278 | The History and Use of Our Earth’s Chemical Elements Properties
278 | The History and Use of Our Earth’s Chemical Elements Properties
Lanthanum is a soft silvery-white metal that, when cut with a knife, forms an oxide with the air (tarnishes) on the exposed area. It is the most reactive of the elements in the series. It reacts slightly with cold water but rapidly with hot water, producing hydrogen gas (H 2 ) and lanthanum oxide (La 2 O 3 ). It directly interacts with several other elements, including nitrogen, boron, the halogens, carbon, sulfur, and phosphorus. Its melting point is 918°C, its boiling point is 3,464°C, and its density is 6.15 g/cm 3 .
Characteristics Lanthanide, as a pure metal, is difficult to separate from its ores, and it is often mixed with
other elements of the series. It is mostly obtained through an ion-exchange process from the sands of the mineral monazite, which can contain as much as 25% lanthanum as well as the oxides of several other elements of the series. The metal is malleable and ductile and can be formed into many shapes. Lanthanum is considered the most basic (alkaline) of the rare-earth elements.
AbundanceandSource The main ore in which lanthanum is found is monazite sands, and it is also found in the
mineral bastnasite. Monazite sands contain all of the rare-earth elements as well as some ele- ments that are not rare-earths. Its ores are found in South Africa, Australia, Brazil, and India and in California, Florida, and the Carolinas in the United States
The prices of lanthanide elements are somewhat reasonable and are less than gold per kilo- gram. (Gold is about $1,800 per kg.) Cesium (Ce), which is relatively common, is often alloyed with La, Nd, and Pr and iron to form misch metal. This alloy has several uses based on its unique ability to spark when scratched. The most common use is as flints for cigarette lighters.
Lanthanum is the fourth most abundant of the rare-earths found on the Earth. Its abun- dance is 18 ppm of the Earth’s crust, making it the 29th most abundant element on Earth. Its abundance is about equal to the abundance of zinc, lead, and nickel, so it is not really rare. Because the chemical and physical properties of the elements of the lanthanide series are so similar, they are quite difficult to separate. Therefore, some of them are often used together as an alloy or in compounds.
History Cerium was the first rare-earth element discovered, and its discovery came in 1803 by Jöns
Jakob Berzelius in Vienna. Johann Gadolin (1760–1852) also studied some minerals that were different from others known at that time. Because they were different from the common “earth elements” but were all very similar to each other, he named them “rare-earth elements.” However, he was unable to separate or identify them. In the 1800s only two rare-earths were known. At that time, they were known as yttria and ceria. Carl Gustav Mosander (1797–1858) and several other scientists attempted to separate the impurities in these two elements. In 1839 Mosander treated cerium nitrate with dilute nitric acid, which yielded a new rare-earth oxide
he called “lanthanum.” Mosander is credited with its discovery. This caused a change in the periodic table because the separation produced two new elements. Mosander’s method for separating rare-earths from a common mineral or from each other led other chemists to use
279 the same process, and by the early nineteenth century, all of the rare-earths were identified.
Guide to the Elements |
Over the years many chemists explored and sought to identify rare-earths. CommonUses
Carl Auer Baron van Welsbach (1858–1929) of Austria developed misch metal as a method of igniting a gas flame. In 1903 he patented an alloy of 70% Ce and 30% Fe that gave off sparks when scratched by steel. Baron van Welsbach is also the inventor of the gas mantle. Today, China manufactures most of the misch metal used in the world. The alloys that China uses consist of Ce, La, and Nd. They use whatever mixture of these elements are found in their ores, and thus there is no need to refine them. Lanthanum is used to make electrodes for high-intensity, carbon-arc lights that are used in motion picture studios and searchlights. It also used in the refining of high-grade europium metal and the creation of glass with a high refractive index as well as for quality lenses in cameras and scientific instruments. It is also used in the manufacture of strong permanent magnets.
Lanthanum is used for electronic instruments, as a rocket fuel, as a reducing agent, and in automobile catalytic converters.
ExamplesofCompounds Lanthanum arsenide (La 1+ + As 1- → LaAs) is used as a binary semiconductor. It is very
toxic. Lanthanum fluoride (La 3+ +3 F 1- → LaF 3 ) is a white powder used to coat the inside of phosphorus lamps and lasers. Lanthanum oxide (2La 3+ + 3O 2- → La 2 O 3 ) is used to make high-quality glass, ceramics, carbon-arc electrodes, and fluorescent lamps. Lanthanum carbide (La 8+ + 2C 4- → LaC 2 ) has an unusual oxidation state of 8+.
