سيريوم

يجب ألا يـُخلـَط بينها وبين caesium or curium.

Cerium, ₅₈Ce

Cerium
المظهر	silvery white
الوزن الذري العياري Ar°(Ce)
Cerium في الجدول الدوري
– ↑ Ce ↓ Th لانثانم ← cerium → پراسيوديميوم
الرقم الذري (Z)	58
المجموعة	n/a
الدورة	period 6
المستوى الفرعي	f-block
التوزيع الإلكتروني	[Xe] 4f1 5d1 6s2
الإلكترونات بالغلاف	2, 8, 18, 19, 9, 2
الخصائص الطبيعية
الطور at د.ح.ض.ق	solid
نقطة الانصهار	1068 K ​(795 °س، ​1463 °F)
نقطة الغليان	3716 K ​(3443 °س، ​6229 °ف)
الكثافة (بالقرب من د.ح.غ.)	6.770 ج/سم³
حين يكون سائلاً (عند ن.إ.)	6.55 ج/سم³
حرارة الانصهار	5.46 kJ/mol
حرارة التبخر	398 kJ/mol
السعة الحرارية المولية	26.94 J/(mol·K)
ضغط البخار P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1992 2194 2442 2754 3159 3705
الخصائص الذرية
الكهرسلبية	مقياس پاولنگ: 1.12
طاقات التأين	الأول: 534.4 kJ/mol الثاني: 1050 kJ/mol الثالث: 1949 kJ/mol (المزيد)
نصف القطر الذري	empirical: 181.8 pm
نصف قطر التكافؤ	204±9 pm
الخطوط الطيفية
خصائص أخرى
التواجد الطبيعي	primordial
البنية البلورية	​double hexagonal close-packed (dhcp) β-Ce
البنية البلورية	​face-centered cubic (fcc) γ-Ce
سرعة الصوت قضيب رفيع	2100 م/ث (عند 20 °س)
قضيب رفيع	11.3 W/(m·K)
التمدد الحراري	γ, poly: 6.3 µm/(m⋅K) (at r.t.)
المقاومة الكهربائية	β, poly: 828 nΩ⋅m (at r.t.)
الترتيب المغناطيسي	مغناطيسية مسايرة
القابلية المغناطيسية	(β) +2450.0×10−6 cm3/mol (293 K)
معامل يونگ	γ form: 33.6 GPa
معامل القص	γ form: 13.5 GPa
معاير الحجم	γ form: 21.5 GPa
نسبة پواسون	γ form: 0.24
صلادة موز	2.5
صلادة ڤيكرز	210–470 MPa
صلادة برينل	186–412 MPa
رقم كاس	7440-45-1
التاريخ
التسمية	على اسم الكوكب القزم سيريس، والذي بدوره مسمى على اسم إلهة الزراعة الرومانية سيريس
الاكتشاف	مارتن هاينريش كلاپروث ويونس ياكوب برزليوس وڤلهلم هيزنگر (1803)
أول عزل	Carl Gustaf Mosander (1838)
نظائر الcerium

السيريوم بالإنجليزية Cerium ، هو عنصر كيميائي له الرمز Ce والعدد الذري 58 في الجدول الدوري للعناصر. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the oxidation state of +3 characteristic of the series, it also has a stable +4 state that does not oxidize water. It is also considered one of the rare-earth elements. Cerium has no known biological role in humans but is not particularly toxic, except with intense or continued exposure.

Despite always occurring in combination with the other rare-earth elements in minerals such as those of the monazite and bastnäsite groups, cerium is easy to extract from its ores, as it can be distinguished among the lanthanides by its unique ability to be oxidized to the +4 state in aqueous solution. It is the most common of the lanthanides, followed by neodymium, lanthanum, and praseodymium. It is the 25th-most abundant element, making up 66 ppm of the Earth's crust, half as much as chlorine and five times as much as lead.

