Promethium

neodymium ← promethium → samarium - ↑ Pm ↓ Np 61Pm Periodic table
Appearance
metallic
General properties
Name, symbol, number	promethium, Pm, 61
Pronunciation	/prɵˈmiːθiəm/ pro-MEE-thee-əm
Element category	lanthanide
Group, period, block	n/a, 6, f
Standard atomic weight	[145]g·mol−1
Electron configuration	[Xe] 4f5 6s2
Electrons per shell	2, 8, 18, 23, 8, 2 (Image)
Physical properties
Phase	solid
Density (near r.t.)	7.26 g·cm−3
Melting point	1315 K, 1042 °C, 1908 °F
Boiling point	3273 K, 3000 °C, 5432 °F
Heat of fusion	7.13 kJ·mol−1
Heat of vaporization	289 kJ·mol−1
Atomic properties
Oxidation states	3 (mildly basic oxide)
Electronegativity	? 1.13 (Pauling scale)
Ionization energies	1st: 540 kJ·mol−1
	2nd: 1050 kJ·mol−1
	3rd: 2150 kJ·mol−1
Atomic radius	183 pm
Covalent radius	199 pm
Miscellanea
Crystal structure	hexagonal
Magnetic ordering	paramagnetic[1]
Electrical resistivity	(r.t.) est. 0.75 µΩ·m
Thermal conductivity	(300 K) 17.9 W·m−1·K−1
Thermal expansion	(r.t.) (α, poly) est. 11 µm/(m·K)
Young's modulus	(α form) est. 46 GPa
Shear modulus	(α form) est. 18 GPa
Bulk modulus	(α form) est. 33 GPa
Poisson ratio	(α form) est. 0.28
CAS registry number	7440-12-2
Most stable isotopes
Main article: Isotopes of promethium
iso NA half-life DM DE (MeV) DP 145Pm syn 17.7 y ε 0.163 145Nd 146Pm syn 5.53 y ε 1.472 146Nd β− 1.542 146Sm 147Pm trace 2.6234 y β− 0.224 147Sm

Promethium (pronounced /prɵˈmiːθiəm/, pro-MEE-thee-əm) is a chemical element with the symbol Pm and atomic number 61. It is notable for being the only other exclusively radioactive element besides technetium which is followed by chemical elements that have stable isotopes.

Contents 1 History 2 Occurrence 3 Characteristics 3.1 Physical properties 3.2 Chemical properties 3.3 Isotopes 4 Applications 5 Precautions 6 Compounds 7 References 8 External links

History

The existence of promethium was first predicted by Bohuslav Brauner in 1902. During his research on the chemical properties of rare earth elements he found that the difference between neodymium and samarium is larger than between the other lanthanides. This prediction was supported in 1914 by Henry Moseley who, having discovered that atomic number was an experimentally measurable property of elements, found that no known element had atomic number 61. With the knowledge of a gap in the periodic table several groups started to search for the predicted element among other rare earths in natural environment.

The first claim of a discovery was published by Italian scientists Luigi Rolla and Lorenzo Fernandes from Florence. After separating a didymium nitrate concentrate from the Brazilian mineral monazite, which contained 70% dysprosium and neodymium with the other lanthanides making up the additional 30%, by fractionated crystallisation, they yielded a solution containing mostly samarium. This solution gave x-ray spectra attributed to samarium and element 61. In honor of their city they named element 61 Florentium. The results were published in 1926, but the scientists claimed that the experiments were done in 1924.^{[2][3][4][5][6][7]} In the same year 1926 a group of scientists University of Illinois at Urbana-Champaign Smith Hopkins and Len Yntema published the discovery of element 61. They named it after the university illinium.^[8][9][10]

These reported discoveries were extraneous: It is now known that there are no stable or long-lived isotopes of promethium, hence there was none to be found in these sources.

Promethium was first produced and characterized at Oak Ridge National Laboratory (ORNL) in 1945 by Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell by separation and analysis of the fission products of uranium fuel irradiated in the Graphite Reactor; however, being too busy with military-related research during World War II, they did not announce their discovery until 1947.^[11] The name promethium is derived from Prometheus, the Titan, in Greek mythology, who stole the fire from Mount Olympus and brought it down to mankind. The name was suggested by Grace Mary Coryell, Charles Coryell's wife, who felt that they were stealing fire from the gods.

In 1963, ion-exchange methods were used at ORNL to prepare about ten grams of promethium from nuclear reactor fuel processing wastes.^[12][13]

Today, promethium is still recovered from the byproducts of uranium fission; it can also be produced by bombarding ¹⁴⁶Nd with neutrons, turning it into ¹⁴⁷Nd which decays into ¹⁴⁷Pm through beta decay with a half-life of 11 days.

