Lanthanoids and Actinoids - details
Lanthanoids and actinoids
are f-block transition
elements, but their
general
properties differ significantly from those of
d-block transition metals. These element
s are placed in separate positions in
the periodic table showing that the periodicity of their electronic structures
differs from the main stream.
Although lanthanoids are called rare-earth
elements, their abundance in the crust is by no means rare and chemistry utilizing
their unique properties is likely to develop significantly in the near future.
Actinoids are closely related to nuclear
chemistry and nuclear energy. Since the amount of superheavy elements
"synthesized" in accelerators is very minute, they are not very
significant from the viewpoint of applied chemistry.
Lanthanoids
The
lanthanum, La (4f0), to lutetium, Lu (4f14), are lanthanoids. Ln may be used as
a general symbol for the lanthanoid elements. Although lanthanoids, scandium,
Sc, and yttrium, Y, are sometimes called rare earth elements, they are
relatively abundant in the earth’s crust.
With the exception of promethium, Pm, which does form a stable isotope,
even the least abundant thulium, Tm, and lutetium, Lu, are as abundant as
iodine. Because lanthanoids have very
similar properties and are difficult to separate from one another, they were
not useful for basic research and application, and hence they were regarded as
rare elements. Since a liquid-liquid
solvent extraction method using tributylphosphine oxide became available in the
1960s, lanthanoid elements have been readily available and widely used not only
for chemical research but also as materials in alloys, catalysts, lasers,
cathode-ray tubes, etc.
Difference
The Fifteen
elements La-Lu are lanthanoids and fourteen elements Ce-Lu without lanthanum
are lanthanides (meaning the elements similar to lanthanum). Occasionally the
names are confused and 15 elements including lanthanum may be called
lanthanides.
Because the
three stages of
ionization enthalpy of lanthanoid
elements are comparatively low,
they are positive elements and readily assume trivalent ionic states. Most
compounds of lanthanoids other than Ce4+ (4f0), Eu2+ (4f7), Yb2+ (4f14), are
usually Ln3+ ones. Ln3+ species are hard
acids, and since f electrons are buried deeply and not used for bonding, they
are hardly influenced by ligands. There
is a tendency for atomic and ionic radii to decrease with the increase of the
atomic number, and this phenomenon is called the lanthanide contraction. This
contraction is due to small shielding effects of 4 f
electrons, which causes the atomic nucleus to draw outer shell electrons
strongly with an increase of atomic number.
Complexes of
lanthanoid metals are 6 to 12 coordinate and especially many 8 and 9 coordinate
compounds are known. Organometallic
compounds with cyclopentadienyl ligands of the types Cp3Ln or Cp2LnX are also
known, all of which are very reactive to oxygen or water.
Actinoids
The fifteen
elements from actinium, Ac, to lawrencium, Lr, are called actinoids .The general symbol of these
elements is An. All the actinoid
elements are radioactive and very poisonous.
Actinoids that exist in nature in considerable amounts are thorium, Th,
protactinium, Pa, and uranium, U, and thorium and uranium are actually isolated
from ores and find application.
Plutonium metal, Pu, is produced in large quantities in nuclear reactors
and its economical efficiency as a fuel for conventional nuclear
reactors and fast breeder reactors, as well as its safety, are being
examined. As isolable amounts of the
elements after americium, Am, are small and their radioactivity is very high,
study of their chemical properties is very limited.
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