Lanthanoids — Core Principles

Core Principles

Lanthanoids are a series of 14 f-block elements (Ce to Lu) characterized by the filling of the 4f subshell. Their general electronic configuration is $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$. The most stable and common oxidation state is +3, though some exhibit +2 or +4 states to achieve stable $4f^0$, $4f^7$, or $4f^{14}$ configurations.

A defining feature is 'lanthanoid contraction,' a gradual decrease in atomic and ionic radii across the series due to the poor shielding effect of 4f electrons. This contraction has significant consequences, such as making the 2nd and 3rd transition series elements of similar size and decreasing the basicity of lanthanoid hydroxides from La to Lu.

Most lanthanoid ions are paramagnetic due to unpaired 4f electrons and exhibit pale colours arising from f-f transitions. They are crucial in modern technology, forming alloys like Mischmetal (used in lighter flints) and serving as catalysts, phosphors, and components in powerful magnets.

Despite being called 'rare earths,' many are relatively abundant.

Important Differences

vs d-block Transition Elements

Aspect	This Topic	d-block Transition Elements
Electronic Configuration	Filling of 4f orbitals ($[Xe] 4f^{1-14} 5d^{0-1} 6s^2$)	Filling of (n-1)d orbitals (e.g., $[Ar] 3d^{1-10} 4s^{1-2}$)
Oxidation States	Primarily +3; some +2, +4 (less common, stabilized by $4f^0, 4f^7, 4f^{14}$)	Wide range of variable oxidation states (e.g., +2 to +7 for Mn)
Colour	Pale colours, sharp absorption bands (f-f transitions)	Intense colours, broad absorption bands (d-d transitions, charge transfer)
Magnetic Properties	Most are paramagnetic (orbital contribution significant due to shielded 4f)	Most are paramagnetic (spin-only contribution often dominant, orbital contribution quenched)
Complex Formation	Low tendency to form complexes due to large size and high charge, ionic bonding	High tendency to form complexes due to small size, high charge, and availability of d-orbitals for bonding
Lanthanoid Contraction	Exhibit significant lanthanoid contraction	Do not exhibit lanthanoid contraction; size trends are different
Basicity of Hydroxides	Basicity decreases from $La(OH)_3$ to $Lu(OH)_3$	Basicity generally decreases with increasing oxidation state and decreasing size

Lanthanoids are f-block elements with distinct properties compared to d-block transition elements. While both can exhibit variable oxidation states and paramagnetism, lanthanoids predominantly show a +3 state, whereas d-block elements display a much wider range. The origin of colour and magnetic properties also differs, with f-f transitions and significant orbital contribution in lanthanoids versus d-d transitions and often quenched orbital contribution in d-block elements. Lanthanoid contraction is a unique feature of the lanthanoids, profoundly impacting the sizes of subsequent elements in the periodic table. Their complex-forming ability is also generally lower than that of transition metals.