The lanthanide or rare earth elements (atomic numbers 57 through 71) typically add electrons to the 4f orbitals as the atomic number increases, but lanthanum (4f 0 ) is usually considered a lanthanide. Scandium and yttrium are also chemically similar to lanthanides. Lanthanide chemistry is typically that of +3 cations, and as the atomic number increases, there is a decrease in radius for each lanthanide, known as the "lanthanide contraction." Because bonding within the lanthanide series is usually predominantly ionic, the lanthanide contraction often determines the differences in properties of lanthanide compounds and ions. Lanthanide compounds often have high coordination numbers between 6 and 12.
Herbert B. Silber
Bünzli, J.-C. G., and Choppin, G. R. (1989). Lanthanide Probes in Life, Chemical, and Earth Sciences. New York: Elsevier.
Cotton, Simon (1991). Lanthanides and Actinides. New York: Oxford University Press.