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Lanthanide contraction is a term used in chemistry to describe different but closely related concepts associated with smaller than expected ionic radii of the elements in the lanthanide series (atomic number 58, Cerium to 71,(Lutetium).
Additional recommended knowledge
Very simply this effect is due to the poor shielding of nuclear charge by 4f electrons.
As a result of the increased attraction of the outer shell electrons across the lanthanide period, the following effects are observed. Each of these effects is sometimes referred to as the lanthanide contraction:
Chemical behavior of the lanthanides
Since the outer shells of the lanthanides do not change within the group, their chemical behaviour is very similar. The differing atomic and ionic radii does affect their chemistry, however. Without the lanthanide contraction, a chemical separation of lanthanides would be extremely difficult. However, this contraction makes the chemical separation of period 5 and period 6 transition metals of the same group rather difficult.
Influence on the post-lanthanides
All elements following the lanthanides in the periodic table are influenced by the lanthanide contraction. The period 6 elements have very similar radii compared with the elements of the period 5 elements in the same group.
For example, the atomic radii of the metal zirconium, Zr, (a period 5 element) is 1.59 Ångström and that of hafnium, Hf, (a period 6 element) is 1.56 Ångström. The ionic radius of Zr4+ is 0.79 Ångström and that of Hf4+ is 0.78 Ångström. The radii are very similar even though the number of electrons increases from 40 to 72 and the atomic mass increases from 91.22 to 178.49 g/mol. The increase in mass and the unchanged radii lead to a steep increase in density from 6.51 to 13.35 g/cm3.
Zirconium and hafnium therefore have very similar chemical behaviour, having closely similar radii and electron configurations. Because of this hafnium is found only in association with zirconium which is much more abundant, and was only discovered as a separate element 134 years later (in 1923) than zirconium (discovered in 1789).
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Lanthanide_contraction". A list of authors is available in Wikipedia.|