Lanthanide contraction

Lanthanide contraction is a term used in chemistry to describe the decrease in ionic radii of the elements in the lanthanide series from atomic number 58, Cerium to 71, Lutetium, which results in smaller than expected ionic radii for the subsequent elements starting with 72, Hafnium.^{[1] [2] [3]}

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. In particular, 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 the 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 the 4f subshell which is filled across these elements is not very effective at shielding the outer shell (n=5 and n=6) 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 for cerium(III) to 86.1 pm for lutetium(III).

About 10% of the lanthanide contraction has been attributed to relativistic effects.^[4]

Effects

The results of the increased attraction of the outer shell electrons across the lanthanide period may be divided into effects on the lanthanide series itself including the decrease in ionic radii, and influences on the following or post-lanthanide elements.

Properties of the lanthanides

The ionic radii of the lanthanides decrease from 103(La³⁺) to 86 pm (Lu³⁺) in the lanthanide series.

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.

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 densest lanthanide and has the highest melting point.

Influence on the post-lanthanides

The elements following the lanthanides in the periodic table are influenced by the lanthanide contraction. The radii of the period-6 transition metals are smaller than would be expected if there were no lanthanides, and are in fact very similar to the radii of the period-5 transition metals, since the effect of the additional electron shell is almost entirely offset by the lanthanide contraction.^[2]

For example, the atomic radii of the metal zirconium, Zr, (a period-5 transition element) is 159 pm and that of hafnium, Hf, (the corresponding period-6 element) is 156 pm. The ionic radius of Zr⁴⁺ is 79 pm and that of Hf⁴⁺ is 78 pm. 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. Radius-dependent properties such as lattice energies, solvation energies, and stability constants of complexes are also similar.^[1] 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

1. ^ ^{a b} Housecroft, C.E. and Sharpe, A.G. "Inorganic Chemistry" 2nd edition, Pearson 2005, pp. 536, 649, 743
2. ^ ^{a b} Cotton F.Albert and Wilkinson Geoffrey "Advanced Inorganic Chemistry" 5th edition, John Wiley 1988, pp. 776, 955.
3. ^ Jolly, William L. "Modern Inorganic Chemistry", McGraw-Hill 1984, p.22
4. ^ Pekka Pyykko (1988). "Relativistic effects in structural chemistry". Chem. Rev. 88: 563-594. doi:10.1021/cr00085a006. 

External links

• Reference Page, See Figure 2 for details
• Simple Definition