Cerium was the first of the lanthanides to be discovered, in Bastnäs, Sweden, by Jöns Jakob Berzelius and Wilhelm Hisinger in 1803, and independently by Martin Heinrich Klaproth in Germany in the same year. In 1839 Carl Gustaf Mosander became the first to isolate the metal. Today, cerium and its compounds have a variety of uses: for example, cerium(IV) oxide is used to polish glass and is an important part of catalytic converters. Cerium metal is used in ferrocerium lighters for its pyrophoric properties. Cerium-doped YAG phosphor is used in conjunction with blue light-emitting diodes to produce white light in most commercial white LED light sources.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

 الخصائص المميزة

يتميز السيريوم بأنه معدن فضي اللون ، وينتمي لمجموعة اللانثنيدات. يشبه الحديد من حيث اللون واللمعان ، وطيع وقابل للطرق أيضا.

وينتمي السيريوم أيضا لمجموعة من العناصر الكيميائية المسامة الأتربة النادرة ، مع أنها ليست كلها نادرة، إذ إن بعضها أكثر انتشاراً في الطبيعة من الرصاص، وقد احتفظت بتسميتها هذه مع أنها تصنف اليوم باسم العناصر اللانتانيدية نسبة إلى أول عناصرها وهو اللانتانيوم La كما يشار إليها أحياناً باسم العناصر الانتقالية الداخلية. وقد وسمت قبل بداية القرن التاسع عشر بالندرة وارتفاع الثمن، إذ لم تكن تسترعي اهتمام إلا قلة من العلماء لم يكن عددهم يتجاوز في العالم آنذاك مئتي عالم، ثم تضافرت الجهود تدريجياً، وازداد توجه الأنظار إلى هذه الأتربة لسبر أغوار عناصرها على الصعيد التجريبي بغية فصل بعضها عن بعض من جهة، وتبين خواصها الفيزيائية والكيمياوية من جهة أخرى.

 Physical

Cerium is the second element of the lanthanide series. In the periodic table, it appears between the lanthanides lanthanum to its left and praseodymium to its right, and above the actinide thorium. It is a ductile metal with a hardness similar to that of silver. Its 58 electrons are arranged in the configuration [Xe]4f¹5d¹6s², of which the four outer electrons are valence electrons. The 4f, 5d, and 6s energy levels are very close to each other, and the transfer of one electron to the 5d shell is due to strong interelectronic repulsion in the compact 4f shell. This effect is overwhelmed when the atom is positively ionised; thus Ce²⁺ on its own has instead the regular configuration [Xe]4f², although in some solid solutions it may be [Xe]4f¹5d¹. Most lanthanides can use only three electrons as valence electrons, as afterwards the remaining 4f electrons are too strongly bound: cerium is an exception because of the stability of the empty f-shell in Ce⁴⁺ and the fact that it comes very early in the lanthanide series, where the nuclear charge is still low enough until neodymium to allow the removal of the fourth valence electron by chemical means.

Cerium has a variable electronic structure. The energy of the 4f electron is nearly the same as that of the outer 5d and 6s electrons that are delocalized in the metallic state, and only a small amount of energy is required to change the relative occupancy of these electronic levels. This gives rise to dual valence states. For example, a volume change of about 10% occurs when cerium is subjected to high pressures or low temperatures. It appears that the valence changes from about 3 to 4 when it is cooled or compressed.

 Chemical properties of the element

Like the other lanthanides, cerium metal is a good reducing agent, having standard reduction potential of E^⦵ = −2.34 V for the Ce³⁺/Ce couple. It tarnishes in air, forming a passivating oxide layer like iron rust. A centimeter-sized sample of cerium metal corrodes completely in about a year. More dramatically, metallic cerium can be highly pyrophoric:

Ce + O₂ → CeO₂

Being highly electropositive, cerium reacts with water. The reaction is slow with cold water but speeds up with increasing temperature, producing cerium(III) hydroxide and hydrogen gas:

2 Ce + 6 H₂O → 2 Ce(OH)₃ + 3 H₂

 Allotropes

Phase diagram of cerium

Four allotropic forms of cerium are known to exist at standard pressure, and are given the common labels of α to δ:

• The high-temperature form, δ-cerium, has a bcc (body-centered cubic) crystal structure and exists above 726 °C.
• The stable form below 726 °C to approximately room temperature is γ-cerium, with an fcc (face-centered cubic) crystal structure.
• The DHCP (double hexagonal close-packed) form β-cerium is the equilibrium structure approximately from room temperature to −150 °C.
• The fcc form α-cerium is stable below about −150 °C; it has a density of 8.16 g/cm³.
• Other solid phases occurring only at high pressures are shown on the phase diagram.
• Both γ and β forms are quite stable at room temperature, although the equilibrium transformation temperature is estimated at 75 °C.

At lower temperatures the behavior of cerium is complicated by the slow rates of transformation. Transformation temperatures are subject to substantial hysteresis and values quoted here are approximate. Upon cooling below −15 °C, γ-cerium starts to change to β-cerium, but the transformation involves a volume increase and, as more β forms, the internal stresses build up and suppress further transformation. Cooling below approximately −160 °C will start formation of α-cerium but this is only from remaining γ-cerium. β-cerium does not significantly transform to α-cerium except in the presence of stress or deformation. At atmospheric pressure, liquid cerium is more dense than its solid form at the melting point.

 Isotopes

المقالة الرئيسية: Isotopes of cerium

Naturally occurring cerium is made up of four isotopes: ¹³⁶Ce (0.19%), ¹³⁸Ce (0.25%), ¹⁴⁰Ce (88.4%), and ¹⁴²Ce (11.1%). All four are observationally stable, though the light isotopes ¹³⁶Ce and ¹³⁸Ce are theoretically expected to undergo double electron capture to isotopes of barium, and the heaviest isotope ¹⁴²Ce is expected to undergo double beta decay to ¹⁴²Nd or alpha decay to ¹³⁸Ba. Additionally, ¹⁴⁰Ce would release energy upon spontaneous fission. None of these decay modes have yet been observed, though the double beta decay of ¹³⁶Ce, ¹³⁸Ce, and ¹⁴²Ce have been experimentally searched for. The current experimental limits for their half-lives are:قالب:NUBASE2016

¹³⁶Ce: >3.8×10¹⁶ y

¹³⁸Ce: >5.7×10¹⁶ y

¹⁴²Ce: >5.0×10¹⁶ y

All other cerium isotopes are synthetic and radioactive. The most stable of them are ¹⁴⁴Ce with a half-life of 284.9 days, ¹³⁹Ce with a half-life of 137.6 days, and ¹⁴¹Ce with a half-life of 32.5 days. All other radioactive cerium isotopes have half-lives under four days, and most of them have half-lives under ten minutes.قالب:NUBASE2016 The isotopes between ¹⁴⁰Ce and ¹⁴⁴Ce inclusive occur as fission products of uranium.قالب:NUBASE2016 The primary decay mode of the isotopes lighter than ¹⁴⁰Ce is inverse beta decay or electron capture to isotopes of lanthanum, while that of the heavier isotopes is beta decay to isotopes of praseodymium.قالب:NUBASE2016 Some isotopes of neodymium can alpha decay or are predicted to decay to isotopes of cerium.

The rarity of the proton-rich ¹³⁶Ce and ¹³⁸Ce is explained by the fact that they cannot be made in the most common processes of stellar nucleosynthesis for elements beyond iron, the s-process (slow neutron capture) and the r-process (rapid neutron capture). This is so because they are bypassed by the reaction flow of the s-process, and the r-process nuclides are blocked from decaying to them by more neutron-rich stable nuclides. Such nuclei are called p-nuclei, and their origin is not yet well understood: some speculated mechanisms for their formation include proton capture as well as photodisintegration. ¹⁴⁰Ce is the most common isotope of cerium, as it can be produced in both the s- and r-processes, while ¹⁴²Ce can only be produced in the r-process. Another reason for the abundance of ¹⁴⁰Ce is that it is a magic nucleus, having a closed neutron shell (it has 82 neutrons), and hence it has a very low cross section towards further neutron capture. Although its proton number of 58 is not magic, it is granted additional stability, as its eight additional protons past the magic number 50 enter and complete the 1g_7/2 proton orbital. The abundances of the cerium isotopes may differ very slightly in natural sources, because ¹³⁸Ce and ¹⁴⁰Ce are the daughters of the long-lived primordial radionuclides ¹³⁸La and ¹⁴⁴Nd, respectively.قالب:NUBASE2016