Occurrence

Promethium can be formed in nature as a product of spontaneous fission of uranium-238 and alpha decay of europium-151. Only trace amounts can be found in naturally occurring ores: a sample of pitchblende has been found to contain promethium at a concentration of four parts per quintillion (10¹⁸) by mass.^[14] It was calculated that the equilibrium mass of promethium in the earth's crust is about 560 g due to uranium fission and about 12 g due to the recently observed alpha decay of europium-151.^[15]

Promethium has also been identified in the spectrum of the star HR 465 in Andromeda, and possibly HD 101065 (Przybylski's star) and HD 965.^[16]

Characteristics

Physical properties

Promethium's longest lived isotope ¹⁴⁵Pm is a soft beta emitter with a half-life of 17.7 years. It does not emit gamma rays, but beta particles impinging on elements of high atomic numbers can generate X-rays, and a sample of ¹⁴⁵Pm does produce some such soft X-ray radiation in addition to beta particles.

Pure promethium exists in two allotropic forms, and its chemistry is similar to other lanthanides. Promethium salts luminesce in the dark with a pale blue or greenish glow, due to their high radioactivity.

Chemical properties

Promethium metal tarnishes slowly in air and burns readily at 150 °C to form promethium(III) oxide:

4 Pm + 3 O₂ → 2 Pm₂O₃

Promethium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form promethium hydroxide:

2 Pm (s) + 6 H₂O (l) → 2 Pm(OH)₃ (aq) + 3 H₂ (g)

Promethium metal reacts with all the halogens:

2 Pm (s) + 3 F₂ (g) → 2 PmF₃ (s)

2 Pm (s) + 3 Cl₂ (g) → 2 PmCl₃ (s)

2 Pm (s) + 3 Br₂ (g) → 2 PmBr₃ (s)

2 Pm (s) + 3 I₂ (g) → 2 PmI₃ (s)

Promethium dissolves readily in dilute sulfuric acid to form solutions containing the pink Pm(III) ions, which exist as a [Pm(OH₂)₉]³⁺ complexes:^[17]

2 Pm(s) + 3 H₂SO₄ (aq) → 2 Pm³⁺ (aq) + 3 (SO₄)^2- (aq) + 3 H₂ (g)

Isotopes

Thirty-six radioisotopes of promethium have been characterized, with the most stable being ¹⁴⁵Pm with a half-life of 17.7 years, ¹⁴⁶Pm with a half-life of 5.53 years, and ¹⁴⁷Pm with a half-life of 2.6234 years. All of the remaining radioactive isotopes have half-lives that are less than 364 days, and the majority of these have half lives that are less than 27 seconds. This element also has 11 meta states with the most stable being ¹⁴⁸Pm^m (T_½ 41.29 days), ¹⁵²Pm^m2 (T_½ 13.8 minutes) and ¹⁵²Pm^m (T_½ 7.52 minutes).

The isotopes of promethium range in atomic weight from 127.9482600 u (¹²⁸Pm) to 162.9535200 u (¹⁶³Pm). The primary decay mode before the longest-lived isotope, ¹⁴⁵Pm, is electron capture, and the primary mode after is beta minus decay. The primary decay products before ¹⁴⁵Pm are neodymium (Nd) isotopes and the primary products after are samarium (Sm) isotopes.

Along with technetium, promethium is one of only two elements with atomic number less than 83 that have only unstable isotopes, which is a rarely occurring effect of the liquid drop model and stabilities of neighbor element isotopes.

Applications

Uses for promethium include:

• As a beta radiation source for thickness gauges.
• As a light source for signals that require reliable, independent operation (using phosphor to absorb the beta radiation and produce light). In particular, Promethium(III) chloride (PmCl₃) mixed with zinc sulfide (ZnS) was used for a time as a major luminous paint for watches after radium was discontinued. This mixture is still occasionally used for some luminous paint applications (though most such uses requiring radioactive materials have switched to tritium for safety reasons).
• In a nuclear battery in which cells convert the beta emissions into electric current, yielding a useful life of about five years, using Pm-147.
• Promethium has possible future uses in portable X-ray sources, and as auxiliary heat or power sources for space probes and satellites (although the alpha emitter plutonium-238 has become standard for most space-exploration related uses – see Radioisotope thermoelectric generators).

Precautions

Promethium must be handled with great care because of its high radioactivity. In particular, promethium can emit X-rays during its beta decay. Its half-life is less than that of plutonium-239 by a factor of about 1350, and its biological toxicity is correspondingly higher. Promethium has no biological role.

Compounds

Promethium compounds include:

• Chlorides
  • PmCl₃
• Bromides
  • PmBr₃
• Oxides
  • Pm₂O₃

References

External links

• WebElements.com – Promethium
• It's Elemental – Promethium

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