Chemistry

An Overview about the Chemical Element Lanthanum



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Lanthanum

Symbol: La

Atomic Number: 57

Atomic Mass: 138.9055 amu (atomic mass units)

Melting Point: 920.0 C (1193.15 K, 1688.0 F)

Boiling Point: 3469.0 C (3742.15 K, 6276.2 F)

Number of Protons: 57

Number of Electrons: 57

Number of Neutrons: 82

Classification: Rare Earth Metal

Crystal Structure: Hexagonal

Density @ 293 K: 6.7 grams per cubic centimeter

Color: white

Lanthanum is the first of the lanthanide or lanthanoid group of rare earth elements and it is from this element that the group gets its name.

It was discovered by the Swish chemist Carl Gustaf Mosander in 1839. At the time he was investigating what he believed were contaminants contained in some compounds of the element cerium. After treating a sample of cerium nitrate with a dilute solution of nitric acid he was left with a compound we now know to be lanthanum oxide. He called this compound lanthana which come from the Greek word "lanthaniea" which means hidden.

Lanthanum is a silvery white, ductile and malleable metal. It is soft and can be cut with a knife. One of the most reactive of the lanthanides, lanthanum will react slowly with cold water but rapidly if the water is heated. If the metal is exposed to air it will oxidize rapidly. Metallic lanthanum will react directly with a number of elements including carbon, boron, phosphorus and the halogens. It exhibits a change in its crystal structure with an increase in temperature - the hexagonal form changes to a face-centered cubic crystal form at 310 C (583.15 K, 590 F) then to a body-centered cubic form at 865 C (1138.15 K, 1589 F).

There is one naturally occurring stable isotope of lanthanum which makes up 99.91% of its total abundance. This isotope is lanthanum-139. The remaining 0.09% of naturally occurring lanthanum is made of unstable lanthanum-138 which has an extremely long half-life. A number of other unstable isotopes of lanthanum have been produced with atomic masses ranging from 117 to 155.

Lanthanum is found in a number of mineral ores but its chief sources are monazite which contains about 25 % lanthanum and bastnasite which contains 38% lanthanum. Monazite also contains the highly radioactive element thorium so care must be taken when extracting rare earth elements from this ore. Rare earth elements are extracted from their mineral by a series of methods such as solvent extraction and ion exchange chromatography. Pure lanthanum can be obtained by the reduction of lanthanum fluoride with calcium metal.

Lanthanum and its compounds have a number of uses.

* The main use for lanthanum is in production of carbon lights such as the arc lamps used in the motion picture industry. About 25% of the total production of all the rare earth elements is used in this manner.

* Like many other rare earth elements lanthanum forms part of misch metal which is used to make cigarette lighter flints.

* Lanthanum is added in small amounts to iron in the production of nodular cast iron.

* Lanthanum oxide is used in the glass industry to improve the resistance of glass to the action of alkalis. It is incorporated in to some optical glass applications such as camera lenses.

* The recent development of hydrogen sponge alloys which also contain lanthanum may find a use in energy conservation. These specialized alloys have the ability to take up about 400 times their own volume in hydrogen gas in a reversible process.

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