Lanthanides

A century of separation

The story of the lanthanides starts in 1787 with a black rock from a quarry at Ytterby, near Stockholm. Johan Gadolin found a new earth in it in 1794, and over the next 120 years chemists discovered that this earth, and another called ceria, were mixtures of more than a dozen almost identical elements.

Separating them meant dissolving and recrystallising the same material hundreds or thousands of times, each step enriching one element very slightly. Many claimed discoveries proved to be mixtures. Only after Henry Moseley’s X-ray work in 1913 could chemists say exactly how many lanthanides there should be.

Why they are so alike

As you move from lanthanum to lutetium, each new electron goes into the 4f subshell, which is buried deep inside the atom beneath the outer electrons. The outer electrons, which control chemical behaviour, hardly change, so the lanthanides behave almost identically and nearly all form 3+ ions.

The 4f electrons are not entirely invisible, though. They give many lanthanide ions sharp colours and strong magnetism, and they pull the atoms slightly smaller across the series, an effect called the lanthanide contraction that also shapes the chemistry of the elements below them.

Rare earths in modern technology

Despite the name, most rare earths are not rare: cerium is about as common as copper. They are “rare” in the sense of being scattered and hard to concentrate and purify. Today they are vital to modern technology.

• Neodymium, with dysprosium: the strongest permanent magnets, in electric car motors, wind turbines and headphones
• Europium and terbium: red and green phosphors in screens and energy-saving lamps
• Erbium: amplifiers in the fibre-optic cables that carry the internet
• Cerium: polishing glass and catalytic converters
• Gadolinium: contrast agents for MRI scans

Mining and supply

Most of the world’s rare earths are mined and processed in China, which makes them a strategic concern for many countries. Mining them can leave radioactive thorium and uranium residues, and new sources and recycling methods are being developed to reduce both the environmental and the supply risks.