Protactinium

Pa. It is an actinide element in group ³ of the periodic table, with atomic number 91 and electron configuration [Rn] ^{5f26d17s2 .} Its atomic weight is 231.03588(2). ²³¹ Pa exists in nature. Its isotopes with mass numbers from 212 to 240 are known. ²³¹ Pa belongs to the actinium decay series (half-life 3.276× ^104y , α decay). The uranium decay series includes ²³⁴ Pa (UZ, half-life 6.70h, β ^- decay) and ^234m Pa ( _UX2 , half-life 1.175min, β ^- decay, metastable state), both of which exist in uranium minerals. In 1913, K. Fajans and OH Göhring discovered ²³⁴ Pa ( _UX2 ), and named it brevium, from the Latin brevis, meaning "short," in reference to its short lifespan. The long-lived ²³¹ Pa was discovered in pitchblende in 1917 by O. Hahn, L. Meitner, and independently by Fajans and F. Soddy. It was named protoactinium from the Greek word πρωτο (protos), meaning "before" actinium, because it decays into actinium by α decay, but in 1949, IUPAC shortened it to the current protactinium. In 1937, A. Grosse converted Pa ₂ O ₅ into iodide PaI ₅ , which was thermally decomposed in a vacuum on a filament heated to a high temperature by passing an electric current through it, and the metal was obtained for the first time. The element is a silver-gray metal that exists in two phases: a tetragonal α phase and a body-centered cubic β phase at temperatures above 1167 °C. Density 15.37 g cm ^-3 (25 °C, calculated value). Melting point 1840 °C. Oxidation numbers 3, 4, 5. Quite stable in air and not easily oxidized. Reacts with oxygen above 1100°C _to produce ^PaⅤ2O5 (colorless) _. Black ^PaⅣO2 also exists. Reaction of _PaO2 with a _H2 -HF mixture produces _PaF4 (brown). Other halides include _PaF5 , _PaCl4 , _PaCl5 , _PaBr5 , _PaI4 , and _PaI5 . Easily hydrolyzed in aqueous solution to form _a colloid. [CAS 7440-13-3]

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

Pa．原子番号91の元素，電子配置[Rn]5f ²6d¹7s²の周期表3族アクチノイド元素．原子量231.03588(2)．天然には ²³¹Pa が存在する．質量数212～240の同位体核種が知られている．²³¹Pa はアクチニウム系崩壊系列に属する(半減期3.276×10⁴ y，α崩壊)．ウラン系崩壊系列には ²³⁴Pa(UZ，半減期6.70 h，β^－ 崩壊)と ^234mPa(UX₂，半減期1.175 min，β^－ 崩壊，準安定状態)があり，いずれもウラン鉱物中に存在する．1913年にK. FajansとO.H. Göhringが ²³⁴Pa(UX₂)を発見，寿命の短いところからラテン語の“短い”brevisをとってbreviumと名づけた．長寿命の ²³¹Pa は1917年にO. Hahn(ハーン)，L. Meitner(マイトナー)，独立にFajans，さらにF. Soddy(ソディ)らによってピッチブレンド中に発見された．α崩壊によりアクチニウムになるので，その前という意味で，ギリシア語の“最初”πρωτο(protos)からプロトアクチニウム(protoactinium)と名づけられたが，1949年にIUPACにより現在のprotactiniumに短縮された．1937年にA. GrosseがPa₂O₅をヨウ化物PaI₅にかえ，真空中で，電流を通じて高温にしたフィラメント上で熱分解してはじめて金属を得た．単体は銀灰色金属で，正方晶のα相と1167 ℃ 以上で体心立方格子のβ相の二相がある．密度15.37 g cm^－3(25 ℃，計算値)．融点1840 ℃．酸化数3，4，5．空気中ではかなり安定で酸化されにくい．1100 ℃ 以上で酸素と反応してPa^Ⅴ₂O₅(無色)を生じる．黒色のPa^Ⅳ O₂も存在する．PaO₂とH₂-HF混合系の反応でPaF₄(褐色)が得られる．ハロゲン化物には，ほかにPaF₅，PaCl₄，PaCl₅，PaBr₅，PaI₄，PaI₅などがある．水溶液中で容易に加水分解してコロイド状になる．[CAS 7440-13-3]

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