Praseodymium (Pr)

• Element Symbol: Pr
• Atomic Number: 59
• Atomic Mass: 140.9077
• Group # in Periodic Table: n/a
• Group Name: Lanthanides
• Period in Periodic Table: 6
• Block of Periodic Table: f-block
• Discovered by: Carl Auer von Welsbach (1885)

Praseodymium is a metallic chemical element in the periodic table. It is a rare earth element, belonging to the lanthanide group of elements, which includes fourteen other rare earth elements, such as lanthanum and neodymium. Rare earth elements, also known as rare earth metals, typically occur together in nature and can be difficult to separate.

In 1839 Swedish chemist Carl G. Mosander successfully extracted lantana, an oxide of lanthanum, which was an element that he had just recently discovered. It was from this oxide that Mosander isolated a pink-colored substance he called didymium. After years of investigation and experimentation, Carl Auer von Welsbach, an Austrian scientist, discovered that didymium was actually composed of two elements— praseodymium and neodymium, which he successfully separated in 1885. The praseodymium that von Welsbach produced was green in color due to oxidation, and he therefore named the element for its hue (the word "praseodymium" is derived from a Greek word that means "green twin").

Physical Properties

Praseodymium has a metallic silvery color. At 298 kelvins (K), praseodymium’s standard state is solid. In this state praseodymium is soft and malleable as well as ductile. Praseodymium has a specific gravity of 6.77. The specific gravity of an element is a measure of its density in comparison with that of water. The melting point of praseodymium is 931 degrees Celsius (°C), and its boiling point is 3520 °C. The specific heat of praseodymium—that is, the amount of energy required to raise its temperature by one degree—is 193 joules per kilogram-kelvin (J/kg·K). Its electrical conductivity is 1.4 × 10⁶ siemens per meter (S/m), and its thermal conductivity is 13 watts per meter-kelvin (W/m·K).

Many rare earth elements are particularly susceptible to corrosion when exposed to air. Praseodymium, however, is much more resistant to corrosion, and it only completely oxidizes after one year of exposure to air. To avoid developing its characteristic green oxide coating, praseodymium is typically stored in mineral oil or sealed inside glass.

Chemical Properties

Common oxidation states of praseodymium are +4 and +3. The element has only one naturally occurring stable isotope, praseodymium-141. Thirty-eight more praseodymium isotopes have been identified, all of which are radioactive and unstable. The most stable radioactive isotopes of praseodymium are praseodymium-143, which has a half-life of 13.57 days, and praseodymium-142, with a half-life of 19.12 hours. The remaining thirty-six radioactive isotopes all have half-lives shorter than six hours, and in most cases shorter than ten minutes. The unstable praseodymium isotopes with mass numbers less than 141 undergo positron emission, a form of beta decay that results in the production of a positron and an electron neutrino. Positron emission is also called beta-plus decay. Unstable praseodymium isotopes that have mass numbers greater than 141 undergo beta-minus decay, which results in the production of an electron and an electron antineutrino. Praseodymium has a simple hexagonal crystal structure.

Applications

Despite its classification as a rare earth element, a name that suggests its scarcity, praseodymium actually occurs naturally in many different sources on Earth. Practically no praseodymium occurs naturally in the atmosphere, but small amounts can be found in Earth’s crust—about 8.7 parts per million (ppm). Praseodymium can be more abundant in soil (as much as 15 ppm) but much less abundant in the ocean (6 × 10⁻⁷ ppm, or 0.6 part per trillion). Praseodymium can also be found in several rare earth minerals, including monazite sand and bastnaesite. It is from monazite that praseodymium is generally acquired through a process known as ion exchange, although praseodymium is characteristically difficult to purify. Additionally, praseodymium makes up approximately 5 percent of misch metal, a material that is found in lighter flints.

Praseodymium is primarily used as an alloy with magnesium for the production of very strong metals. An alloy is a mixture of metals, usually combined to create a sturdier material than the elements alone. This particular alloy is used in aircraft engines. In addition, praseodymium cores can be found in carbon arc lights, which are used in the lights of studios and projectors in the motion picture industry. Praseodymium is also used for its electrical conduction abilities, and it is added to fiber optic cables in order to increase their signal strengths. Together with neodymium, praseodymium can create powerful magnets that possess a strong and long-lasting magnetic force.

In the 1920s praseodymium began to be investigated for its potential use in the coloring of glass. The result of the first of these efforts was a glass called prasemit, which was notable for its yellow-green hue. Later, a commercially viable use for purified praseodymium was found—namely, the element provided a yellow-orange stain that was used in ceramics and was known as praseodymium yellow. This stain is still popular today. In addition praseodymium is useful in the goggles worn by welders and glassblowers. When mixed with didymium, the praseodymium creates a particular color of blue that is able to filter out the yellow glow produced by molten glass. By wearing goggles with this glass, glassblowers can see their work more clearly.

Bibliography

Atwood, David A., ed. The Rare Earth Elements: Fundamentals and Applications. Hoboken: Wiley, 2012. Print.

"Facts about Praseodymium." LiveScience. Purch, 21 June 2013. Web. 3 Aug. 2015.

Haynes, William M., ed. CRC Handbook of Chemistry and Physics. 95th ed. Boca Raton: CRC, 2014. Print.

Lucas, Jacques, et al. Rare Earths: Science, Technology, Production and Use. Waltham: Elsevier, 2015. Print.

Pecharsky, Vitalij K. "Praseodymium (Pr)." Encyclopædia Britannica. Encyclopædia Britannica, 3 July 2013. Web. 3 Aug. 2015.

"Technical Data for Praseodymium." The Photographic Periodic Table of the Elements. Element Collection, n.d. Web. 3 Aug. 2015.