Cerium

Cerium
From Wikipedia, the free encyclopedia
Jump to: navigation, search
58 lanthanum ← cerium → praseodymium
-
↑
Ce
↓
Th
Periodic Table - Extended Periodic Table


General
Name, Symbol, Number cerium, Ce, 58
Chemical series lanthanides
Group, Period, Block n/a, 6, f
Appearance silvery white

Standard atomic weight 140.116(1)  g·mol−1
Electron configuration [Xe] 4f1 5d1 6s2
Electrons per shell 2, 8, 18, 19, 9, 2
Physical properties
Phase solid
Density (near r.t.) 6.770  g·cm−3
Liquid density at m.p. 6.55  g·cm−3
Melting point 1068 K
(795 °C, 1463 °F)
Boiling point 3716 K
(3443 °C, 6229 °F)
Heat of fusion 5.46  kJ·mol−1
Heat of vaporization 398  kJ·mol−1
Heat capacity (25 °C) 26.94  J·mol−1·K−1
Vapor pressure P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 1992 2194 2442 2754 3159 3705

Atomic properties
Crystal structure cubic face centered
Oxidation states 3, 4
(mildly basic oxide)
Electronegativity 1.12 (scale Pauling)
Ionization energies
(more) 1st:  534.4  kJ·mol−1
2nd:  1050  kJ·mol−1
3rd:  1949  kJ·mol−1
Atomic radius 185  pm
Miscellaneous
Magnetic ordering no data
Electrical resistivity (r.t.) (β, poly) 828 nΩ·m
Thermal conductivity (300 K) 11.3  W·m−1·K−1
Thermal expansion (r.t.) (γ, poly)
6.3 µm/(m·K)
Speed of sound (thin rod) (20 °C) 2100 m/s
Young's modulus (γ form) 33.6  GPa
Shear modulus (γ form) 13.5  GPa
Bulk modulus (γ form) 21.5  GPa
Poisson ratio (γ form) 0.24
Mohs hardness 2.5
Vickers hardness 270  MPa
Brinell hardness 412  MPa
CAS registry number 7440-45-1
Selected isotopes
Main article: Isotopes of cerium iso NA half-life DM DE (MeV) DP
134Ce syn 3.16 days ε 0.500 134La
136Ce 0.185% Ce is stable with 78 neutrons
138Ce 0.251% Ce is stable with 80 neutrons
139Ce syn 137.640 days ε 0.278 139La
140Ce 88.450% Ce is stable with 82 neutrons
141Ce syn 32.501 days β- 0.581 141Pr
142Ce 11.114% > 5×1016 years β-β- unknown 142Nd
144Ce syn 284.893 days β- 0.319 144Pr

References
Cerium (IPA: /ˈsiːriəm, ˈsɪəriəm/) is a chemical element in the periodic table that has the symbol Ce and atomic number 58.

Contents [hide]
1 Notable characteristics
2 Applications
3 History
4 Occurrence
5 Compounds
6 Isotopes
7 Precautions
8 References
9 External links



[edit] Notable characteristics
Cerium is a silvery metallic element, belonging to the lanthanide group. It is used in some rare-earth alloys. It resembles iron in color and luster, but is soft, and both malleable and ductile. It tarnishes readily in the air. Only europium is more reactive than cerium among rare earth elements. Alkali solutions and dilute and concentrated acids attack the metal rapidly. The pure metal can ignite if scratched. Cerium oxidizes slowly in cold water and rapidly in hot water.

Although cerium belongs to chemical elements group called rare earth metals, it is not rare at all. Cerium is available in relatively large quantities (68 ppm in Earth’s crust); in fact it is more common than lead.

Cerium in the +3 oxidation state is referred to as cerous, while the metal in the +4 oxidation state is called ceric.

Cerium(IV) salts are orange red or yellowish, whereas cerium(III) salts are usually white or colorless. Both oxidation states of cerium absorb ultraviolet light strongly. Cerium(III) can be used to make glasses that are colorless, yet absorb ultraviolet light almost completely. Cerium can be readily detected in rare earth mixtures by a very sensitive qualitative test: addition of ammonia and hydrogen peroxide to an aqueous solution of lanthanides produces a characteristic dark brown color if cerium is present.

