Promethium – facts and properties

Position in the periodic table

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Occurrence and extraction

Promethium is practically absent on Earth. All of its isotopes are radioactive. The longest-lived, promethium-145, has a half-life of about eighteen years. Measured against the age of the Earth that is tiny, and any stock from the planet's formation decayed long ago. Only traces form continually, when uranium in ores undergoes spontaneous fission. According to estimates, the whole Earth's crust holds only a few hundred grams.

Promethium and technetium are the only two elements among the first eighty-two with no stable isotope. For promethium this is because, for every possible nuclear mass, one of its neighbours neodymium or samarium has a more stable nucleus. Every promethium nucleus therefore sooner or later changes into one of those two elements by beta decay or electron capture.

For a long time the missing element between neodymium and samarium was sought in minerals without success. Several claimed finds could not be confirmed. Proof came only in 1945 at Oak Ridge in the USA, where Jacob Marinsky, Lawrence Glendenin and Charles Coryell separated the element by ion exchange from the fission products of uranium. The name Prometheus alludes to the fact that nuclear fission opened up a new source of energy, much as the myth describes fire.

Properties and reactions

Promethium, the fifth lanthanide, is in period 6. Neodymium lies to its left and samarium to its right. Its electron configuration is [Xe] 4f⁵ 6s². The metal has a density of 7.26 g/cm³, melts at 1042 °C and boils at 3000 °C. Because only tiny amounts could be studied, some of these values are less well established than for the other lanthanides. The atomic mass is given as 145 u. That is the mass number of the longest-lived isotope, since there is no natural mixture with a fixed atomic mass.

Chemically promethium behaves like its neighbours. It is a base metal and occurs almost only as a triply charged ion. Its salts are pink. Because of their own radiation, promethium compounds can glow faintly blue or greenish in the dark.

The most important isotope, promethium-147, is a beta emitter with a half-life of about two and a half years. Its beta radiation is low in energy and is stopped by thin layers of glass, metal or plastic, and it emits hardly any gamma radiation. It becomes dangerous mainly when the substance enters the body, for example by inhalation or swallowing, and settles mainly in the liver and bones. Handling promethium requires a licence and belongs in radiation-protection areas.

Research and synthesis

Promethium is made in nuclear reactors. It is separated from spent fuel, where it forms as a fission product, or produced deliberately by irradiating neodymium-146 with neutrons. Both happen in specialised facilities and in small amounts.

In the past the isotope promethium-147 had several uses. As a luminous paint it made a zinc sulfide phosphor glow. It thereby replaced the more hazardous radium on dials and instruments and needed no outside light source. A beta emitter can also be used to measure thickness, because the radiation weakens as it passes through foil or paper. Small nuclear batteries turned the radiation directly into electric current, for example in heart pacemakers in the 1970s.

Today promethium has largely been displaced by other substances and is used mainly in research, for example to study the chemistry of the lanthanides without a gap. Because of the radiation and the short half-life there is no everyday use.

Quick question

What is the atomic number of promethium (Pm)?

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