セリウム (かもしれません 2024)
セリウム（Ce）、化学元素、希土類金属の中で最も豊富。
クイズ
周期表クイズ
Hs
商用グレードのセリウムは、色が鉄灰色で、純粋な形の場合は銀色で、スズと同じくらい柔らかくて延性があります。室温で空気中で酸化し、CeO 2を形成します。金属は水とゆっくりと反応し、希フッ化水素酸（HF）を除いて希酸にすばやく溶解しますが、フッ化水素酸（HF）は保護フッ化物（CeF 3）金属表面の層。セリウムの切りくず（金属をやすりがけ、研磨、または機械加工したとき）は、空気中で容易に自然発火し、白熱します。その自然発火性は、軽量フリントでの重要な冶金学的用途の1つを占めています。金属は、真空または不活性雰囲気のいずれかで保管する必要があります。金属は、室温の上下両方で適度に強い常磁性体であり、13 K（-260°C、または-436°F）未満では反強磁性になります。 20 kbarを超える圧力でミリケルビン範囲で超伝導になります。
Cerium as the oxide (ceria) was discovered in 1803 by Swedish chemists Jöns Jacob Berzelius and Wilhelm Hisinger, working together, and independently by German chemist Martin Klaproth. It was named after the asteroid Ceres, which was discovered in 1801. Cerium occurs in bastnasite, monazite, and many other minerals. It also is found among the fission products of uranium, plutonium, and thorium. Cerium is about as abundant as copper and nearly three times as abundant as lead in the igneous rocks of Earth’s crust.
Four isotopes occur in nature: stable cerium-140 (88.48 percent) and radioactive cerium-142 (11.08 percent), cerium-138 (0.25 percent), and cerium-136 (0.19 percent). Excluding nuclear isomers, a total of 38 radioactive isotopes of cerium have been characterized. They range in mass from 119 to 157 with half-lives as short as 1.02 seconds for cerium-151 and as long as 5 × 1016 years for cerium-142.
The metal is prepared by electrolysis of the anhydrous fused halides or by metallothermic reduction of the halides with alkali or alkaline-earth metals. It exists in four allotropic (structural) forms. The α-phase is face-centred cubic with a = 4.85 Å at 77 K (−196 °C, or −321 °F). The β-phase forms just below room-temperature and is double close-packed hexagonal with a = 3.6810 Å and c = 11.857 Å. The γ-phase is the room temperature form and is face-centred cubic with a = 5.1610 Å at 24 °C (75 °F). The δ-phase is body-centred cubic with a = 4.12 Å at 757 °C (1,395 °F).
Cerium compounds have a number of practical applications. The dioxide is employed in the optics industry for fine polishing of glass, as a decolourizer in glass manufacturing, in petroleum cracking catalysts, and as a three-way automotive emission catalyst that makes use of its dual valence (3+/4+) characteristics. Together with the other rare-earth metals, cerium is a constituent of numerous ferrous alloys to scavenge sulfur and oxygen and to nodulize cast iron. It is also used in nonferrous alloys, most commonly to improve high-temperature oxidation resistance of superalloys. Misch metal (typically 50 percent cerium, 25 percent lanthanum, 18 percent neodymium, 5 percent praseodymium, and 2 percent other rare earths) is primarily used for lighter flints and alloying additions.
セリウムはプラセオジムやテルビウムとともに、他の希土類とは異なり、酸化状態が+4の化合物を形成します。これは、溶液中で+4の酸化状態を示す唯一の希土類です。強力であるが安定した酸化剤であるCe 4+イオンの塩（セリウム塩）は、分析化学で第一鉄（酸化状態が+2の鉄）などの酸化可能な物質を決定するために使用されます。+3の酸化状態にあるセリウムは、典型的な希土類として振る舞います。
要素のプロパティ
| 原子番号 | 58 |
|---|---|
| 原子量 | 140.115 |
| 融点 | 798°C（1,468°F） |
| 沸点 | 3,443°C（6,229°F） |
| 比重 | 6.7704（24°C、または75°F） |
| 酸化状態 | + 3、+ 4 |
| 電子配置 | [Xe] 4f 2 6s 2 |
