What causes the lanthanide contraction in f-block elements?

The lanthanide contraction is caused by the poor shielding effect of the 4f electrons in the lanthanide series.

The lanthanide contraction refers to the phenomenon where the atomic radii of the elements in the lanthanide series (from lanthanum to lutetium) decrease steadily with increasing atomic number. This is a somewhat unexpected trend, as typically within a period of the periodic table, atomic radii increase with increasing atomic number. The cause of this contraction is rooted in the unique electron configuration of the lanthanide elements.

The lanthanides are part of the f-block of the periodic table, and their electron configuration involves the filling of the 4f subshell. The 4f electrons, due to their shape and distance from the nucleus, are not very effective at shielding the outer electrons from the nuclear charge. This poor shielding effect means that as protons are added to the nucleus (increasing the atomic number), the outer electrons are drawn closer to the nucleus, resulting in a smaller atomic radius.

Furthermore, the 4f electrons themselves are also drawn closer to the nucleus due to the increasing nuclear charge. This further contributes to the decrease in atomic radius, as the 4f subshell becomes more compact. This effect is cumulative, meaning that the atomic radius continues to decrease across the lanthanide series, despite the addition of electrons to the 4f subshell.

In addition to the poor shielding effect of the 4f electrons, there is also a degree of electron-electron repulsion within the 4f subshell. This repulsion can cause the electrons to be pushed closer to the nucleus, further decreasing the atomic radius.

Overall, the lanthanide contraction is a complex phenomenon that results from the interplay of electron shielding, nuclear charge, and electron-electron repulsion. It is a key concept in understanding the chemistry of the lanthanide elements and their unique properties.