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Promethium (Pm): structure, properties, obtaining, uses

This element is found naturally in very low concentrations (about 4 x 10 -15 grams per Kg) in a type of uraninite, which is a uranium mineral, in which promethium is present due to the spontaneous fission of uranium. -238.

Symbol, atomic number and atomic mass of promethium. Source: me / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5). Source: Wikimedia Commons.

Despite its scarcity in the earth’s crust, it has been detected in the spectrum of the star HR465 of the constellation Andromeda, on whose surface it is estimated that it has been forming recently, since no isotope of Pm has a half-life longer than 17 ,7 years.

It was useful for a time on glow-in-the-dark objects, but due to its radioactivity it was discontinued on these. It is used to measure thickness of materials and in nuclear energy batteries.

Discovery

The existence of the promise was suspected in 1902 by the Czech chemist Bohuslav Brauner. Later, in 1914, the English physicist Henry Moseley found in his studies that an element was missing between neodymium and samarium.

Later, although several groups of researchers (Italians in 1924, Americans in 1926 and 1938) announced its discovery, the difficulty of separating it from other elements and reproducing the experiences meant that none could take the credit.

Jacob A. Marinsky. ORNL / Public domain. Source: Wikimedia Commons.

Finally, in 1944 researchers Marinsky JA, Glendenin LE, and Coryell CD tested its presence and isolated it from the by-products of uranium fission in the nuclear reactor at Clinton Laboratories in Oak Ridge in Tennessee, United States.

Larry E. Glendenin. ORNL / Public domain. Source: Wikimedia Commons.

However, they did not claim priority in discovery until 1946 as they were fully dedicated to secret investigations related to defense activities during World War II.

Assigning the name

Although the researchers had proposed calling it “clintonium” in honor of the laboratories where they worked, Coryell’s wife convinced them to call it “promise” as a souvenir of the Greek titan Prometheus or Prometheus.

Charles D. Coryell. Photo credit: Christian A. Coryell / GFDL (http://www.gnu.org/copyleft/fdl.html). Source: Wikimedia Commons.

According to some interpretations of Greek mythology, Prometheus stole fire from the gods to give it to humanity, for which he was cruelly punished for all eternity. Other legends say that he provided human beings with mathematics, science, agriculture and medicine, and even that he created man.

Representation of Prometheus by a German sculptor. © El Grafo / CC-BY-SA-4.0 / Public domain. Source: Wikimedia Commons.

These doubts about what Prometheus’s contribution to humanity was resembles the uncertainty about who discovered the element. On the other hand, according to Coryell’s wife, the name was justified as the researchers had “stolen the fire of the gods” when they discovered it during the secret atomic bomb program.

Electronic structure

The electron configuration of this element is:

2 ; 2 2 2 6 ; 3 2 3 6 3 10 ; 4 2 4 6 4 10 ; 5 2 5 6 ; 4 5 6 2 ;

which can also be expressed as:

[Xe] 4 5 6 2 .

Electronic structure of promethium according to Bohr’s improved atomic model. Ahazard.sciencewriter / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

Nomenclature

  • Promethium, symbol Pm.
  • Promethium-147, Pm-147 or 147 Pm: isotope of promethium with atomic mass 147.

Promethium properties

Physical state

Silver look metallic solid, hexagonal crystals.

Atomic mass

144.91

Melting point

1042 ºC

Boiling point

3000 ºC

Density

7.26 g / cm 3

Chemical properties

It belongs to the family of lanthanides, lantanoids or “rare earths”, so called because in the earth’s crust they are always found in the form of their oxides. They are part of period 6 of the periodic table of elements.

According to consulted sources, Pm, like the other lanthanides, reacts directly with water, slowly when cold, rapidly when heating, releasing hydrogen (H 2 ). It darkens quickly in air and burns easily producing its rust.

More than 30 promethium compounds have been prepared and most have some type of coloration. Its oxidation state in these compounds is +3. It does not form tetravalent cations.

Formation of some compounds

Dissolving it in hydrochloric acid (HCl), the promethium forms PmCl 3 , which is yellow and soluble in water. In nitric acid (HNO 3 ), Pm (NO 3 ) 3 is generated, which is pink in color and also soluble in water.

