Rare Earth Elements: What They Are and Why the World Needs Them

The motor in an electric car, the speaker in your earphones, the screen on your phone and the turbine in a wind farm all depend on a group of elements most people have never heard of. The rare earth elements are a set of 17 metals that are chemically very similar, surprisingly common, and extraordinarily difficult to separate.

Which elements are they?

The rare earths are:

• the 15 lanthanides — elements 57 to 71: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium;
• plus scandium and yttrium, which sit above them and occur in the same ores with similar chemistry.

On most periodic tables the lanthanides are shown as a separate row at the bottom. That’s purely to keep the table from becoming impractically wide. See the s, p, d and f blocks.

“Rare” but not rare

Every stable rare earth is more abundant in the Earth’s crust than silver or gold, and several — cerium, lanthanum, neodymium and yttrium — are more abundant than lead. Cerium is about as abundant as copper. The name is historical: in the late 18th and 19th centuries they were isolated from unusual minerals as oxides — “earths” in the old sense — that seemed uncommon.

Four elements (yttrium, terbium, erbium and ytterbium) are named after a single village: Ytterby in Sweden, where the mineral they came from was quarried. Gadolinium, scandium, holmium and thulium also have Scandinavian or chemist-related names.

What is rare is finding them concentrated in deposits that are worth mining. They’re usually spread thinly through rock.

Why they’re so hard to separate

The lanthanides are chemically almost identical. They nearly all form +3 ions, and those ions are very similar in size. The reason is in their electron configurations: moving across the series, electrons are added to the deep 4f orbitals, which sit inside the atom and barely affect how it bonds. The outer electrons — the ones that do the chemistry — stay much the same.

Separating them took more than a century of painstaking work. Early chemists used repeated fractional crystallization, sometimes thousands of cycles. Today industry uses solvent extraction, passing solutions through hundreds of mixer-settler stages, each exploiting tiny differences between neighbouring elements. It’s expensive and generates a lot of chemical waste, which is part of why production is concentrated in a few countries.

The lanthanide contraction

Across the lanthanide series, atoms and ions get steadily smaller, because 4f electrons shield the growing nuclear charge poorly. This lanthanide contraction has consequences beyond the rare earths: it makes the metals after them (hafnium, tantalum, tungsten…) almost the same size as the ones above them in period 5, and much denser.

What they’re used for

• Permanent magnets. Neodymium-iron-boron magnets are the strongest permanent magnets available, often with dysprosium or terbium added to keep them working at high temperatures. They’re in electric vehicle motors, wind turbine generators, hard drives, phones, headphones and speakers. Samarium-cobalt magnets are used where heat resistance matters.
• Lighting and displays. Europium gives red (and some blue) phosphors; terbium gives green. They’ve been used in screens and energy-efficient lighting.
• Catalysts. Cerium oxide in catalytic converters; lanthanum in the catalysts that crack crude oil.
• Glass and polishing. Cerium oxide is the standard compound for polishing glass and lenses; lanthanum improves camera and telescope lenses.
• Lasers and fibre optics. Erbium-doped fibre amplifiers boost the signals in undersea internet cables; neodymium and ytterbium are used in lasers.
• Medicine. Gadolinium compounds are MRI contrast agents.
• Batteries. Lanthanum was a key ingredient in nickel-metal hydride batteries in early hybrid cars.

Why they’re called “critical”

Because they’re essential to electric vehicles, renewable energy, electronics and defence, and because mining and processing are concentrated in a small number of countries, many governments list rare earths as critical raw materials. Recycling them from old magnets and electronics, and opening new processing plants, are active areas of research and investment.

Quick answers

How many rare earth elements are there? Seventeen: the 15 lanthanides plus scandium and yttrium.

Are rare earth elements radioactive? Only promethium, which has no stable isotopes. But rare earth ores often contain thorium and uranium, which is a challenge for waste handling.

What’s the most important rare earth? By value and strategic importance, neodymium (for magnets) is usually named first.