Lanthanum – facts and properties

Position in the periodic table

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Occurrence and extraction

Lanthanum is one of the rare earths: scandium, yttrium and the fifteen lanthanides from lanthanum to lutetium. The name is misleading, because these elements are not rare in the Earth's crust. Lanthanum is more common there than lead. What is rare are the deposits in which they become concentrated. Chemists once called hard-to-melt metal oxides "earths".

The main ores are monazite, a phosphate, and bastnaesite, a fluoride carbonate. Both contain the light lanthanides side by side, usually with lanthanum and cerium in the largest amounts. Monazite often contains thorium too and is therefore weakly radioactive. Large deposits lie in China, the USA and Australia.

Separating the lanthanides is laborious because their chemistry is almost identical. Nearly all form triply charged positive ions, and the size of those ions changes only minimally from one element to the next. Industry therefore uses processes that repeat in very many stages, above all solvent extraction, in which the metal ions each distribute themselves slightly differently between a water-based and an organic solution. The metal itself is then made by molten-salt electrolysis or by reducing a compound with calcium.

The element was discovered in 1839 by the Swedish chemist Carl Gustaf Mosander. He was studying cerium compounds from the mineral cerite and found another, until then hidden, oxide in them. Hence the name.

Properties and reactions

Lanthanum is in period 6 and opens the lanthanide series, which tables usually show as a separate row below the main body. To its right in that row comes cerium. Its electron configuration is [Xe] 5d¹ 6s² and its electronegativity is 1.1. The metal has a density of 6.15 g/cm³, melts at 918 °C and boils at 3464 °C. It is so soft that it can be cut with a knife.

In lanthanum the 4f subshell is still empty. From cerium onwards it fills element by element. These elements are called lanthanides, meaning lanthanum-like, because they behave chemically like lanthanum. Something unusual happens along the way: the extra 4f electrons shield the growing nuclear charge poorly, so the atoms do not get bigger along the row but smaller. The atomic radius falls, by and large steadily, from 195 pm at lanthanum to lutetium. This lanthanide contraction explains why the lanthanides resemble one another so closely. It is also the reason why hafnium, below zirconium, is almost exactly as large as zirconium.

Lanthanum is a base metal and reacts more vigorously than most lanthanides. It tarnishes quickly in air, forms hydrogen and lanthanum hydroxide with water, and is stored under oil or protective gas. In compounds it is a triply charged ion that is colourless, since there are no 4f electrons. Lanthanum oxide, La₂O₃, is a white solid.

Uses in everyday life and technology

In optics, lanthanum oxide makes glass strongly refractive without splitting light into colours much. With lanthanum glass, camera objectives, binoculars and spectacles need thinner and fewer lenses and show fewer colour fringes.

In hybrid cars lanthanum is found in nickel-metal-hydride batteries. The negative electrode is an alloy that soaks up hydrogen like a sponge and releases it again, and lanthanum is a main component of that alloy.

In refineries, catalytic cracking breaks heavy parts of crude oil into petrol and diesel. The catalysts are zeolites, porous minerals stabilised with lanthanum so that they withstand heat and steam better. This is one of the largest uses of the metal.

Other uses: lanthanum carbonate binds phosphate in the gut of dialysis patients. Hot cathodes of lanthanum hexaboride give electron microscopes a fine beam. Lanthanum salts show only low toxicity.

Quick question

What is the atomic number of lanthanum (La)?

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