Gadolinium – facts and properties

Position in the periodic table

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Occurrence and extraction

Gadolinium is one of the rare earths covered on the lanthanum page. It occurs in monazite, bastnaesite and xenotime and is more common in the Earth's crust than arsenic. Separating it from its neighbours works by solvent extraction or ion exchange, and the metal is then made by reducing the fluoride with calcium.

The name leads to Ytterby near Stockholm. A black mineral was found there that the Finn Johan Gadolin examined in 1794. In it he discovered a new oxide, yttria, the first rare-earth oxide ever recognised as such. The mineral was later named gadolinite. The element gadolinium makes up only a small part of it.

The element itself was discovered by the Swiss chemist Jean Charles Galissard de Marignac in 1880. In material from samarskite he found spectral lines that belonged to an unknown substance. It was named after the mineral, and so after Gadolin.

Properties and reactions

Gadolinium is the eighth lanthanide. In period 6 it borders europium on the left and terbium on the right. The configuration [Xe] 4f⁷ 5d¹ 6s² shows a peculiarity: one electron sits in the 5d subshell, because the half-filled 4f subshell with seven electrons is especially stable. With an electronegativity of 1.2 the element is a typical metal. It melts at 1313 °C, boils at 3273 °C and has a density of 7.9 g/cm³.

Seven unpaired electrons in the 4f subshell make the Gd³⁺ ion strongly paramagnetic. The metal itself is ferromagnetic below about 20 degrees Celsius, like iron, cobalt and nickel, and no longer above that. This limit is called the Curie temperature. Gadolinium is the only lanthanide whose Curie temperature lies near room temperature.

Near the Curie temperature the magnetocaloric effect is especially strong. When a magnetic field is applied, the atomic magnets line up and the metal warms by a few degrees. When the field is removed, it cools again. If the heat is released in between, cooling without a compressor becomes possible, a principle being researched for magnetic refrigerators.

In dry air the metal is fairly stable, in moist air it tarnishes and forms a loose oxide layer. It reacts slowly with water, faster when warm, giving hydrogen. In compounds Gd³⁺ is colourless.

Uses in everyday life and technology

In MRI, the hydrogen nuclei of the body's water provide the signal. The Gd³⁺ ion, with its seven unpaired electrons, speeds up the return of these nuclei to their starting state, and nearby tissue looks brighter. The free ion is toxic, partly because it is about as large as Ca²⁺ and disturbs its jobs, so it is injected as a firmly bound complex with an organic molecule. The kidneys excrete the complexes within hours.

With normal kidney function the contrast agents are considered well tolerated. In people with severe kidney failure, older and less stable agents caused cases of nephrogenic systemic fibrosis, a serious disease of the connective tissue. Small amounts of gadolinium can remain in bone and, as shown in 2014, in the brain. Harm to health has not so far been shown in people with healthy kidneys. European authorities have restricted some of the less stable agents. Traces of the stable complexes now turn up in rivers and in the tap water of big cities, far below the amounts at which damage is expected.

Gadolinium is also a neutron absorber. Its isotopes gadolinium-155 and gadolinium-157 capture slow neutrons more strongly than any other stable isotope. In nuclear power stations, gadolinium oxide is mixed into part of the fuel so that fresh fuel assemblies do not react too fiercely.

Gadolinium silicate crystals also serve as scintillators in PET scanners.

Quick question

What is the atomic number of gadolinium (Gd)?

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