Praseodymium is a lanthanide rare earth element denoted by the atomic symbol Pr. It has an atomic number of 59, and the average atomic weight of its isotopes is 140.90765. It is a soft, yellow metal that forms an oxide film when exposed to air.

Early chemists were often confused about which materials were true elements and which were not. Without today's periodic table of elements to refer to, they had a hard time sorting out the relationships among new elements, especially the metals that came to be called rare earths. These rare earth elements, which include praseodymium, are actually not so rare--millions of pounds of them have since been found--but they were first extracted from scarce materials.

Before 1885 chemists believed that praseodymium and a similar rare earth metal were a single element, which was then called didymium. But an Austrian chemist named Karl Auer (1858-1929) thought differently. His intense interest in the rare earths led him to isolate the metal oxides of two separate elements from didymium. He named these metal elements praseodymium ("green twin," from the most prominent color in its light waves) and neodymium ("new twin"). A decade earlier, Swedish mineralogist Per Cleve (1840-1905) had suspected what Auer proved. Praseodymium's atomic weight was later determined by Czech chemist Bohuslav Brauner (1855-1935).

At first, praseodymium was used mainly to color glass, from clear yellow to green. Today, it is used to make sunglasses, art objects, protective goggles, ceramic tile, and optical filters. In mixtures with other rare earths, praseodymium can be used as a catalyst the oil refining process, and it is an ingredient of the sparking device in a cigarette lighter. Alloyed with cobalt, praseodymium has unsurpassed magnetic properties; a new family of permanent magnets using such alloys are improving technology in power generation and electronic communication.