7439-99-8]

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

Np．原子番号93の元素．電子配置[Rn]5f ⁴6d7s²の周期表3族超ウラン元素の一つ．アクチノイド元素の一つ．質量数237，半減期2.14×10⁶ y でα崩壊するもっとも長寿命の核種のほか，質量数225～244の放射性核種が知られている．1940年，E. McMillan(マクミラン)とP. Abelsonにより，カリフォルニア大学バークレー校のサイクロトロンでBeを重陽子で照射して発生させた中性子による天然ウランの衝撃で質量数239のβ崩壊核種(半減期2.4 d)が合成された．ウランの次の元素であるところから，太陽系の天王星uranusの次の惑星・海王星neptuneにちなみ，ネプツニウムと命名された．
²³⁷Npは原子炉中で²³⁸Uの(n,2n)反応あるいは²³⁵Uの2段階中性子捕獲によりつくられる．人工放射性元素であるが，ウラン鉱石中にごく微量存在する．三フッ化物を金属バリウムで還元すると，銀白色のネプツニウム金属が得られる．α，β，γの3変態があり，α相(斜方晶系)は，278 ℃ でβ相(正方晶系)に，577 ℃ でγ相(立方晶系)に転移する．融点640 ℃，沸点3900 ℃．密度(g cm^－3)：α 20.25(20 ℃)，β 19.36(313 ℃)，γ 18.0(600 ℃)．第一イオン化エネルギー6.266 eV．常温では空気中で安定である．かなり反応性に富み，水素と反応し水素化物NpH₃をつくる．酸化数は3～7で，もっとも安定な酸化数は5(ウランは6)．超ウラン元素のなかでは化学的性質は一番ウランに似ているが，ウランに比べて低酸化状態が安定である．塩酸に可溶，硫酸，硝酸，酢酸には難溶．酸化物Np^Ⅳ O₂(若緑色)は，金属の酸素雰囲気中での燃焼で得られる．さらに高温で燃焼するとNp^Ⅴ₂O₅(暗褐色)となる．Np^Ⅵ O₃･xH₂O(褐色)は高温の溶融塩中でNp^Ⅳ O₂に O₃ を反応させるなどの強力な酸化条件を必要とする．酸化数3～6のハロゲン化物が知られていて，Np^ⅢはNpF₃(紫色)からNpI₃(褐色)まで，Np^ⅣはNpF₄(緑色)からNpBr₄(暗赤色)まで，Np^Ⅴ，Np^Ⅵはフッ化物NpF₅，NpF₆のみで，高い酸化数ほど不安定となる．NpF₆(橙色)は融点53 ℃ で分解するきわめて腐食性の化合物である．酸性水溶液中では酸化状態に応じた特有の色を示す：Np^3＋ 青～紫色，Np^4＋ 黄緑色，Np^Ⅴ O₂^＋緑色，Np^Ⅵ O₂^2＋淡赤～赤色，Np^Ⅶ O₅^3－暗緑色(アルカリ性溶液)．化合物，水溶液の色の多様性はアクチノイドの特徴である．[CAS 7439-99-8]

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