Cerium (Ce)

• Element Symbol: Ce
• Atomic Number: 58
• Atomic Mass: 140.116
• Group # in Periodic Table: n/a
• Group Name: Lanthanides
• Period in Periodic Table: 6
• Block of Periodic Table: f-block
• Discovered by: Jöns Jacob Berzelius, Wilhelm Hisinger, Martin Heinrich Klaproth (1803)

Cerium is a silvery-gray metal. It is part of the lanthanide series, which includes the elements with atomic numbers between 57 and 71 in the periodic table. Like the other elements in this series, cerium is a rare earth element. It can be found with other rare earth elements in certain minerals within Earth’s crust. Minerals that may contain cerium include bastnaesite and monazite. Cerium can also be found in naturally occurring minerals that contain mostly cerium, such as cerite (cerium silicate). Cerium metal is not found in its pure form in nature.

Cerium was discovered in 1803 by the Swedish chemist Jöns Jacob Berzelius and mine owner Wilhelm Hisinger. Berzelius used electrolysis to examine a mineral in a sample from one of Hisinger’s mines. This mineral contained a new element that Berzelius called cerium, after the newly discovered dwarf planet Ceres. Ceres is also the name of the Roman goddess of agriculture.

Physical Properties

Cerium is silvery gray and solid in its standard state—that is, at a temperature of 298 kelvins (K). It is a relatively soft, ductile rare earth metal. Its Mohs hardness number is 2.5. Cerium has a density of 6.689 grams per cubic centimeter (g/cm³) at standard state. Its melting point is 798 degrees Celsius (ºC), and its boiling point is 3360 ºC. The specific heat of cerium is 192 joules per kilogram-kelvin (J/kg·K). Cerium is a good conductor of both heat and electricity. Its thermal conductivity is 11 watts per meter-kelvin (W/m·K), its electrical conductivity is 1.4 × 10⁶ siemens per meter (S/m), and its resistivity is 7.4 × 10⁻⁷ meter-ohms (m·Ω). Cerium becomes a superconductor at extremely low temperature and high pressure. At most temperatures, it is paramagnetic, meaning that it has only a small response to a magnetic field.

Chemical Properties

Cerium has different structural forms, depending on the temperature. At low temperatures its crystal structure is face-centered cubic. Just below room temperature, its structure shows a close-packed hexagonal form. At room temperature the structure returns to a face-centered cubic form. At much higher temperatures the crystal structure of cerium becomes body-centered cubic.

Cerium forms cerium oxide in the presence of air. It produces sparks at room temperature when the metal is scratched or ground. Cerium reacts with water and dissolves in most dilute acids. Like other lanthanides, cerium does not dissolve in hydrofluoric acid. It instead forms a protective cerium trifluoride layer on the acid’s surface.

The electron affinity of cerium is 50 kilojoules per mole (kJ/mol). Cerium has four valence electrons. Its ionization energies are 534.4, 1050, 1949, 3547, 6325, and 7490 kJ/mol. It has two oxidation states: +3 and +4. While an oxidation state of +3 is common for rare earth elements, cerium is the only rare earth element with an oxidation state of +4.

Cerium has four naturally occurring isotopes. The most common of them is cerium-140, which is a stable isotope that represents around 88 percent of the cerium on Earth. The other three natural isotopes appear stable but are theorized to undergo slow radioactive decay. These include cerium-142, cerium-138, and cerium-136. Thirty-five radioactive isotopes of cerium have been produced during nuclear fission. The half-lives of these radioisotopes range from about one second to 285 days. This element has an electron configuration of [Xe]4f¹5d¹6s².

Applications

Cerium is the most abundant lanthanide and a very common element in general. Its concentration in Earth’s crust is around 46 parts per million. Cerium is similar to copper or zinc in its abundance on Earth. Because it is widely available, this element is relatively inexpensive. Late in the first decade of the twenty-first century, cerium was selling for as low as $6 per pound. China is the largest producer of cerium and of rare earth elements in general.

Cerium oxide can be obtained by crushing and heating mineral ores such as bastnaesite and then adding hydrochloric acid to the crushed ore. Cerium metal can be isolated through the electrolysis of cerium oxide or by applying heat to a mixture of calcium and cerium(III) fluoride.

Cerium is a component of misch metal, which is an alloy containing a variety of rare earth metals. The name of the alloy comes from the German word Mischmetall, which means "mixed metal." Misch metal generally contains about 50 percent cerium, 25 percent lanthanum, and 15 percent neodymium. It is added to steel to make the metal easier to cast. It is also added to magnesium to increase the strength of that metal. Misch metal is used in metal hydride batteries (such as NiMH batteries). It is also used in vacuum tubes, where the rare earth elements help remove any oxygen.

Some products containing misch metal take advantage of the spark-producing properties of cerium and lanthanum. Misch metal is used to make the flints in lighters. It is also used to make rods that generate huge sparks when dragged or subjected to a cutting wheel. These sparking rods are used to produce numerous special effects in movies and amusement parks.

Cerium sulfide is a common pigment in red paints. Because this compound is nontoxic, it serves as an alternative to other red-producing compounds that contain heavy metals, such as cadmium, lead, or mercury.

Cerium oxide has a wide variety of uses. As a coating in self-cleaning ovens, it oxidizes baked-on food. As a catalyst in catalytic converters, it helps convert carbon monoxide to carbon dioxide. In powdered form, it is used to grind and polish optical glass as well as to polish screens of LED and plasma televisions. In nanoparticle form and in combination with oleic acid, it has been investigated for use as an antioxidant in humans. Nanoparticles of cerium oxide have also been investigated as additives to diesel fuel to reduce particle emissions.

Cerium used to be found in cathode-ray television sets. This element protected the screen from damage that could occur over time as a result of the impact of high-energy electrons.

Bibliography

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