 المركبات

Cerium exists in two main oxidation states, Ce(III) and Ce(IV). This pair of adjacent oxidation states dominates several aspects of the chemistry of this element. Cerium(IV) aqueous solutions may be prepared by reacting cerium(III) solutions with the strong oxidizing agents peroxodisulfate or bismuthate. The value of E^⦵(Ce⁴⁺/Ce³⁺) varies widely depending on conditions due to the relative ease of complexation and hydrolysis with various anions, although +1.72 V is representative. Cerium is the only lanthanide which has important aqueous and coordination chemistry in the +4 oxidation state.

 الهاليدات

Cerium forms all four trihalides CeX₃ (X = F, Cl, Br, I) usually by reaction of the oxides with the hydrogen halides. The anhydrous halides are pale-colored, paramagnetic, hygroscopic solids. Upon hydration, the trihalides convert to complexes containing aquo complexes [Ce(H₂O)_8-9]³⁺. Unlike most lanthanides, Ce forms a tetrafluoride, a white solid. It also forms a bronze-colored diiodide, which has metallic properties. Aside from the binary halide phases, a number of anionic halide complexes are known. The fluoride gives the Ce(IV) derivatives CeF4−
8 and CeF2−
6. The chloride gives the orange CeCl2−
6.

 Oxides and chalcogenides

Cerium(IV) oxide ("ceria") has the fluorite structure, similarly to the dioxides of praseodymium and terbium. Ceria is a nonstoichiometric compound, meaning that the real formula is CeO_2−x, where x is about 0.2. Thus, the material is not perfectly described as Ce(IV). Ceria reduces to cerium(III) oxide with hydrogen gas. Many nonstoichiometric chalcogenides are also known, along with the trivalent Ce₂Z₃ (Z = S, Se, Te). The monochalcogenides CeZ conduct electricity and would better be formulated as Ce³⁺Z²⁻e⁻. While CeZ₂ are known, they are polychalcogenides with cerium(III): cerium(IV) derivatives of S, Se, and Te are unknown.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

 معقدات السيريوم الرباعي

Ceric ammonium nitrate

The compound ceric ammonium nitrate ("CAN") (NH
₄)
₂[Ce(NO
3)
6] is the most common cerium compound encountered in the laboratory. The six nitrate ligands bind as bidentate ligands. The complex [Ce(NO
3)
6]²⁻ is 12-coordinate, a high coordination number which emphasizes the large size of the Ce⁴⁺ ion. CAN is a popular oxidant in organic synthesis, both as a stoichiometric reagent and as a catalyst. It is inexpensive, and easily handled. It operates by one-electron redox. Cerium nitrates also form 4:3 and 1:1 complexes with 18-crown-6 (the ratio referring to that between cerium and the crown ether). Classically CAN is a primary standard for quantitative analysis. Cerium(IV) salts, especially cerium(IV) sulfate, are often used as standard reagents for volumetric analysis in cerimetric titrations.

A white LED in operation: the diode produces monochromatic blue light but the Ce:YAG phosphor converts some of it into yellow light, the combination is perceived as white by the human eye.

Due to ligand-to-metal charge transfer, aqueous cerium(IV) ions are orange-yellow. Aqueous cerium(IV) is metastable in water and is a strong oxidizing agent that oxidizes hydrochloric acid to give chlorine gas. In the Belousov–Zhabotinsky reaction, cerium oscillates between the +4 and +3 oxidation states to catalyze the reaction.

