Lanthanide contraction

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).

Cause

Very simply, the effect results from poor shielding of nuclear charge by 4f electrons.

In single-electron atoms, the average separation of an electron from the nucleus is determined by the subshell it belongs to, and decreases with increasing charge on the nucleus; this in turn leads to a decrease in atomic radius. In multi-electron atoms, the decrease in radius brought about by an increase in nuclear charge is partially offset by increasing electrostatic repulsion among electrons. Particularly, a "shielding effect" operates: i.e., as electrons are added in outer shells, electrons already present shield the outer electrons from nuclear charge, making them experience a lower effective charge on the nucleus. The shielding effect exerted by the inner electrons decreases in order "s" > "p" > "d" > "f". Usually, as a particular subshell is filled in a period, atomic radius decreases. This effect is particularly pronounced in the case of lanthanides, as their 4"f" subshells are being filled across the period and they are less and less able to shield the outer (5th and 6th) shell electrons. Thus the shielding effect is less able to counter the decrease in radius caused by increasing nuclear charge. This leads to "lanthanide contraction". The ionic radius drops from 102 pm in case of cerium(III) to 86.1 pm in the case of lutetium(III).

About 10% of the lanthanide contraction has been attributed to relativistic effects. [Pekka Pyykko. Relativistic effects in structural chemistry. "Chem. Rev." 1988, "88", 563-594.]

Effects

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:

* The atomic radii of the lanthanides are smaller than would normally be expected.

* The ionic radii of the lanthanides decrease from 1.17 Ångström (La³⁺) to 1.00 Ångström (Lu³⁺) in the lanthanide period.

* The third row of "d" block elements have only marginally larger atomic radii than the second transition series.

* The radii of the elements following the lanthanides are smaller than would be expected if there were no "f"-transition metals.

* There is a general trend of increasing Vickers hardness, Brinell hardness, density and melting point from cerium to lutetium (with ytterbium being the most notable exception). Lutetium is the hardest and most dense lanthanide and has the highest melting point.

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 similarity hafnium is found only in association with zirconium, which is much more abundant, and was discovered as a separate element 134 years later (in 1923) than zirconium (discovered in 1789).

References

External links

* [http://www.hull.ac.uk/php/chsajb/heavy_ho/lanthanides.html Technical reference page]
* [http://chemcases.com/cisplat/cisplat06.htm Reference Page, See Figure 2 for details]
* [http://www.chemicool.com/definition/lanthanide_contraction.html Simple Definition]