Chemical Science Short Notes - General Trend of f-Block Elements!!! (Download PDF)

Study Notes on General Trend of f-Block Elements 

Lanthanoid Chemistry:

All lanthanoids are electropositive metals having remarkable chemical properties. Between two lanthanoids, there exists a significant difference only in their atomic or ionic radius. For example, the electronic and magnetic properties of the material generally depend on the exact separation of the atoms and the extent of overlap of various atomic orbitals.

General trends:

Generally, all elements from La to Lu are highly electropositive having standard potentials of the Ln3+/Ln couple like that of Mg²⁺/Mg. They can easily form an oxidation state Ln (III). Some properties change significantly. Like, the radii of Ln³⁺ ions contract steadily from 116 pm for La³⁺ to 98 pm for Lu³⁺. The decrease in atomic radius takes place due to an increase in effective atomic number, Zeff, as electrons are added to the poorly shielding 4f subshell.

The given below graph shows that the decrease in ionic radius corresponds to two gentle curves intersecting at Gd³⁺ (f⁷). The effect can be compared with d block and later can be traced to the splitting of the f-orbital energies by the crystal field generated by the ligand environment, but this splitting is less as compared to d block. On the left of the series, the lower energy f orbitals (which point away from the ligands) are occupied as the number of f electrons increases (f¹ to f*). For later configurations, such as f⁶ and f⁷, the higher energy f orbitals are occupied, and as these point towards the ligands, the increased electron-ligand repulsion results in a smaller than expected decrease in ionic radius.

Due to the 18 per cent decrease in ionic radius from La³⁺ to Lu³⁺, an increase in the hydration enthalpy across the series takes place. The reason behind the occurrence of the lanthanoids as Ln (III) is that once the s and d electrons are removed, the f electrons will be held tightly by the nucleus. A further consequence of the burying of the f electrons is that an Ln³⁺ ion has no frontier orbitals with directional preference, so ligand-field stabilization plays only a small part in the properties of lanthanoid complexes. For a few complexes weak stabilization energies (of a few kilojoules per mole) are observed for configurations such as f⁴, f¹⁰, and f¹¹ when compared with f⁰, f⁷, and f¹⁴.

Superimposed on the common occurrence of Ln³⁺, there are some atypical oxidation states that are most prevalent when the ion can attain the relatively more stable empty (f⁰), half-filled (f⁷), or filled (f¹⁴) subshell So, due to this, Ce³⁺ (f¹) can be oxidized to Ce⁴⁺ (f⁴⁺) and the latter is a strong and useful oxidizing agent. The next most common of the atypical oxidation states is Eu²⁺ (f⁷), and there are several stable Eu²⁺ compounds, including Eul₂, EuSO₄, and EuCO₃, and solutions of this ion are generally stable. The ions Sm²⁺ and Yb²⁺ also have extensive chemistry but get reduced from water to hydrogen in aqueous solutions. Recently, Dy (II), Nd (II), and Tm (II) have been produced in molecular complexes in solution; an example is NdI₂(THF)₅. Other reasonably stable oxidation states include Pr (IV) and Tb (IV), and the oxides of these elements formed in air, Pr6O11, and Tb4O7, contain mixtures of Ln (III) and Ln (IV). Under very strongly oxidizing conditions, Dy (IV) and Nd (IV) can be obtained. An Ln3+ ion is a hard Lewis acid as it binds with F⁻ and oxygen-containing ligands and its occurrence with PO_4^3- minerals. 

Element names, symbols, and selected properties of the lanthanoids