Oxidation States and Lanthanoid Contraction — Core Principles

Core Principles

Lanthanoids are 14 f-block elements (Ce to Lu) primarily characterized by a +3 oxidation state. This is due to the loss of two 6s electrons and one 5d or 4f electron. However, some lanthanoids exhibit +2 or +4 oxidation states to achieve stable f$^0$, f$^7$, or f$^{14}$ electronic configurations.

For instance, Ce$^{4+}$ (f$^0$) is a strong oxidizer, while Eu$^{2+}$ (f$^7$) and Yb$^{2+}$ (f$^{14}$) are strong reducers. A key phenomenon is 'lanthanoid contraction,' the gradual decrease in atomic and ionic radii across the series.

This contraction is caused by the poor shielding effect of the 4f electrons, which allows the increasing nuclear charge to pull the outer electrons more strongly. Consequences include the similar sizes and chemical properties of 4d and 5d transition elements (e.

g., Zr and Hf), increased densities of 5d elements, and a decrease in the basicity of lanthanoid hydroxides from La to Lu.

Important Differences

vs General Trend of Atomic Radii Decrease (across a period)

Aspect	This Topic	General Trend of Atomic Radii Decrease (across a period)
Cause	Lanthanoid Contraction: Poor shielding of 4f electrons leading to increased effective nuclear charge.	General Trend: Increasing effective nuclear charge due to addition of protons, while electrons are added to the same valence shell, leading to stronger nuclear pull.
Magnitude	Lanthanoid Contraction: Significant cumulative decrease over 14 elements (e.g., ~21 pm for M$^{3+}$ ions from La to Lu).	General Trend: Gradual and less pronounced decrease across a typical main group period (e.g., ~30 pm from Na to Cl).
Effect on Subsequent Elements	Lanthanoid Contraction: Causes 4d and 5d transition elements in the same group to have nearly identical sizes and chemical properties.	General Trend: Primarily affects the elements within that specific period; does not cause size similarities between different periods in the same group.
Electron Shell Involved	Lanthanoid Contraction: Involves the filling of inner 4f subshell, which poorly shields outer electrons.	General Trend: Involves the filling of the outermost valence shell.

While both lanthanoid contraction and the general trend of atomic radii decrease across a period are driven by an increasing effective nuclear charge, their underlying causes and consequences differ significantly. Lanthanoid contraction is specifically due to the exceptionally poor shielding by the deeply embedded 4f electrons, leading to a substantial cumulative reduction in size over 14 elements. This unique phenomenon has a profound impact on the elements that follow the lanthanoids, particularly the 5d transition metals, making them remarkably similar in size and properties to their 4d counterparts. The general periodic trend, in contrast, involves the filling of the valence shell and results in a more moderate size decrease within a given period, without such dramatic effects on subsequent groups.