**1. Rare earth element: inorganic chemistry**

The rare earth elements (REEs) are the 14 elements comprising the lanthanide series: cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) [1]. Lanthanum (La) is associated with the rare earth elements because of its trivalent chemical affinity and periodic table position. The lanthanide series are elements characterized as having one or more electrons in the 4f electronic orbitals for their ground state configuration. Promethium undergoes rapid radioactive decay and is absent in the environment.

The typically trivalent REE elements have considerable ionic bonding character [1]. The chemical attributes of the REEs are influenced by the regular decrease

in the ionic radii on progression from La to Lu, the so-called lanthanide contraction. The "lanthanide contraction" occurs because of the incomplete electric field shielding by the f orbitals and increases in atomic number, supporting greater chemical affinity for hydrolysis and chelate/complex stability on progression across the lanthanide series [1]. The LREE are the light rare earth elements, comprised of the elements La to Eu, and the HREE are the heavy rare earth elements, comprised of the elements Gd to Lu. In some cases, The REEs have been partitioned as (i) the light REE (LREE includes La, Ce, Pr), (ii) the middle REE (MREE includes Nd, Sm, Eu, and Gd), and (iii) the heavy REE (HREE includes Tb to Lu).