If a solution containing Pm 3+ ions is made alkaline with NH 3 , that is, the pH is increased, the light brown hydroxide (Pm (OH) 3 ) precipitates .

Sulfate Pm 2 (SO 4 ) 3 is slightly soluble in water. If the Promethium oxalate is heated, it turns into the white or lavender Pm 2 O 3 oxide .

Radioactive release

It emits beta radiation in the form of electrons that leave the nucleus of its atoms. When emitted, it becomes the element samarium (Sm). On the other hand, it can be converted to neodymium (Nd) by removing a positron from the nucleus and capturing an electron in it. Pm does not emit gamma rays.

Isotopes

So far 14 isotopes of promethium are known, with atomic masses ranging from 134 to 155. All are unstable (radioactive), which differentiates it from other lanthanides.

Promethium-147 is the most useful and has a half-life of 2.6 years.

Isotope with longer half-life

The isotope with the longest half-life is Promethium-145 with 17.7 years, which decays through electronic capture, which consists of an atomic electron combining with a proton and forming a neutron and a neutrino.

However, it is the only isotope of Pm that can also undergo alpha decay (emission of 2 neutrons). This is because it has 84 neutrons and the emission of 2 of them makes it promethium-141 with 82 neutrons, which is a stable configuration of the nucleus. But this is a type of decay that occurs very little.

Current obtaining

In 1963 about 10 grams of promethium was prepared using ion exchange chromatography methods on atomic reactor fuel waste. Today it is still recovered in this way as 147 Pm 3+ salts .

According to certain sources, it was also prepared in those years through the reduction of fluoride (PmF 3 ) with lithium vapor (Li). It is also currently prepared by the reduction of chloride (PmCl 3 ) with calcium (Ca) at high temperatures.

It can also be obtained from neodymium-146. This isotope is subjected to neutron bombardment so that each atom captures one neutron and becomes neodymium-147. The latter takes about 11 days to undergo beta decay, thus transforming into promethium-147.

Applications

Isotope 147 ( 147 Pm) is the one that has been used the most, being the only one that can be found commercially. It does not emit gamma rays and is supplied in the form of oxide or chloride. However, their transport and handling require radiation protection.

It had some uses that it no longer has today, as it is currently only used in special applications.

In batteries

Promethium-147 is used in long-life atomic batteries. In this case, small pieces of the Pm are placed in the matrix of a semiconductor to convert beta emissions into electricity.

These types of nuclear power batteries can provide energy for about 5 years and have been used in all kinds of devices, from missiles to pacemakers, which are devices placed on patients with heart problems.

Pacemakers can have long-life promethium-based batteries. Author: 2427999. Source: Pixabay.

In medical applications

According to certain sources, promethium oxide is used in medicine as a source of radiation.

In the industry

It has been used to measure the thickness of some materials, which is done by measuring the beta radiation that passes through the samples.

Potential applications

It could be used as a transportable X-ray source, but apparently this application has not yet been commercially developed.

X-ray image obtained in 1963 with a small device (approximately 10 cm x 5 cm x 1 cm) containing promethium-147. ENERGY.GOV / Public domain. Source: Wikimedia Commons.

It would also serve to provide power to space probes and satellites or to make devices that allow communication with submarines.

Discontinued use in light signals

Initially, Promethium-147 was used to replace the radio element in the luminous dials, as radio was known to be too dangerous. For this, the PmCl 3 was mixed with luminophores, which glow from greenish yellow to blue when the radiation of the promethium (beta particles) hits them.

A few dozen years ago the hands of wristwatches could be seen in the dark because of the glow caused by the radioactivity of the promethium. Author: MonacoCannes. Source: Pixabay.

This property was used in wristwatches or dials to glow in the dark. Also in the power button of some fluorescent lamps. In addition, it did not damage the luminophores as occurs with other sources of beta radiation, extending the life of the devices.

However, when it was discovered that Promethium is also dangerous, its use in the glow-in-the-dark market was stopped.

Risks

Pm is a radioactive element, so it must be handled with extreme caution and with due protection. In addition to the beta particles that it emits, these can impact elements with a higher atomic number and generate X-rays.

Promethium samples must be transported within a thick lead shield to avoid the damaging effects of the radiation they emit. ENERGY.GOV / Public domain. Source: Wikimedia Commons.

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