Lanthanide contraction or lanthanoid contraction is the greater-than-expected decrease in the ionic radius of the lanthanide series elements (atomic number 57-71) and the subsequent elements (starting with atomic number 72, hafnium), such as mercury. Norwegian chemist Victor Goldschmidt coined the term “lanthanide contraction” in his 1925 publication on geochemical distribution laws of the elements.
Here is a look at what lanthanide contraction is, why it occurs, and whether a similar contraction occurs in other element series.
Lanthanide Contraction
Decreasing atomic and ionic radius size moving from left to right across an element period is one of the periodic table trends. The reason is that the number of protons increases moving across a period, while the number of electron shells remains constant. The greater effective nuclear charge draws the electrons in more tightly, shrinking the atoms. So, there is an expected decrease in ionic radius, but lanthanide contraction means the ionic radius is much smaller than you would expect, based solely on the number of protons in the atomic nucleus.
Reasons for Lanthanide Contraction
A couple of factors account for lanthanide contraction. First, the electron configuration of the elements has a filled 4f subshell. The geometry of the 4f shell poorly shields valence electrons from the positive nuclear charge. Essentially, the 6s electrons spend time closer to the atomic nucleus than the 4f electrons do. Relativistic effects account for about 10% of lanthanide contraction. The lanthanide atoms are so large that electrons move at relativistic speeds orbiting the nucleus. This makes them act as if they were much more massive, which also draws them closer to the nucleus.
| Element | Electron Configuration | Ln3+ Radius (pm) |
|---|---|---|
| La | [Xe]5d16s2 | 103 |
| Ce | [Xe]4f15d16s2 | 102 |
| Pr | [Xe]4f36s2 | 99 |
| Nd | [Xe]4f46s2 | 98.3 |
| Pm | [Xe]4f56s2 | 97 |
| Sm | [Xe]4f66s2 | 95.8 |
| Eu | [Xe]4f76s2 | 94.7 |
| Gd | [Xe]4f75d16s2 | 93.8 |
| Tb | [Xe]4f96s2 | 92.3 |
| Dy | [Xe]4f106s2 | 91.2 |
| Ho | [Xe]4f116s2 | 90.1 |
| Er | [Xe]4f126s2 | 89 |
| Tm | [Xe]4f136s2 | 88 |
| Yb | [Xe]4f146s2 | 86.8 |
| Lu | [Xe]4f145d16s2 | 86.1 |
Actinide Contraction
Similarly, the actinides experience actinide contraction. Actinide contraction is even greater than lanthanide contraction. The ionic radius of actinides decreases steadily from thorium to lawrencium because the 5f electrons very poorly shield the valence electrons and because of even more pronounced relativistic effects.
Contraction in Other Series of Elements
Although contraction is most apparent in the lanthanides and actinides, it also occurs in the transition metals. The effect is not as pronounced because the atomic nuclei are smaller, but they still experience relativistic effects.
Consequences of Lanthanide Contraction
For both the lanthanides and the actinides, the ion sizes of elements within each series are comparable in size. This means each of the lanthanides reacts chemical much like other lanthanides. Actinides similarly readily substitute in reactions for other actinides. This makes the lanthanides or rare earths difficult to isolate from one another.
However, lanthanide and actinide electronegativity and covalency increase moving from left to right across the period. For example, lanthanum compounds are less covalent than europium compounds. Californium compounds are more covalent than actinium compounds.
The effect of small ion size with increasing nuclear charge means the tendency to form coordinate complexes increases moving across the group. So, La3+ forms fewer coordination complexes than Lu3+.
As covalency increases, basicity decreases. For example, La(OH)3 is more basic than Eu(OH)3. Ac(OH)3 is more basic than Cf(OH)3.
All these factors affect the physical properties of the lanthanides. Density, melting point, Vickers hardness, and Brinell hardness increase from lanthanum to lutetium. So, lutetium is the densest lanthanide and has the highest melting point.
References
- Cotton, F. Albert; Wilkinson, Geoffrey (1988). Advanced Inorganic Chemistry (5th ed.). New York: Wiley-Interscience. ISBN 0-471-84997-9.
- Goldschmidt, Victor M. (1925). “Geochemische Verteilungsgesetze der Elemente”, Part V “Isomorphie und Polymorphie der Sesquioxyde. Die Lanthaniden-Kontraktion und ihre Konsequenzen”. Oslo.
- Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. ISBN 978-0-13-039913-7.
- Pekka Pyykko (1988). “Relativistic effects in structural chemistry”. Chem. Rev. 88 (3): 563–594. doi:10.1021/cr00085a006
- Tatewaki, H.; Yamamoto, S.; Hatano, Y. (2017). “Relativistic Effects in the Electronic Structure of Atoms.” ACS Omega 2(9): 6072-6080. doi:10.1021/acsomega.7b00802