Rare Earth Elements

Rare earth elements (REEs) are a group of 17 chemically similar metallic elements consisting of the 15 lanthanides, plus scandium and yttrium. These elements are classified as inner transition metals because their distinguishing feature is the progressive filling of the 4f subshell. Despite their name, rare earth elements are relatively abundant in Earth’s crust. However, they rarely occur in concentrated, economically extractable deposits and are difficult to separate from one another due to their nearly identical chemical behavior. Rare earth elements are essential to modern technology, including permanent magnets, electric vehicles, wind turbines, smartphones, lasers, catalysts, and military systems.

Key Takeaways: Rare Earth Elements

• Rare earth elements include the 15 lanthanides plus scandium and yttrium.
• They are not rare in abundance, but rare in concentrated deposits.
• Most form stable +3 oxidation states.
• Their similar ionic radii make separation difficult.
• They are critical materials for electronics, renewable energy, and defense.
• Their chemistry is governed by the lanthanide contraction.

What Are Rare Earth Elements?

The term “rare earth” originated in the 18th and 19th centuries when early chemists isolated unusual oxide compounds, called “earths,” from uncommon minerals. As more of these elements were discovered, they were grouped together due to their chemical similarity.

The rare earth elements consist of:

• Lanthanides (atomic numbers 57–71)
• Scandium (21)
• Yttrium (39)

Scandium and yttrium are included because they share similar ionic radii and form stable +3 ions, and they frequently occur in the same mineral deposits as the lanthanides.

List of the Rare Earth Elements

Electron Configuration and the 4f Subshell

The defining feature of the lanthanides is the filling of the 4f orbitals.

General electron configuration:

[Xe]2f^1-145d^0-16s²

The 4f orbitals are poorly shielded from nuclear charge. As protons are added across the series, the effective nuclear charge increases, causing the atomic and ionic radii to decrease steadily.

This phenomenon is called the lanthanide contraction.

Lanthanide Contraction

Lanthanide contraction is the gradual decrease in atomic and ionic size from lanthanum to lutetium.

Consequences:

• Nearly identical chemical behavior
• Increasing density across the series
• Difficulty separating elements
• Influence on later transition metals in the periodic table

This contraction is one of the most important periodic trends in inorganic chemistry.

Oxidation States and Chemistry

The most common oxidation state of rare earth elements is +3.

Notable exceptions include:

• Cerium (+4)
• Europium (+2)
• Samarium (+2)
• Ytterbium (+2)

These alternative oxidation states influence catalytic activity and magnetic properties.

Physical Properties

General trends across the lanthanides:

• Silvery, reactive metals
• High melting points
• Good electrical conductivity
• Strong paramagnetism in many members
• Increasing density across the series

Magnetic and optical properties are especially important in modern technology.

Light Rare Earth Elements (LREE) and Heavy Rare Earth Elements (HREE)

The rare earth metals are grouped into the light rare earth elements (light-group rare earth elements or LREE) and the heavy rare earth elements (heavy-group rare earth elements or HREE). The LREE are lanthanum through gadolinium. The HREE are yttrium and then terbium through lutetium.

The classification depends on the electron configuration of the atoms of the element. The LREE do not have any paired electrons, while the HREE do have paired electrons. The element yttrium is an HREE because it has similar chemical properties to the other heavy rare earth metals and a comparable ionic radius. Scandium forms a trivalent cation, like the other rare earth elements, but it does not meet the electron configuration criterion for an LREE and does not share enough chemical properties with the HREE to be considered one of them.

Uses of the Rare Earth Elements

The rare earth metals are used in a variety of materials and products, including:

• superconductors
• extremely powerful magnets
• phosphors and pigments
• catalysts
• lasers and masers
• steel and other alloys

Abundance and Geology

Rare earth elements are more abundant in Earth’s crust than many industrial metals.

For example:
• Cerium is more abundant than copper
• Neodymium is more abundant than nickel

However, they rarely occur in concentrated ore bodies. They are typically found dispersed in minerals such as:

• Bastnäsite
• Monazite
• Xenotime
• Loparite

Major producing countries include China, the United States, and Australia.

Extraction and Separation

Rare earth elements occur together in minerals and require separation after mining.

Separation is difficult because:

• Nearly identical ionic radii
• Same +3 oxidation state
• Similar chemical reactivity

Industrial separation methods include:

• Solvent extraction
• Ion exchange
• Fractional crystallization

Modern processing plants sometimes use hundreds of extraction stages to isolate individual elements.

Environmental Issues

Rare earth mining and processing produces:

• Acidic waste streams
• Radioactive tailings (due to thorium and uranium impurities)
• Soil and water contamination

Environmental regulations strongly influence global production patterns.

Are Rare Earth Elements Radioactive?

Most rare earth elements are not radioactive. However, their ores often contain thorium and uranium, which create environmental and regulatory challenges during mining and processing.

Promethium is the only rare earth element with no stable isotopes.

FAQs

Why are rare earth elements called rare?
They are not rare in abundance, but rare in economically concentrated deposits.

Which rare earth element is most abundant?
Cerium is the most abundant rare earth element in Earth’s crust.

Why does China dominate rare earth production?
China developed large-scale mining and refining capacity early and invested heavily in separation technology.

References and Further Reading

• Brownlow, Arthur H (1996). Geochemistry. Upper Saddle River, N.J.: Prentice Hall. ISBN 978-0-13-398272-5.
• Jébrak, Michel; Marcoux, Eric; Laithier, Michelle; Skipwith, Patrick (2014). Geology of Mineral Resources (2nd ed.). St. John’s, NL: Geological Association of Canada. ISBN 978-1-897095-73-7.
• Liu, Shuang-Liang; Fan, Hong-Rui; et al. (2023). “Global rare earth elements projects: New developments and supply chains”. Ore Geology Reviews. 157: 105428. doi:10.1016/j.oregeorev.2023.105428
• Zhou, Baolu; Li, Zhongxue; Chen, Congcong (2017). “Global Potential of Rare Earth Resources and Rare Earth Demand from Clean Technologies”. Minerals. 7 (11): 203. doi:10.3390/min7110203

Photos of the Rare Earths