The D Block Elements
The lanthanides are located in block 5d of the periodic table. The first 5d transition element is either lanthanum or lutetium, depending on how you interpret the periodic trends of the elements. Sometimes only the lanthanides, and not the actinides, are classified as rare earths. The lanthanides are not as rare as was once thought; even the scarce rare earths (e.g., europium, lutetium) are more common than the platinum-group metals. Several of the lanthanides form during the fission of uranium and plutonium.
The lanthanides have many scientific and industrial uses. Their compounds are used as catalysts in the production of petroleum and synthetic products. Lanthanides are used in lamps, lasers, magnets, phosphors, motion picture projectors, and X-ray intensifying screens. A pyrophoric mixed rare-earth alloy called Mischmetall (50% Ce, 25% La, 25% other light lanthanides) or misch metal is combined with iron to make flints for cigarette lighters. The addition of <1% Mischmetall or lanthanide silicides improves the strength and workability of low alloy steels.
Common Properties of the Lanthanides
Lanthanides share the following common properties:
- Silvery-white metals that tarnish when exposed to air, forming their oxides.
- Relatively soft metals. Hardness increases somewhat with higher atomic number.
- Moving from left to right across the period (increasing atomic number), the radius of each lanthanide 3+ ion steadily decreases. This is referred to as 'lanthanide contraction'.
- High melting points and boiling points.
- Very reactive.
- React with water to liberate hydrogen (H2), slowly in cold/quickly upon heating. Lanthanides commonly bind to water.
- React with H+ (dilute acid) to release H2 (rapidly at room temperature).
- React in an exothermic reaction with H2.
- Burn easily in air.
- They are strong reducing agents.
- Their compounds are generally ionic.
- At elevated temperatures, many rare earths ignite and burn vigorously.
- Most rare earth compounds are strongly paramagnetic.
- Many rare earth compounds fluoresce strongly under ultraviolet light.
- Lanthanide ions tend to be pale colors, resulting from weak, narrow, forbidden f x f optical transitions.
- The magnetic moments of the lanthanide and iron ions oppose each other.
- The lanthanides react readily with most nonmetals and form binaries on heating with most nonmetals.
- The coordination numbers of lanthanides are high (greater than 6; usually 8 or 9 or as high as 12).