The lanthanides are a sequence of elements that are gathered together in a single strip below the rest of the periodic table. They are also (together with scandium and yttrium) known as the ‘rare earths’ because these chemicals were all isolated as oxides (also known as earths) and they derive from rare minerals (although the elements themselves are mostly not that rare). They are grouped together in a single spot in the main table because of their similarity and also because of a quirk in the way their electrons are arranged: they all have the same number of electrons in their outer orbit, which is the orbit that reacts with other atoms, and thus defines the chemical properties of the element. Each separate lanthanide has a different number of electrons, but as the atomic number goes up, the extra electron is added to an inner orbit and the same set of three electrons remains in the outer shell, so they all have similar properties.
It is unwise to suggest to a chemist that any particular element is ‘boring’ – you’ll probably find they spent years working on a PhD thesis concerning some minor property of that exact element – but it would be somewhat repetitive to give every lanthanide equal treatment, so, instead, here is a table of their key information, with a summary of key points about each below.
The Lanthanides’ General Properties
Most of the lanthanides are silvery metals, soft enough to be cut with a knife. Lanthanum, cerium, praseodymium, neodymium and europium are all highly reactive, rapidly forming an oxide coating. The other lanthanides are all prone to corrosion if mixed with other metals, and to becoming brittle if they are contaminated by nitrogen or oxygen. They react more rapidly with hot water than cold and produce hydrogen in the reaction. They tend to burn fairly easily in air. The lanthanides are almost always found in the two minerals monazite and bastnäsite – they tend to be mixed together in fairly steady proportions in these (with about 25–38 per cent being lanthanum) and increasingly small amounts of the lanthanides with higher atomic numbers, which are heavier and so sank deeper into the Earth’s mantle in the volatile past.
Lanthanum
Lanthanum was discovered in 1839 by Swedish chemist Carl Gustav Mosander, and isolated in 1923. As an alloy it shares palladium’s ability to act as a ‘hydrogen sponge’, absorbing the gas at high density – although lanthanum is probably too heavy for this property to have any commercial value. As 25 per cent of the alloy ‘mischmetal’ (which also contains 50 per cent cerium, 18 per cent neodymium and assorted other lanthanides), it is used in flints in cigarette lighters. It can neutralize phosphorus, so is used in ponds to prevent the unwanted growth of algae.
Cerium
Cerium was discovered by Jöns Jacob Berzelius and his colleague Wilhelm Hisinger in 1803. Whereas most of the lanthanides were found together in monazite or bastnäsite, cerium was found separately in cerium silicate, a cerium salt. The most common lanthanide in the Earth’s crust, it has some nice environmentally friendly uses: it produces a red pigment, for example, that is much safer than those obtained from cadmium, mercury or lead in paints; and a small amount added to fuel can reduce the number of polluting particulates produced in exhaust. It is also used to coat the walls of self-cleaning ovens, converting cooking residues to an ashy substance that is relatively easily wiped away. If you file or scrape filings of cerium off a block, they spontaneously combust – a property known as ‘pyrophorism’.
Praseodymium
When Carl Gustav Mosander discovered lanthanum, there was a residue left behind that he suspected was another element, which he called didymium. In 1885, the Austrian chemist Carl Auer von Welsbach finally showed that this was a mixture of (mostly) two elements – praseodymium and neodymium. The main use of praseodymium is in aircraft engine parts as part of a high-strength alloy with magnesium. Like other lanthanides, it is used in carbon arc electrodes for studio lights. It can also be used to give glass and enamel a strong yellow colour, and to produce the glass in welders’ goggles, where it filters out yellow light and infrared radiation.
Neodymium
Neodymium – the other part of Mosander’s ‘didymium’ – was isolated in 1925. Its most important role is in extremely strong ‘NIB’ magnets (made from an alloy of neodymium, iron and boron). These are widely used in the magnets that power motors in electric cars. Neodymium is also used to make the glass in welder’s goggles, and in sunbeds, where it transmits tanning UV, but not infrared, rays.
Promethium
Most elements with an atomic number lower than that of bismuth (the chemical element of atomic number 83: a brittle, reddish-tinged grey metal) have a stable form. The two exceptions are technetium (see here) and promethium, whose isotopes have a half-life of eighteen years at most. As a result, promethium no longer occurs naturally on Earth (although large amounts are being manufactured by a star in the Andromeda system, for unknown reasons). Its first confirmed discovery was in the fission products of uranium fuel taken from a nuclear reactor in 1945. It can also be created by bombarding neodymium and praseodymium in a particle accelerator. Briefly used to replace radium in the luminous dials of watches, it is only really utilized today for research purposes. It affords, however, another example of the periodic table being used to find a ‘missing element’, as it was predicted by the Czech chemist John Bohuslav Branner in 1902, and also by Henry Moseley (see here) after he rearranged the periodic table in 1913, that the gap between neodymium and samarium would eventually be filled.
Samarium
Samarium was the first element named (indirectly) after a person. Colonel Samarsky, a Russian mine official, granted mineralogist Gustav Rose access to some samples, one of which turned out to be a new mineral, which, in gratitude to the colonel, Rose called samarskite. In 1879, Paul-Émile Lecoq de Boisbaudran (discoverer of gallium) extracted didymium from the mineral, but he also managed to extract a new element, which he called samarium. It has some specialized uses in lasers, glass production and lighting. When alloyed with cobalt it makes strong magnets, although these have been superseded by NIB versions.
Europium
Europium was isolated and named by French chemist Eugène-Anatole Demarçay in 1901, having been identified by several different scientists independently. Its most useful properties are connected to phosphorescence: phosphors are the substances used to produce a glow when stimulated by electrons in, for instance, traditional televisions. Red phosphors used to be the weakest – but when they are ‘doped’ with a small amount of europium, the output light is much stronger. Europium is also used (in a gas combination) in white fluorescent lightbulbs – and it is the key element involved in making a phosphorescent anti-forgery mark on euro banknotes.