Hazards In powder form, lanthanum will ignite spontaneously. If ingested, it can cause liver damage
and prevent blood from clotting. Many of its compounds are toxic. CERIUM
SYMBOL:Ce PERIOD:6 SERIESNAME:Lanthanide NUMBER:58 ATOMICMASS:140.116amuVALENCE:3and4 OXIDATIONSTATE:+3and+4
NATURALSTATE:Solid ORIGINOFNAME:NamedfortheasteroidCeres,whichwasdiscoveredtwoyearsbefore theelement. ISOTOPES:Thereare44isotopesofcerium,fourofwhichareconsideredstable.Ce-140 accountsformostofthecerium(88.450%)foundintheEarth’scrust,andCe-138 makesupjust0.251%oftheelementinthecrust.Therearetwoisotopeswithhalf-lives longenoughtobeconsideredstable:Ce-136(0.185%),withahalf-lifeof0.7×10 +14 years,andCe-142(11.14%),withahalf-lifeof5×10 +16 years.Alltheotherisotopesare radioactivewithhalf-livesrangingfrom150nanosecondsto137.641days.Allaremade artificially.
280 | The History and Use of Our Earth’s Chemical Elements
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=19
s2,p6,d10,p1
5-O=9
s2,p6,d1
6-P=2
s2
Properties Cerium is a grayish/iron-colored, very reactive metallic element that is attacked by both
acids and alkalies. Pure cerium will ignite if scratched with a knife, but it can be combined safely with many other elements and materials. It is relatively soft and both malleable and ductile.
Its melting point is 798°C, its boiling point is 3,443°C, and its density is 6.770g/cm 3 . Characteristics As a pure metal, cerium is unstable and will decompose rapidly in moist air. It also decom-
poses in hot water to form hydrogen. Its oxide compounds and halides are stable and have a number of uses.
Cerium is separated from other rare-earth elements by an ion-exchange process in which it reacts with fluoride. This compound is then reduced with calcium metal (3Ca +2CeF 3 → 2Ce + 3CaF 3 ). Cerium can also be produced by the electrolysis of molten cerium salts. The metal ion collects at the cathode, and the chlorine or fluorine gases of the salt compound at the anode.
AbundanceandSource Cerium is the 25th most abundant element on Earth. It is also the most abundant rare-
earth metal in the lanthanide series. Its major ores are monazite and bastnasite. Cerium is found in the Earth’s crust in 46 ppm, which is about 0.0046% of the Earth’s crust. Cerium is mixed with other elements in its ores, making it difficult to find, isolate, and identify. Its existence was unknown until about 1803
Monazite sands contain most of the rare-earths. The sands of the beaches of Florida and parts of California contain monazite. Monazite is also found in South Africa, India, and Brazil. Bastnasite is found in southern California and New Mexico.
History Similar to the discovery of many other elements, cerium was detected simultaneously by
several different scientists. In 1803 the Swedish chemist Jöns Jakob Berzelius (1770–1848)
281 and the German chemists Wilhelm Hisinger (1766–1852) and Martin Klaproth (1743–1817)
Guide to the Elements |
identified cerium as an impurity of bastnasite. However, it was not until 1875 that W. F. Hillebrand and T. H. Norton purified the metal cerium.
CommonUses The compound cerium oxide (either Ce 2 O 3 or CeO 2 ) is used to coat the inside of ovens
because it was discovered that food cannot stick to oven walls that are coated with cerium oxide. Cerium compounds are used as electrodes in high-intensity lamps and film projec- tors used by the motion picture industry. Cerium is also used in the manufacturing and polishing of high-refraction lenses for cameras and telescopes and in the manufacture of incandescent lantern mantles. It additionally acts as a chemical reagent, a misch metal, and
a chemical catalyst. Cerium halides are an important component of the textile and photo- graphic industries, as an additive to other metals, and in automobile catalytic converters. Cerium is also used as an alloy to make special steel for jet engines, solid-state instruments, and rocket propellants.
ExamplesofCompounds There are two oxides of cerium with oxidation states of 3 and 4. Cerium can also react with
fluorine and chlorine.
Cerium (III) Oxide: 2Ce 3+ + 3O 2- → 2Ce 2 O 3 .
Cerium (IV) oxide: Ce 4+ + 2O 2- → CeO 2. Following is an example of a Ce 8+ ion compound:
Cerium carbide (CeC 2 ): Ce 8+ + 2C 4- → CeC 2 .
There is one radioactive isotope of cerium that is used in medicine. It is Ce-141, with a half- life of 32,641 days.
