Samarium – facts and properties

Position in the periodic table

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Occurrence and extraction

What the rare earths are is explained on the lanthanum page. Samarium is more common in the Earth's crust than tin, but it always occurs as a small share alongside other lanthanides, mainly in monazite and bastnaesite. There it makes up only a few per cent of the rare earths, far less than cerium, lanthanum or neodymium.

Making the metal takes advantage of the fact that samarium evaporates easily. The oxide is heated in a vacuum together with lanthanum or mischmetal. The lanthanum takes up the oxygen, and the samarium that is set free distils off and is collected as metal.

Paul-Émile Lecoq de Boisbaudran, a French chemist, discovered samarium in 1879. He examined didymium from the mineral samarskite and saw lines in its spectrum that fitted no known substance. What didymium turned out to be is told on the praseodymium page. The chemist Heinrich Rose had named samarskite in the middle of the nineteenth century after Vasili Samarsky-Bykhovets, a Russian mining colonel who had made samples from the Urals available to him and his brother Gustav. So samarium carries a person's name, but only by way of the mineral.

Properties and reactions

As the sixth lanthanide, samarium sits in period 6. Along the lanthanide row, promethium is on its left and europium on its right. The electron configuration reads [Xe] 4f⁶ 6s², and the electronegativity is 1.17. The metal has a density of 7.52 g/cm³, becomes liquid at 1074 °C and boils at 1794 °C, and its atoms have a radius of 181 pm.

In air samarium tarnishes only slowly, but turnings catch fire at around 150 degrees Celsius. With water it reacts sluggishly when cold and faster when warm, giving hydrogen and samarium hydroxide. It is kept under oil or protective gas. In compounds it is usually triply charged, and the salts are pale yellow. Unlike most of its neighbours, however, it also forms doubly charged ions. These give up an electron easily and are therefore strong reducing agents.

Natural samarium contains the isotope samarium-147, which is weakly radioactive. Its half-life is more than a hundred billion years, so its radiation plays no part in handling the metal. Geologists use its decay to neodymium-143 for samarium-neodymium dating, which gives the age of rocks, meteorites and lunar samples. Another isotope, samarium-149, forms as a fission product in nuclear reactors and captures neutrons very readily, so it counts among the reactor poisons.

Uses in everyday life and technology

Samarium-cobalt magnets were developed as early as the 1960s, before neodymium magnets. They are alloys such as SmCo₅ and Sm₂Co₁₇. Slightly weaker than neodymium magnets, they have two advantages: they keep their magnetisation up to several hundred degrees Celsius, and they hardly rust. That is why they are used where it gets hot or where a failure is costly, for example in servo motors, sensors, microwave tubes, aircraft and spacecraft.

In medicine, samarium-153 is used against the pain caused by bone metastases. The isotope is made in a reactor and attached to a carrier molecule that collects in bone where remodelling is most active, which is exactly where metastases are. Its beta radiation travels only a few millimetres in tissue, so it mainly hits those spots. The half-life is just under two days, so after about two weeks the activity has largely died away. The treatment belongs in specialised hospitals under radiation protection.

In organic chemistry, samarium(II) iodide, SmI₂, is a versatile reagent. Its deep blue solution in tetrahydrofuran hands over single electrons. With it, chemists can form carbon-carbon bonds under mild conditions and reduce halogen compounds or ketones. The French chemist Henri Kagan introduced the reagent in 1977.

Samarium oxide is also found in special glass that absorbs infrared radiation. Samarium and its compounds are considered to have low toxicity.

Quick question

What is the atomic number of samarium (Sm)?

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