Lanthanide contraction

In the lanthanide is the phenomenon that the ionic radius in the lanthanides from lanthanum (57) to lutetium (71 ) out decreases. The term was first " Geochemical distribution laws of the elements " used by the geochemist Victor Moritz Goldschmidt in his famous series. The decrease in the ionic radius within the group of lanthanides distinguishes this from the other subgroup elements. There they observed only a slight decrease of the radii and towards the end of the period even an increase. A similar behavior is found for the lanthanide for the actinides, this process is called Actinoidenkontraktion.

Theoretical Considerations

The cause of lanthanide contraction lies in the properties of the 4f subshell founded: the f atomic orbitals are in shape to very large and diffuse, so that the electrons contained there are less localized than in the other s-, p- and d- orbitals. This means that the lanthanum to lutetium increasing the nuclear charge is poorly shielded - a stronger attraction of the 5s and 5p electrons is the result: the (triple positive ) ions shrink.

Relativistic effects

For heavy elements such as the lanthanides relativistic effects must be considered. So these, around 10 % contribute to the lanthanide contraction. The closer to the core located electrons as that of the 1s orbital have high speeds, leading to a mass increase of relativistic electrons and a contraction of the inner shells. The nuclear charge is screened and the 4f orbitals are destabilized relativistic. For this reason, the 4f electrons nuclear charge shield worse and a further contraction of the outer radii of 5s and 5p electron is the result.

Effects

The lanthanide contraction causes, some that subgroup elements have very similar radii within a group. So among themselves standing in the periodic table metals zirconium and hafnium are almost the same size and have a very similar chemical behavior. This is also the reason why hafnium could be detected by X-ray spectroscopy in 1923 in a zirconium ore.