Gadolinium
Like samarium, gadolinium was extracted from a sample of didymium by Paul-Émile Lecoq de Boisbaudran – this was in 1886, six years after an oxide of the element had been identified by Swiss chemist Jean Charles Galissard de Marignac. It is often used in alloys; for instance, to make iron and chromium easier to work with. It is the best-known neutron absorber among the elements, so is used in nuclear reactors. It is also used in MRI scanning, where an injected gadolinium compound can enhance the image produced.
Terbium
We’ve seen how yttrium came from a mine near the Swedish village of Ytterby. Three more elements would be discovered directly in the ores taken from the area and named after the village, in one of the most confusing and boring pieces of naming in the history of the periodic table: these were erbium (1842), terbium (1842), and ytterbium (1878). And to make things worse, three more elements owed their name indirectly to the village: holmium (1878) was named after the Swedish capital Stockholm, thulium (1879) after Thule, the mythical name for Scandinavia, and, just for a bit of variation, gadolinium was named after Johan Gadolin, who had identified the mineral from Ytterby containing yttrium in the first place.
Confused yet?
Anyhow, terbium is mainly used in compounds in solid-state devices, in low-energy lightbulbs, X-ray machines and lasers. Its most interesting use is in the alloy of terbium, dysprosium and iron, which flattens out in a magnetic field. This property can be used to create loudspeakers that you attach to a flat surface, such as a window pane, turning that flat surface into an amplifier of the sound.
Dysprosium
Dysprosium was discovered as what at first appeared to be an impurity in another lanthanide – in this case erbium. Over a period of years, it was shown that the impurity contained, as well as dysprosium, at least two more separate elements: holmium and thulium. It took some exceptionally patient and repetitive experiments by Paul-Émile Lecoq de Boisbaudran to isolate dysprosium, so he named it after the Greek word dysprositos, meaning ‘hard to get’. It can be used in nuclear reactor control rods, as it absorbs neutrons well. More importantly, it is added to alloys to make neodymium-based magnets designed to be used at high temperatures, at which it retains its magnetism well. Such magnets can be used in electric cars and wind turbines, both of which represent growing markets. Dysprosium, then, faces potential supply problems in the years ahead – it is the most expensive lanthanide and remains as hard to acquire today as it was when de Boisbaudran first named it.
Holmium
Holmium’s main practical role is in high-performance lasers used to vaporize certain types of tumours with minimal damage to the surrounding tissue – these need yttrium aluminium crystals with a trace of holmium added. It is also used in high-strength magnets. In 2009, French scientists spectacularly claimed to have found holmium titanate crystals that behaved like monopoles (hypothetical particles that have only one magnetic pole – they are of interest to science geeks because the Nobel Prize-winning physicist Paul Dirac suggested that they must exist for the grand unified theory of physics to hold). The claim was heavily disputed, as the two poles of the crystals were really just extremely close together and not identical. In 2017, IBM made the more credible yet still astonishing claim that they had created a technique whereby a bit of data could be stored on a single holmium atom.
Erbium
In certain forms, erbium has very particular optical fluorescent properties, which are used in lasers. When it is added to the glass in fibre-optic cables, it amplifies the broadband signal being carried. It can also be alloyed to metals such as vanadium and used in infrared-absorbing glass, like several other lanthanides.
The discovery of this lanthanide and that of holmium are both generally credited to Swedish scientist Per Teodor Cleve, although there were parallel investigations going on in several countries between 1878 and 1879. It is the second least common lanthanide (after promethium, which is self-destructing and only produced in nuclear reactions!). This means that, while it is not as rare as some other elements, it is expensive to produce, and as most of its properties are mirrored in cheaper lanthanides, it is not often used. However, one of its isotopes is employed in lightweight, portable X-ray machines, and it can also be used in surgical lasers.
Ytterbium
Sometimes described as the final element in the lanthanide sequence, ytterbium was identified in 1878 by Jean Charles Galissard de Marignac. It was discovered by heating erbium nitrate so that it decomposed into two oxides: erbium oxide and a white substance, mostly made up of a new element that he named ytterbium (although a pure sample was only created in 1953). It is mainly researched as a substitute or possible improvement on the uses of other lanthanides that it resembles. However, it might be used in the future to create even more accurate atomic clocks than those we already have – the isotope ytterbium-174 could in theory perform better than a caesium clock (which is already accurate to about a second every 100 million years!).
It eventually turned out that the sample of ytterbium produced by de Marignac was still not completely pure. The problem with the lanthanides is that their similarities make it extremely difficult to be sure you have isolated one. (The American chemist Theodore William Richards, in 1911, had to perform 15,000 successive recrystallizations of a sample of thulium bromate in order to isolate pure thulium with certainty.) In 1907, the French chemist Georges Urbain followed the same convoluted series of extractions that de Marignac had performed, and then showed that you could still extract a new element from the remaining sample of ytterbium: this was lutetium. Some chemists argue that the latter should be classed as a transition metal and included in the main body of the periodic table rather than as a lanthanide. The difficulty of extracting lutetium has meant that it is rarely used in isolation, although it has some commercial uses. For instance, it can be used in oil refineries as a catalyst for cracking hydrocarbons (in other words, breaking them down into simpler molecules).
Languid Centaurs
The lanthanides’ chemical symbols are La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Chemistry students struggling to remember this for an exam sometimes learn the following mnemonic: Languid Centaurs Praise Ned’s Promise of Small European Garden Tubs; Dinosaurs Hobble Erratically Thrumming Yellow Lutes.