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.

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.

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.

The table below show summary of the latinoids properties

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.

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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