d-and f-Block Elements - The Lanthanoids

The Lanthanoids consist of 14 elements from Cerium ($Z=58$) to Lutetium ($Z=71$), following Lanthanum ($Z=57$).

General Electronic Configuration: The general configuration is represented as $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$.

Lanthanoid Contraction: The steady decrease in atomic and ionic radii ($Ln^{3+}$) with increasing atomic number. This is due to the imperfect shielding of one $4f$ electron by another, leading to an increase in effective nuclear charge.

Oxidation States: The most common and stable oxidation state is $+3$. Elements like Cerium ($Ce^{4+}$) and Europium ($Eu^{2+}$) show other states to achieve stable $f^0, f^7,$ or $f^{14}$ configurations.

Consequences of Lanthanoid Contraction: It results in the similarity of atomic radii between 4d and 5d transition series (e.g., $Zr$ and $Hf$), making their separation difficult.

Chemical Reactivity: Lanthanoids are highly electropositive. They react with acids to liberate $H_2$, with oxygen to form $Ln_2O_3$, and with water to form $Ln(OH)_3$.

Color and Magnetic Properties: Many $Ln^{3+}$ ions are colored due to $f-f$ transitions. They exhibit paramagnetism (except $La^{3+}$ and $Lu^{3+}$) because of the presence of unpaired electrons in $4f$ orbitals.