Lanthanides

The lanthanides are a series of fifteen metallic elements, from lanthanum (La, Z = 57) to lutetium (Lu, Z = 71). They are placed in a separate row beneath the main body of the periodic table to keep the table compact. Together with scandium and yttrium, they are often called the rare earth elements — although most are not actually rare in the Earth's crust.

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Position in the Periodic Table

The lanthanides belong to period 6 and fill the 4f electron subshell. In the full-length periodic table, they would be inserted between barium (Ba, Z = 56) and hafnium (Hf, Z = 72). However, this would make the table impractically wide, so they are conventionally displayed as a separate row below.

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The Lanthanide Elements

Element	Symbol	Atomic Number	Electron Configuration	Key Application
Lanthanum	La	57	[Xe] 5d¹ 6s²	Camera lenses, catalysts
Cerium	Ce	58	[Xe] 4f¹ 5d¹ 6s²	Catalytic converters, glass polishing
Praseodymium	Pr	59	[Xe] 4f³ 6s²	Aircraft engines, green glass
Neodymium	Nd	60	[Xe] 4f⁴ 6s²	Powerful permanent magnets
Promethium	Pm	61	[Xe] 4f⁵ 6s²	Nuclear batteries, luminous paint
Samarium	Sm	62	[Xe] 4f⁶ 6s²	Magnets, cancer treatment
Europium	Eu	63	[Xe] 4f⁷ 6s²	Red and blue phosphors
Gadolinium	Gd	64	[Xe] 4f⁷ 5d¹ 6s²	MRI contrast agents
Terbium	Tb	65	[Xe] 4f⁹ 6s²	Green phosphors, solid-state devices
Dysprosium	Dy	66	[Xe] 4f¹⁰ 6s²	High-power magnets, nuclear reactors
Holmium	Ho	67	[Xe] 4f¹¹ 6s²	Strongest known magnetic field generator
Erbium	Er	68	[Xe] 4f¹² 6s²	Fibre optic amplifiers
Thulium	Tm	69	[Xe] 4f¹³ 6s²	Portable X-ray devices
Ytterbium	Yb	70	[Xe] 4f¹⁴ 6s²	Atomic clocks, metallurgy
Lutetium	Lu	71	[Xe] 4f¹⁴ 5d¹ 6s²	PET scan detectors, catalysts

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Common Properties

Despite spanning fifteen elements, the lanthanides share remarkably similar properties:

• Silvery-white metals that tarnish when exposed to air
• Soft and malleable — can be cut with a knife
• Strong reducing agents — they readily lose electrons
• Most commonly adopt the +3 oxidation state (Ln³⁺)
• Some also adopt +2 (Eu²⁺, Sm²⁺, Yb²⁺) or +4 (Ce⁴⁺, Tb⁴⁺) oxidation states
• Their similar chemistry makes separation extremely difficult — historically requiring thousands of fractional crystallisations, now done using solvent extraction and ion exchange chromatography
• Many produce brightly coloured compounds and are used as phosphors

The Lanthanide Contraction

As atomic number increases across the lanthanide series, the atomic radius decreases slightly — this is the lanthanide contraction. It occurs because the 4f electrons poorly shield each other from the nuclear charge, so each additional proton pulls the electron cloud inward. This contraction has profound consequences:

• Hafnium (Z = 72) has almost the same atomic radius as zirconium (Z = 40), despite being a full period below
• It makes hafnium and zirconium very difficult to separate
• It affects the chemistry of all period-6 elements that follow the lanthanides

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Notable Lanthanides in Detail

Lanthanum (La, Z = 57)