LANTHANUM AND THE LANTHANIDES

The lanthanides are 14 components following lanthanum in the periodic table, and associated with the filling of the seven orbitals of the 4f shell. The symbol Ln is usually used to denote these elements collectively. Atomic designs are complex with electrons in 4f, 5d and 6s orbitals outside the Xe core. The first three ionization energies are relatively less, leading to electropositive metals with chemistry dominated by the Ln3+ state in solution and in ionic solids. All Ln3 + ions have electron design (4f)n, but the 4f orbitals are highly contracted in size and do not overlap significantly with neighboring ions. Unlike the case with the d orbitals in the transition components, magnetism and spectra associated with 4f orbitals in Ln3+ compounds are same to those found in free gas-phase ions. Chemical bonding and ligand field effects associated with incomplete 4f orbitals are very small and hardly detectable in chemical trends.

The chemistry of all Ln3+ ions is therefore same and differentiated only by the gradual contraction in radius associated with increasing nuclear charge. The lanthanide contraction is also important for the transition components of the 5d series. The Ln3+ state is the most stable under normal conditions for all components in the series. Oxides Ln2O3and halides LnX3 are known for all components, as well as an extensive range of oxo salts having mixed and hydrated compounds such as Ln2(SO4)3.3Na2(SO4).12H2O. Ionic radii vary from 104 pm (La3+) to 86 pm (Lu3+) and this relatively bigger size for 3+ ions (cf. Al3+ 53 pm) is associated with correspondingly high coordination numbers in solid compounds. LnF3 elements for earlier elements have nine-coordination, Ln2O3 are seven-coordinate. For later Ln components the decrease in radius goes to changes in structure with reduction in coordination.