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Name: Cerium |
Boiling Point: 3699oK, 3426oC, 6199oF Melting Point: 1071oK, 798oC, 1468oF Electrons Energy Level: 2, 8, 18, 19, 9, 2 Isotopes 35 + 4 Stable + 5 meta states Heat of Vaporization: 414 kJ/mol Heat of Fusion: 5.46 kJ/mol Density 6.757 g/cm3 @ 300oK Specific Heat 0.19 J/gK Atomic Radius: 2.7Å Ionic Radius: 1.034Å Electronegativity: 1.12 (Pauling), 1.06 (Allrod Rochow) |
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1s2 2s2p6 3s2p6d10 4s2p6d10f1 5s2p6d1 6s2
Cerium was discovered, in 1803, in Sweden by Jöns Jakob Berzelius and Wilhelm von Hisinger, and independently in Germany by Martin Heinrich Klaproth, but not isolated as a metal until 1875 by Hillebrand and Norton. Cerium was so named by Berzelius after the dwarf planet Ceres, discovered two years earlier in 1801.
Martin Heinrich Klaproth
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 is likely to ignite if scratched with a knife. Cerium oxidizes slowly in cold water and rapidly in hot water.
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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 Earths 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.
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.
Metallic cerium is prepared by metallothermic reduction techniques, such as by reducing cerous fluoride with calcium, or by electrolysis of molten cerous chloride or other cerous halides. The metallotherrnic technique is used to produce high-purity cerium. Cerium is especially interesting because of its variable electronic structure. The energy of the inner 4f level is nearly the same as that of the outer or valence electrons, and only small amounts of energy are required to change the relative occupancy of these electronic levels. This gives rise to dual valency 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. The low temperature behavior of cerium is complex. Four allotropic modifications are thought to exist: cerium at room temperature and at atmospheric pressure is known as gamma cerium. Upon cooling to -16oC, gamma cerium changes to beta cerium. The remaining gamma cerium starts to change to alpha cerium when cooled to -172oC, and the transformation is complete at -269oC. Alpha Cerium has a density of 8.16; delta cerium exists above 726oC. At atmospheric pressure, liquid cerium is more dense than its solid form at the melting point. Except for europium, cerium is the most reactive of the "rare-earth" metals. It slowly decomposes in cold water, and rapidly in hot water. Alkali solutions and dilute and concentrated acids attack the metal rapidly. The pure metal is likely to ignite if scratched with a knife. Ceric salts are orange red or yellowish; cerous salts are usually white.
Large deposits of monazite found on the beaches of Travancore, India, in river sands in Brazil, and deposits of allanite in the western United States, and bastnasite in Southern California will supply cerium, thorium, and the other rare-earth metals for many years to come.
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 oxidizing 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.
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). 30 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).
Isotope | Atomic Mass |
Half-Life |
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119Ce | 118.95276 | ~200 ms |
120Ce | 119.94664 | ~250 ms |
121Ce | 120.94342 | 1.1 seconds |
122Ce | 121.93791 | ~2 seconds |
123Ce | 122.93540 | 3.8 seconds |
124Ce | 123.93041 | 9.1 seconds |
125Ce | 124.92844 | 9.3 seconds |
126Ce | 125.92397 | 51.0 seconds |
127Ce | 126.92273 | 29 seconds |
128Ce | 127.91891 | 3.93 minutes |
129Ce | 128.91810 | 3.5 minutes |
130Ce | 129.91474 | 22.9 minutes |
131Ce | 130.91442 | 10.2 minutes |
131mCe | 5.0 minutes | |
132Ce | 131.911460 | 3.51 hours |
133Ce | 132.911515 | 97 minutes |
133mCe | 4.9 hours | |
134Ce | 133.908925 | 3.16 days |
135Ce | 134.909151 | 17.7 hours |
135mCe | 20 seconds | |
136Ce | 135.907172 | Stable |
137Ce | 136.907806 | 9.0 hours |
137mCe | 34.4 hours | |
138Ce | 137.905991 | Stable |
139Ce | 138.906653 | 137.641 days |
139mCe | 56.54 seconds | |
140Ce | 139.9054387 | Stable |
141Ce | 140.9082763 | 32.508 days |
142Ce | 141.909244 | Stable |
143Ce | 142.912386 | 33.039 hours |
144Ce | 143.913647 | 284.91 days |
145Ce | 144.91723 | 3.01 minutes |
146Ce | 145.91876 | 13.52 minutes |
147Ce | 146.92267 | 56.4 seconds |
148Ce | 147.92443 | 56 seconds |
149Ce | 148.9284 | 5.3 seconds |
150Ce | 149.93041 | 4.0 seconds |
151Ce | 150.93398 | 1.02 seconds |
152Ce | 151.93654 | 1.4 seconds |
153Ce | 152.94058 | ~500 ms |
154Ce | 153.94342 | ~300 ms |
155Ce | 154.94804 | ~200 ms |
156Ce | 155.95126 | ~150 ms |
157Ce | 156.95634 | ~50 ms |
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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.
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It is much more reactive than iron, however, readily oxidizing in moist air and releasing hydrogen from boiling water. Friction from abrading a sample can cause it to ignite. |
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.
*Mischmetal (from German : Mischmetall - "mixed metals") is an alloy of rare earth elements in various naturally-occurring proportions. It is also called cerium mischmetal, rare earth mischmetal or misch metal. A typical composition includes approximately 50% cerium and 45% lanthanum, with small amounts o neodymium and praseodymium. Its most common use is in the "flint" ignition device of many lighters and torches, although an alloy of only rare-earth elements would be too soft to give good sparks. For this purpose, it is blended with iron oxide and magnesium oxide to form a harder material known as ferrocerium.
Mischmetal is used in the preparation of virtually all rare earth elements. This is because such elements are nearly identical in most chemical processes, meaning that ordinary extraction processes do not distinguish them. Highly specialized processes, such as those developed by Carl Auer von Welsbach, exploit subtle differences in solubility to separate mischmetal into its constituent elements, with each step producing only an incremental change in composition. Such processes later informed Marie Curie in her search for new elements.
Mischmetal is widely applied in steel foundaries for making FeSiMg alloy and it is used to remove free oxygen and sulfur by forming stable oxysulfides and by tying up undesirable trace elements, such as lead and antimony.
Mischmetal can be further processed to various shapes of ingots, wires, slabs, rods, and discs.
Cerium Data |
Atomic Radius (Å): 2.7Å Electrochemical Equivalents: 1.7426 g/amp-hr Atomic Mass Average: 140.115 Fusion Heat: 5.2 kJ/mol |