 مركبات السيريوم العضوي

Organocerium chemistry is similar to that of the other lanthanides, often involving complexes of cyclopentadienyl and cyclooctatetraenyl ligands. Cerocene (Ce(C₈H₈)₂) adopts the uranocene molecular structure. The 4f electron in cerocene, Ce(C 8H 8) 2, is poised ambiguously between being localized and delocalized and this compound is considered intermediate-valent. Alkyl, alkynyl, and alkenyl organocerium derivatives are prepared from the transmetallation of the respective organolithium or Grignard reagents, and are more nucleophilic but less basic than their precursors.

 التطبيقات

إستعمالات السيريوم:

• في علم المعادن:
  • يستعمل السيريوم في عمل سبائك الألومنيوم.
  • يمكن الحصول على الحديد المطاوع بإضافة السيريوم إلى الحديد الزهر.
  • يستخدم السيريوم عند صناعة الحديد الصلب للحد من تكون أكسيد الحديد و كبريتيد الحديد.
  • يستخدم السيريوم في صناعة الحديد المقاوم للصدأ كعنصر لزيادة الصلابة.
  • 3 to 4% cerium added to magnesium alloys, along with 0.2 to 0.6% zirconium, helps refine the grain and give sound casting of complex shapes. It also adds heat resistance to magnesium castings.
  • Cerium is used in alloys that are used to make permanent magnets.
  • Cerium is used as an alloying element in tungsten electrodes for gas tungsten arc welding.
  • Cerium is a major component of ferrocerium, also known as "lighter flint". Although modern alloys of this type generally use Mischmetal rather than purified cerium, it still is the most prevalent constituent.
  • Cerium is used in carbon-arc lighting, especially in the motion picture industry.
• Cerium oxalate is an anti-emetic drug.
• Cerium(IV) oxide
  • The oxide is used in incandescent gas mantles, such as the Welsbach mantle, where it was combined with thorium, lanthanum, magnesium or yttrium oxides .
  • The oxide is emerging as a hydrocarbon catalyst in self cleaning ovens, incorporated into oven walls.
  • Cerium(IV) oxide has largely replaced rouge in the glass industry as a polishing abrasive.
  • Cerium(IV) oxide is finding use as a petroleum cracking catalyst in petroleum refining.
  • Cerium(IV) additives to diesel fuel cause that to burn more cleanly, with less resulting air-pollution.
  • In glass, cerium(IV) oxide allows for selective absorption of ultraviolet light.
• Cerium(IV) sulfate is used extensively as a volumetric oxidizing agent in quantitative analysis.
• Ceric ammonium nitrate is a useful one-electron oxidant in organic chemistry, used to oxidatively etch electronic components, and as a primary standard for quantitative analysis.
• Cerium compounds are used in the manufacture of glass, both as a component and as a decolorizer.
• Cerium in combination with titanium gives a golden yellow color to glass.
• Cerium compounds are used for the coloring of enamel.
• Cerium(III) and cerium(IV) compounds such as cerium(III) chloride have uses as catalysts in organic synthesis.

 التاريخ

The dwarf planet and asteroid Ceres, after which cerium is named.

Jöns Jakob Berzelius, one of the discoverers of cerium

Cerium was discovered in Bastnäs in Sweden by Jöns Jakob Berzelius and Wilhelm Hisinger, and independently in Germany by Martin Heinrich Klaproth, both in 1803. Cerium was named by Berzelius after the asteroid Ceres, discovered two years earlier. The asteroid is itself named after the Roman goddess Ceres, goddess of agriculture, grain crops, fertility and motherly relationships.

Cerium was originally isolated in the form of its oxide, which was named ceria, a term that is still used. The metal itself was too electropositive to be isolated by then-current smelting technology, a characteristic of rare-earth metals in general. After the development of electrochemistry by Humphry Davy five years later, the earths soon yielded the metals they contained. Ceria, as isolated in 1803, contained all of the lanthanides present in the cerite ore from Bastnäs, Sweden, and thus only contained about 45% of what is now known to be pure ceria. It was not until Carl Gustaf Mosander succeeded in removing lanthana and "didymia" in the late 1830s that ceria was obtained pure. Wilhelm Hisinger was a wealthy mine-owner and amateur scientist, and sponsor of Berzelius. He owned and controlled the mine at Bastnäs, and had been trying for years to find out the composition of the abundant heavy gangue rock (the "Tungsten of Bastnäs", which despite its name contained no tungsten), now known as cerite, that he had in his mine. Mosander and his family lived for many years in the same house as Berzelius, and Mosander was undoubtedly persuaded by Berzelius to investigate ceria further.