Cerium has the longest liquid range of any non-radioactive element: 2648°C (795°C to 3443°C).


Phase diagram of cerium



[edit] Applications
Uses of cerium:

In metallurgy:
Cerium is used in making aluminium alloys.
Adding cerium to cast irons opposes graphitization and produces a malleable iron.
In steels, cerium degasifies and can help reduce sulfides and oxides.
Cerium is used in stainless steel as a precipitation hardening agent.
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(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 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.

[edit] History
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 so named by Berzelius after the dwarf planet Ceres, discovered two years earlier (1801). As originally isolated, cerium was in the form of its oxide, and was named "ceria", a term that is still used. The metal itself was too electopositive to be isolated by then-current smelting technology, a characteristic of "earths" in general. But the development of electrochemistry by Humphry Davy was only five years into the future, and then the earths were well on their way to yielding up their contained metals. Ceria, as isolated in 1803, contained all of the lanthanides present in the cerite ore from Bastnaes, Sweden, and thus only contained about 45% of what is now known to be pure ceria. It was not until Mosander succeeded in removing lanthana and "didymia" in the late 1830s that ceria was obtained pure. As an historical aside: Wilhelm Hisinger was a wealthy mine owner and amateur scientist, and sponsor of Berzelius. He owned or controlled the mine at Bastnaes, and had been trying for years to find out the composition of the abundant heavy gangue rock (the "Tungstein of Bastnaes") 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 was undoubtedly persuaded by the latter to investigate ceria further. When the rare earths were first discovered, since they were strong bases like the oxides of calcium or magnesium, they were thought to be divalent. Thus, "ceric" cerium was thought to be trivalent, and the oxidation state ratio was therefore thought to be 1.5. Berzelius was extremely annoyed, to keep on getting the ratio 1.33. He was after all one of the finest analytical chemists in Europe. But he was a better analyst than he thought, since 1.33 was the correct answer!


[edit] Occurrence
Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight. It is found in a number of minerals including allanite (also known as orthite)—(Ca, Ce, La, Y)2(Al, Fe)3(SiO4)3(OH), monazite (Ce, La, Th, Nd, Y)PO4, bastnasite(Ce, La, Y)CO3F, hydroxylbastnasite (Ce, La, Nd)CO3(OH, F), rhabdophane (Ce, La, Nd)PO4-H2O, zircon (ZrSiO4), and synchysite Ca(Ce, La, Nd, Y)(CO3)2F. Monazite and bastnasite are presently the two most important sources of cerium.

Cerium is most often prepared via an ion exchange process that uses monazite sands as its cerium source.

Large deposits of monazite, allanite, and bastnasite will supply cerium, thorium, and other rare-earth metals for many years to come.

See also Category:Lanthanide minerals


[edit] Compounds

Cerium(IV)-sulfateCerium 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 reside 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.

See also Category:Cerium compounds


[edit] Isotopes
Main article: isotopes of cerium
Naturally occurring cerium is composed of 3 stable isotopes and 1 radioactive isotope; 136Ce, 138Ce, 140Ce, and 142Ce with 140Ce being the most abundant (88.48% natural abundance). 27 radioisotopes have been characterized with the most {abundant and/or stable} being 142Ce with a half-life of greater than 5×1016 years, 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 isotopes of cerium range in atomic weight from 123 u (123Ce) to 152 u (152Ce).


[edit] Precautions
Cerium, like all rare earth metals, is of low to moderate toxicity. Cerium is a strong reducing agent and 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. Animals injected with large doses of cerium have died due to cardiovascular collapse.

Cerium(IV) oxide is a powerful oxidizing agent at high temperatures and will react with combustible organic materials. While cerium is not radioactive, the impure commercial grade may contain traces of thorium, which is radioactive. Cerium serves no known biological function.


[edit] References
Los Alamos National Laboratory – Cerium
Lattice and spin dynamics of gamma-Ce

[edit] External links
Wikimedia Commons has media related to:
CeriumLook up cerium in
Wiktionary, the free dictionary.WebElements.com – Cerium
It's Elemental – The Element Cerium
Cerium Properties and Applications
Retrieved from "http://en.wikipedia.org/wiki/Cerium"
Categories: Chemical elements | Lanthanides | Cerium compounds