lanthanides

The lanthanides are a group of fifteen metallic elements with atomic numbers 57 through 71, spanning from lanthanum to lutetium. They are f-block elements, characterized by the progressive filling of the 4f subshell. Promethium (atomic number 61) is the only lanthanide not found in nature; the remaining members occur naturally in minerals such as monazite or bastnäsite, though often in tiny, dispersed quantities. The name comes from the Greek lanthanein, meaning to lie hidden, reflecting their chemical similarity and historical difficulty in distinguishing them.

Chemically, the lanthanides share many properties: most form triply charged cations (the +3 oxidation state is the most stable), and their chemistry is dominated by ionic bonding in many compounds. A characteristic feature is the lanthanide contraction—a gradual decrease in ionic radii and atomic sizes across the series due to imperfect shielding of the nuclear charge by 4f electrons. This affects bond lengths, crystal structures, and the separation of neighboring elements.

Occurrence and applications: They occur together in rare-earth minerals and are difficult to separate; isolation involves multiple separation steps. The elements have extensive applications: magnet materials such as NdFeB and SmCo magnets; phosphors used in display and lighting technologies (europium-doped compounds for red, terbium-doped for green); catalysts, especially cerium oxide used in oxidation reactions; and in some cases neutron absorbers in nuclear reactors (gadolinium, samarium).

Taxonomy and relation: The lanthanides are part of the larger group of rare earth elements. While historically called rare earths, many are relatively abundant in the earth's crust, but they occur dispersed and are difficult to mine and refine.