What do peridot, twins and absolute zero have in common?
1.5 grams praseodymium under argon, in 0.5 cm pieces.
Image: Jurii (Creative Commons Attribution 3.0 Unported license.)

This week's element is one that most of you have probably never heard of, or forgot you'd heard about shortly after you took the exam; praseodymium. Praseodymium has the symbol Pr and atomic number 59. As you can see in the image above, it is silvery in colour, and it is also soft, malleable and ductile. It gets its name from the Greek for "green twin" because it was mistakenly thought to be the element, didymium, which later was discovered to be comprised of praseodymium along with a second element, neodymium.

But why the name "didymium"? Carl Mosander, the chemist who discovered didymium in 1841, was the father of two sets of twins, so his name choice for this "element" may have been suggested by his personal circumstances at the time. Although didymium was originally accepted as a true element for more than 40 years, suspicions arose about its true nature. Some chemical detective work revealed that didymium's chemical spectrum was not produced by a pure element and so the race was on to isolate didymium's components. In 1885, didymium was split into its two component elements and much later, in 1931, a pure metallic sample of praseodymium was isolated.

The prosios, or green, part of praseodymium's name comes from the lovely oxide coating that forms on the surface of this element. Instead of forming a protective layer, this oxide flakes off, which then exposes more fresh metal to the oxidizing effects of air. In fact, unless it's stored under argon or is coated with plastic or light mineral oil, a 1 cm piece of praseodymium will completely oxidize away in a year.

Even though praseodymium is a "rare earth element", this metal is not at all rare. It is one of the more abundant rare earth elements; it is, for example, four times more abundant than tin. The most important ores that contain praseodymium are monazite and bastnäsite [a particularly beautiful crystal of bastnäsite is pictured at right, courtesy of Rob Lavinsky/iRocks (Creative Commons Attribution-Share Alike 3.0 Unported license)], and large reserves of praseodymium are present in China, USA, Brazil, India, Australia and Greenland. These reserves are estimated to exceed 2 million tonnes.

Although praseodymium is too reactive to be found in its native state in the wild, it is used in a variety of familiar applications, including to provide the yellow-green colouring in cubic zirconia that simulate the birthstone for August, peridot; it colours "didymium glass", which is used in certain types of welder's and glass blower's goggles to filter out infrared radiation; it also is a source of yellow colouring for glass used in stained glass work and for a variety of brilliant enamel colourings used in ceramics. But in my opinion, the coolest use of praseodymium is as a nickel alloy, PrNi5, which, when exposed to a changing magnetic field, it responds by dropping its temperature -- by a lot! By harnessing this unusual quality, scientists have been able to approach within one thousandth of a degree of absolute zero (-273.15oC).

Here's a short video telling us a little more about the wonders of praseodymium:

[video link].

Since praseodymium is frequently misspelled, I thought I'd leave you with this final comment: there are no n's in "praseodymium".



You've already met these elements:

Cerium: Ce, atomic number 58
Lanthanum: La, atomic number 57
Barium: Ba, atomic number 56
Cæsium: Cs, atomic number 55
Xenon: Xe, atomic number 54
Iodine: I, atomic number 53
Tellurium: Te, atomic number 52
Antimony: Sb, atomic number 51
Tin: Sn, atomic number 50
Indium: In, atomic number 49
Cadmium: Cd, atomic number 48
Silver: Ag, atomic number 47
Palladium: Pd, atomic number 46
Rhodium: Rh, atomic number 45
Ruthenium: Ru, atomic number 44
Technetium: Tc, atomic number 43
Molybdenum: Mo, atomic number 42
Niobium: Ni, atomic number 41
Zirconium: Zr, atomic number 40
Yttrium: Y, atomic number 39
Strontium: Sr, atomic number 38
Rubidium: Rr, atomic number 37
Krypton: Kr, atomic number 36
Bromine: Br, atomic number 35
Selenium: Se, atomic number 34
Arsenic: As, atomic number 33
Germanium: Ge, atomic number 32
Gallium: Ga, atomic number 31
Zinc: Zn, atomic number 30
Copper: Cu, atomic number 29
Nickel: Ni, atomic number 28
Cobalt: Co, atomic number 27
Iron: Fe, atomic number 26
Manganese: Mn, atomic number 25
Chromium: Cr, atomic number 24
Vanadium: V, atomic number 23
Titanium: Ti, atomic number 22
Scandium: Sc, atomic number 21
Calcium: Ca, atomic number 20
Potassium: K, atomic number 19
Argon: Ar, atomic number 18
Chlorine: Cl, atomic number 17
Sulfur: S, atomic number 16
Phosphorus: P, atomic number 15
Silicon: Si, atomic number 14
Aluminium: Al, atomic number 13
Magnesium: Mg, atomic number 12
Sodium: Na, atomic number 11
Neon: Ne, atomic number 10
Fluorine: F, atomic number 9
Oxygen: O, atomic number 8
Nitrogen: N, atomic number 7
Carbon: C, atomic number 6
Boron: B, atomic number 5
Beryllium: Be, atomic number 4
Lithium: Li, atomic number 3
Helium: He, atomic number 2
Hydrogen: H, atomic number 1



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