Rare earth elements got their name because there was a time when it was a very difficult task to purify these metals from their oxides. However, since 1945, easier methods like ion-exchange, and solvent extraction processes are used to obtain these elements in a pure form at a low cost.
Most of these elements are present together in the same minerals in nature. The characteristics of these elements are so similar, that it becomes difficult to differentiate them from one another. There are slight variations in solubility, and complex formation which are used to identify them. In the periodic table, these elements belong to group 3 and the 6th and 7th periods. They are divided into two blocks. One is the lanthanide series, and the other is the actinide series. Properties of these elements are very similar to that of transition metals. Hence, they are often called special transition metals.
The position of lanthanides is the Block 4f of the periodic table. The first element of this series is Lanthanum. Metals in this series are usually soft, although metals with higher atomic numbers are comparatively harder. At room temperature, lanthanides burn in air, and burn vigorously at higher temperatures. In general, elements of this series adhere to water. When they react with water, Hydrogen gas is released. Also, they readily react with dilute acids at room temperature to give out Hydrogen. On reaction with Hydrogen, large amount of heat is released by these elements. They possess strong reducing properties. Most of the compounds formed by them are ionic. When heated with nonmetals, they form binaries.
Elements of the actinide series have individual distinctive structures. All the elements of this series are radioactive. A large number of allotropes of these metals are found. Electrons are present in the f-orbits in these elements. They are electropositive elements, and tend to donate the electrons present in their outermost orbits. Metals of this series have high density. They readily reacts with most non-metallic elements. On reaction with boiling water, they give out Hydrogen gas.
Some of the properties of rare earth elements are common in both the lanthanide as well as actinide series. In the pure form, they are highly lustrous substances, but when exposed to air they tend to get tarnished very easily. Colors of such elements vary from silver, silvery-white or gray. All of them are good conductors of electricity.
Elements of lanthanides have a number of scientific as well as industrial uses. Compounds of these elements act as catalysts during the production of petroleum, other synthetic products. Lasers, some types of lamps, magnets, motion picture projectors, X-ray screens, etc., use elements from this series. Elements of actinides are used in nuclear fission.
In the lanthanide series, those elements which are followed by the Lanthanum, and in the actinide series, elements that come after Actinium are rare earth elements. That is why chemists often argue that the names of these two elements should be removed from this group.