The Bottom of the Periodic Table
When you look at the Periodic Table, there is a block of two rows of elements located below the main body of the chart. These elements, plus lanthanum (element 57) and actinium (element 89), are known collectively as the rare earth elements or rare earth metals. Actually, they aren't particularly rare, but prior to 1945, long and tedious processes were required to purify the metals from their oxides. Ion-exchange and solvent extraction processes are used today to quickly produce highly pure, low-cost rare earths, but the old name is still in use. The rare earth metals are found in group 3 of the periodic table, and the 6th (5d electronic configuration) and 7th (5f electronic configuration) periods. There are some arguments for starting the 3rd and 4th transition series with lutetium and lawrencium rather than lanthanum and actinium.
There are two blocks of rare earths, the lanthanide series and the actinide series. Lanthanum and actinium are both located in group IIIB of the table. When you look at the periodic table, notice that the atomic numbers make a jump from lanthanum (57) to hafnium (72) and from actinium (89) to rutherfordium (104). If you skip down to the bottom of the table, you can follow the atomic numbers from lanthanum to cerium and from actinium to thorium, and then back up to the main body of the table. Some chemists exclude lanthanum and actinium from the rare earths, considering the lanthanides to start following lanthanum and the actinides to start following actinium. In a way, the rare earths are special transition metals, possessing many of the properties of these elements.
Common Properties of the Rare Earths
These common properties apply to both the lanthanides and actinides.
- The rare earths are silver, silvery-white, or gray metals.
- The metals have a high luster, but tarnish readily in air.
- The metals have high electrical conductivity.
- The rare earths share many common properties. This makes them difficult to separate or even distinguish from each other.
- There are very small differences in solubility and complex formation between the rare earths.
- The rare earth metals naturally occur together in minerals (e.g., monazite is a mixed rare earth phosphate).
- Rare earths are found with non-metals, usually in the 3+ oxidation state. There is little tendency to vary the valence. (Europium also has a valence of 2+ and cerium also a valence of 4+.)
Metalloids or Semimetals