The element played a role in the Manhattan Project, where cerium compounds were investigated in the Berkeley site as materials for crucibles for uranium and plutonium casting. For this reason, new methods for the preparation and casting of cerium were developed within the scope of the Ames daughter project (now the Ames Laboratory). Production of extremely pure cerium in Ames commenced in mid-1944 and continued until August 1945.

 التواجد

يعتبر السيريوم من أكثر العناصر وفرة من مجموعة الأتربة النادرة ، حيث تصل نسبته 0.0046% في القشرة الأرضية من حيث الوزن. وقد وجد مجموعة من المعادن تشمل ألانيت والذي يعرف بإسم أورثيت ، ومعدن المونازيت والباستناسيت وهذين الأخيرين من أهم المصادر لإستخلاص السيريوم.

عادة ما يحضر السيريوم بواسطة عملية التبادل الأيوني التي تستعمل رمال المونازيت كمصدر للسيريوم.

انظر أيضاتصنيف:معادن لانثنيدات

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

 المركبات

Cerium(IV)-sulfate

Cerium has two common oxidation states, +3 and +4. The most common compound of cerium is cerium(IV) oxide (CeO₂), which is used as "jeweller's rouge" as well as in the walls of some self-cleaning ovens. Two common oxidising agents used in titrations are ammonium cerium(IV) sulfate (ceric ammonium sulfate, (NH₄)₂Ce(SO₄)₃) and ammonium cerium(IV) nitrate (ceric ammonium nitrate or CAN, (NH₄)₂Ce(NO₃)₆). Cerium also forms a chloride, CeCl₃ or cerium(III) chloride, used to facilitate reactions at carbonyl groups in organic chemistry. Other compounds include cerium(III) carbonate (Ce₂(CO₃)₃), cerium(III) fluoride (CeF₃), cerium(III) oxide (Ce₂O₃), as well as cerium(IV) sulfate (ceric sulfate, Ce(SO₄)₂) and cerium(III) triflate (Ce(OSO₂CF₃)₃).

The two oxidation states of cerium differ enormously in basicity: cerium(III) is a strong base, comparable to the other trivalent lanthanides, but cerium(IV) is weak. This difference has always allowed cerium to be by far the most readily isolated and purified of all the lanthanides, otherwise a notoriously difficult group of elements to separate. A wide range of procedures have been devised over the years to exploit the difference. Among the better ones:

1. Leaching the mixed hydroxides with dilute nitric acid: the trivalent lanthanides dissolve in cerium-free condition, and tetravalent cerium remains in the insoluble residue as a concentrate to be further purified by other means. A variation on this uses hydrochloric acid and the calcined oxides from bastnaesite, but the separation is less sharp.
2. Precipitating cerium from a nitrate or chloride solution using potassium permanganate and sodium carbonate in a 1:4 molar ratio.
3. Boiling rare-earth nitrate solutions with potassium bromate and marble chips.

Using the classical methods of rare-earth separation, there was a considerable advantage to a strategy of removing cerium from the mixture at the beginning. Cerium typically comprised 45% of the cerite or monazite rare earths, and removing it early greatly reduced the bulk of what needed to be further processed (or the cost of reagents to be associated with such processing). However, not all cerium purification methods relied on basicity. Ceric ammonium nitrate [ammonium hexanitratocerate(IV)] crystallization from nitric acid was one purification method. Cerium(IV) nitrate (hexanitratoceric acid) was more readily extractable into certain solvents (e.g. tri-n-butyl phosphate) than the trivalent lanthanides. However, modern practice in China seems to be to do purification of cerium by counter-current solvent extraction, in its trivalent form, just like the other lanthanides.

