Lanthanum
|
 |
Atomic Number: |
57 |
|
Melting
Point: |
918 ºC |
| Atomic Symbol: |
La |
|
Boiling
Point: |
1897 ºC |
| Atomic Weight: |
138.9055
amu |
|
Density: |
6146
kg/m 3
|
| Atomic
Radius: |
187.7 pm
|
|
Oxidation States: |
2 |
| Covalent Radius: |
169
pm |
|
Electron
Configuration: |
[Xe]6s25d1 |
| van der Waals
Radius: |
-- |
|
State of Matter: |
solid |
History
(Greek lanthanein: to lie hidden) Mosander in 1839 extracted lanthana
from impure cerium nitrate and recognized the new element.
Lanthanum was isolated in relatively pure form in 1923. Iron exchange and
solvent extraction techniques have led to much easier isolation of the so-called
"rare-earth" elements.
Properties
Lanthanum is silvery white, malleable, ductile, and soft enough to be cut
with a knife. It is one of the most reactive of the rare-earth metals. It
oxidizes rapidly when exposed to air. Cold water attacks lanthanum slowly, while
hot water attacks it much more rapidly.
The metal reacts directly with elemental
carbon,
nitrogen,
boron,
selenium,
silicon,
phosphorus,
sulfur, and with
halogens.
At 310ºC, lanthanum changes from a hexagonal to a face-centered cubic
structure, and at 865ºC it again transforms into a body-centered cubic
structure.
Sources
Lanthanum is found in rare-earth minerals such as cerite, monazite, allanite,
and bastnasite. Monazite and bastnasite are principal ores in which lanthanum
occurs in percentages up to 25 percent and 38 percent respectively. Misch metal,
used in making lighter flints, contains about 25 percent lanthanum.
The availability of lanthanum and other rare earths has improved greatly in
recent years. The metal can be produced by reducing the anhydrous fluoride with
calcium .
Uses
Lanthanum is added to steel to improve its malleability, resistance to impact
and ductility. It is also added to molybdenum to decrease the hardness and
sensitivity to temperature variations. Mischmetal, a pyrophoric alloy used in
lighter flints, contains 25% to 45% lanthanum.
There is current interest in hydrogen sponge alloys containing lanthanum.
These alloys take up to 400 times their own volume of hydrogen gas, and the
process is reversible. Every time they take up the gas, heat energy is released;
therefore these alloys have possibilities in an energy conservation system.
The oxide and the boride are used in electronic vacuum tubes.
Hydrogen sponge alloys can contain lanthanum. These alloys are
capable of storing up to 400 times their own volume of hydrogen
gas in a reversible adsorption process.
Lanthanum is also used
in as a petroleum cracking catalyst, in gas lantern mantles, and
as a glass and lapidary polishing compound. It is used in age
dating of rocks and ores as well.
Rare-earth compounds containing lanthanum are extensively used in carbon
lighting applications, especially by the motion picture industry for studio
lighting and projection. This application consumes about 25 percent of the
rare-earth compounds produced. La2O3 improves the alkali
resistance of glass, such as infrared absorbing glass, camera and telescope
lenses. It is also used in making special optical glasses. Small amounts of
lanthanum, as an additive, can be used to produce nodular cast iron.
Isotopes
Natural lanthanum is a mixture of two stable isotopes, 138La and
139La. Twenty three other radioactive isotopes are recognized. All of
the radioactive isotopes have half-lifes that are less than 24 hours and the
majority of these have half-lifes that are less than 1 minute. This element also
has 3 meta states.
Hazards
Lanthanum and its compounds have a low to moderate acute toxicity rating;
therefore, care should be taken in handling them. In animals, the injection of
lanthanum solutions produces glycohemia, low blood pressure, degeneration of the
spleen and hepatic alterations.
It is a flammable solid. As such it reacts with water and dilute acids to
produce flammable, explosive hydrogen gas. Dusts in high concentrations may
cause skin, eye and respiratory tract irritation.