Hazards Most of the compounds of cerium are toxic if ingested or if the fumes are inhaled. Cerium
will ignite when heated.
PRASEODYMIUM SYMBOL:Pr PERIOD:6 SERIESNAME:Lanthanide ATOMICNO:59
ATOMICMASS:140.9075amu VALENCE:3 OXIDATIONSTATE:+3 NATURALSTATE: Solid ORIGINOFNAME: ThenameisderivedfromtwoGreekwords,prasiosanddidymos, whichtogethermean“greentwins.” ISOTOPES:Thereare45isotopesofpraseodymium.Allareartificiallyproducedandradio- activewithhalf-livesrangingfromseveralhundrednanosecondsto23.6days.Onlyone isstable(Pa-141),anditmakesup100%ofthepraseodymiumfoundintheEarth’s crust.
282 | The History and Use of Our Earth’s Chemical Elements
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=21
s2,p6,d10,f3
5-O=8
s2,p6
6-P=2
s2
Properties Praseodymium is a silvery-white, soft metal that is easily formed into various shapes. When
the pure metal is exposed to the air, a green oxide coating forms on its surface. To prevent oxidation, praseodymium is usually kept in oil in a covered container.
Its melting point is 931°C, its boiling point is 3,520°C, and its density is 6.77g/cm 3 . Characteristics As a metal, Pr is hygroscopic (adsorbs water) and tarnishes in the atmosphere. It will react
with water to liberate hydrogen. It is soluble in acids and forms greenish salts, along with some other rare-earths. It is used to fabricate the electrodes for high-intensity lights.
AbundanceandSource Praseodymium is the 41st most abundant element on Earth and is found in the ores of mona-
zite, cerite, bastnasite, and allanite along with other rare-earths. Praseodymium is also the stable isotope resulting from the process of fission of some other heavy elements, such as uranium.
Praseodymium is mainly found in monazite sands and bastnasite ores. The monazite sands contain all of the rare-earths and are found in river sand in India and Brazil as well as in Florida beach sand. A large deposit of bastnasite exists in California.
Praseodymium is separated from its ore and other rare-earths by a process called ion exchange, which exchanges one type of ion for another.
History At first praseodymium was called didymium, which is Greek for “twin,” because it was
always found with another rare-earth element. Using spectroscopic analysis, the two different color bands, one green and one yellow, indicated that there were two elements in didymium, but no one could identify the new elements.
In 1885 Carl Auer Baron von Welsbach (1858–1929) separated the oxides of two similar elements from didymium. He named one praseodymium from the Greek word prasios, which means “green” or the “green twin,” and he named the other element “neodymium,” which is derived from “new” and “dymium” and means “new twin.”
283 greenish color, another important use of praseodymium is as a yellow pigment to color glass
Guide to the Elements |
and ceramics. Along with several other rare-earths, it is also used to form the electrodes for high-intensity arc lamps.
It is used to manufacture safety goggles that filter out strong yellow light (used in welding, for example). Misch metal uses about 5% Pr in the manufacture of cigarette lighter flints.
ExamplesofCompounds Since the main oxidation state of praseodymium is +3, most of its stable compounds are
built by the Pr 3+ ion. A major example follows: Praseodymium (III) oxide: 2Pr 3+ + 3O 2- → Pr 2 O 3 . This compound is a yellow powder used to color glass and ceramics. Under certain conditions, the oxidation state of +4 exists for Pr, as in the following example:
Praseodymium (IV) dioxide: Pr 4+ + 2O 2- → PrO 2 .
Hazards If praseodymium gets wet or is submerged in water, the hydrogen released may explode. It
must be kept dry and protected from the atmosphere. NEODYMIUM
SYMBOL:Nd PERIOD:6SERIESNAME:Lanthanide ATOMICNO:60 ATOMICMASS:144.24amu VALENCE:3 OXIDATIONSTATE:+3
NATURALSTATE:Solid ORIGINOFNAME:DerivedfromthetwoGreekwordsneosanddidymos.Whencom- bined,theymean“newtwin.” ISOTOPES:Thereare47isotopesofneodymium,sevenofwhichareconsideredstable. TogetherthestableisotopesmakeupthetotalabundanceintheEarth’scrust.Two oftheseareradioactivebuthavesuchlonghalf-livesthattheyareconsideredstable becausetheystillexistonEarth.TheyareNd-144(half-lifeof2.29×10 +15 years)and Nd-150(half-lifeof6.8×10 +18 years).Alltheotherisotopesaresyntheticandhave half-livesrangingfrom300nanosecondsto3.37days.