Cerium(IV) is a strong oxidant under acidic conditions, but stable under alkaline conditions, when it is cerium(III) that becomes a strong reductant, easily oxidized by molecular dioxygen (or air). This ease of oxidation under alkaline conditions leads to the occasional geochemical parting of the ways between cerium and the trivalent light lanthanides under supergene weathering conditions, leading variously to the "negative cerium anomaly" or to the formation of the mineral cerianite. Air-oxidation of alkaline cerium(III) is the most economical way to get to cerium(IV), which can then be handled in acid solution.

انظر أيضاتصنيف:مركبات سيريوم

 النظائر المشعة

المقالة الرئيسية: isotopes of cerium

Naturally-occurring cerium is composed of 4 stable isotopes; ¹³⁶Ce, ¹³⁸Ce, ¹⁴⁰Ce, and ¹⁴²Ce with ¹⁴⁰Ce being the most abundant (88.48% natural abundance). ¹³⁶Ce and ¹⁴²Ce are predicted to be double beta active but no signs of activity were ever observed (for ¹⁴²Ce, the lower limit on half-life is 5×10¹⁶ years). 26 radioisotopes have been characterized with the most long-lived being ¹⁴⁴Ce with a half-life of 284.893 days, ¹³⁹Ce with a half-life of 137.640 days, and ¹⁴¹Ce with a half-life of 32.501 days. All of the remaining radioactive isotopes have half-lives that are less than 4 days and the majority of these have half-lives that are less than 10 minutes. This element also has 2 meta states.

The known isotopes of cerium range in atomic weight from 123 u (¹²³Ce) to 152 u (¹⁵²Ce).

Cerium 144 is a high-yield product of nuclear fission; the ORNL Fission Product Pilot Plant separated substantial quantities of cerium-144 from reactor waste, and it was used in the Aircraft Nuclear Propulsion and SNAP programs.

 الدور الحيوي والاحتياطات

سيريوم

المخاطر
ن.م.ع. مخطط تصويري
ن.م.ع. كلمة الاشارة	Danger
ن.م.ع. عبارات المخاطر	H228
ن.م.ع. العبارات الاحتياطية	P210
NFPA 704 (معيـَّن النار)	0 2 0

The early lanthanides have been found to be essential to some methanotrophic bacteria living in volcanic mudpots, such as Methylacidiphilum fumariolicum: lanthanum, cerium, praseodymium, and neodymium are about equally effective. Cerium is otherwise not known to have biological role in any other organisms, but is not very toxic either; it does not accumulate in the food chain to any appreciable extent. Because it often occurs together with calcium in phosphate minerals, and bones are primarily calcium phosphate, cerium can accumulate in bones in small amounts that are not considered dangerous.

Cerium nitrate is an effective topical antimicrobial treatment for third-degree burns, although large doses can lead to cerium poisoning and methemoglobinemia. The early lanthanides act as essential cofactors for the methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV, for which lanthanum, cerium, praseodymium, and neodymium alone are about equally effective.

Like all rare-earth metals, cerium is of low to moderate toxicity. A strong reducing agent, it ignites spontaneously in air at 65 to 80 °C. Fumes from cerium fires are toxic. Water should not be used to stop cerium fires, as cerium reacts with water to produce hydrogen gas. Workers exposed to cerium have experienced itching, sensitivity to heat, and skin lesions. Cerium is not toxic when eaten, but animals injected with large doses of cerium have died due to cardiovascular collapse. Cerium is more dangerous to aquatic organisms, on account of being damaging to cell membranes; this is an important risk because it is not very soluble in water, thus causing contamination of the environment .

 وصلات خارجية

ابحث عن cerium في
قاموس المعرفة.

• WebElements.com - Cerium
• It's Elemental - The Element Cerium
• Cerium Properties and Applications

 المصادر

• Los Alamos National Laboratory - Cerium
• Lattice and spin dynamics of gamma-Ce