Lanthanides

Like many metals, the lanthanides have a bright silvery appearance. Five of the elements (La, Ce, Pr, Nd, Eu) are very reactive and when exposed to air react with oxygen to form an oxide coating that tarnishes the surface. For this reason these metals are stored under mineral oil. The remainder of the lanthanides are not as reactive, and some (Gd, Lu) retain their silvery metallic appearance for a long time. When contaminated with nonmetals, such as oxygen or nitrogen, the lanthanides become brittle. They will also corrode more easily if contaminated with other metals, such as calcium. Their melting points, which range from about 1,506.2°F (819°C) (Yb) to about 3,025.4°F (1,663°C) (Lu), are also sensitive to contamination. The lanthanides form alloys with many other metals, and these alloys exhibit a wide range of physical properties.

The lanthanides react slowly with cold water (more rapidly with hot water) to form hydrogen gas, and readily burn in air to form oxides. Oxides are substances in which a metal and oxygen have chemically combined to form a compound. For example, samarium and oxygen combine to form the compound samarium oxide. Yttrium has a natural protective oxide coating, making it much more resistant. The lanthanides form compounds with many nonmetals, such as hydrogen, fluorine, phosphorous, sulfur, and chlorine, and heating may be required to induce these reactions.

The arrangement of electrons in an atom (the electron configuration) influences the atom's reactivity with other substances. In particular, it is the outer or valence electrons-those furthest away from the center of the atom-that are most involved in reactions since these are exposed to the surrounding environment. All the lanthanides, from cerium to lutetium, have a similar arrangement of their outer electrons. This explains why they are all found in nature together and why they all react similarly. When they react with other elements to form compounds, most lanthanides lose three of their outer electrons to form tripositive ions. For most compounds of the lanthanides, this is the most stable ion. Some lanthanides form ions with a positive two or four charge, but these are usually not as stable. A comparison of the sizes of the lanthanide atoms, and their ions, reveals a progressive decrease in going from lanthanum to lutetium and is referred to as the lanthanide contraction. Compounds containing positive and negative ions are called ionic compounds. Most ionic lanthanide compounds are soluble in water. Compounds of lanthanides with the element fluorine (lanthanide fluorides), however, are insoluble. Adding fluoride ions to a solution of tripositive lanthanide ions can generally be used as a characteristic test for the presence of the lanthanides. Likewise, lanthanide oxalates (oxalate is the negative ion C_zO₄^-2) have low solubility.