# Protactinium

properties
[ Rn ] 5 f 2 6 d 1 7 s 2
91 Pa
General
Name , symbol , atomic number Protactinium, Pa, 91
Element category Actinoids
Group , period , block Ac , 7 , f
Appearance bright, silvery, shiny metallic
CAS number 7440-13-3
EC number 616-087-9
ECHA InfoCard 100.122.906
Mass fraction of the earth's envelope 9 · 10 −8  ppm
Atomic
Atomic mass 231.03588 (2) et al
Electron configuration [ Rn ] 5 f 2 6 d 1 7 s 2
1. Ionization energy 5.89 (12) eV568 kJ / mol
2. Ionization energy 11.9 (4) eV1 150 kJ / mol
3. Ionization energy 18th.6 (4) eV1 790 kJ / mol
4. Ionization energy 30th.9 (4) eV2 980 kJ / mol
5. Ionization energy 44.3 (4) eV4 270 kJ / mol
Physically
Physical state firmly
Crystal structure tetragonal
density 15.37 g / cm 3
Melting point 1841 K (1568 ° C)
Molar volume 15.18 10 −6 m 3 mol −1
Heat of evaporation 470 kJ / mol
Heat of fusion 15 kJ mol −1
Electric conductivity 5.56 · 10 6 A · V −1 · m −1
Thermal conductivity 47 W m −1 K −1
Chemically
Oxidation states 5
Electronegativity 1.5 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
229 Pa {syn.} 1.50 d ε 0.316 229 Th
α 5,841 225 Ac
230 Pa {syn.} 17.4 d ε 1,310 230 th
β - 0.563 230 U
α 5.439 226 Ac
231 Pa 100% 32,760 a α 5.149 227 Ac
232 Pa {syn.} 1.31 d β - 1.337 232 U
ε 0.495 232 Th
233 Pa {syn.} 26,967 d β - 0.571 233 U
234 Pa in traces 6.70 h β - 2.197 234 U
234 m Pa in traces 1.17 min β - 2.271 234 U
IT 0.074 234 Pa
For other isotopes see list of isotopes
Hazard and safety information

GHS hazard labeling
no classification available
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Protactinium (originally shortened from Proto-actinium (Greek πρώτος (prõtos) = first and actinium); hyphenation Pro | tac | ti | ni | um or Pro | t | ac | ti | ni | um) is a chemical element with the element symbol Pa and the atomic number 91. In the periodic table it is in the group of actinides ( 7th period , f-block ). It is silvery metallic and becomes superconducting below 1.4 K. It is radioactive and occurs extremely rarely in nature. The greatest amount of protactinium is produced artificially.

## history

In 1871, Dmitri Mendeleev postulated the existence of an element between thorium and uranium . The series of actinide elements was still unknown at the time. Therefore, uranium was placed below the tungsten , and thorium below the eka - zirconium (the also undiscovered element hafnium at that time ), whereby the space below the tantalum remained free. The periodic table was represented in this form until the 1950s. For a long time chemists looked for eka-tantalum with similar chemical properties to tantalum.

Mendeleev's periodic table from 1871 with a gap for protactinium at the bottom, between thorium ( Th = 231 ) and uranium ( U = 240 )

In 1900, William Crookes isolated a highly radioactive material from uranium; however, he could not characterize it as a new chemical element and named it Uranium-X (UX). Crookes dissolved uranyl nitrate in ether , the remaining aqueous phase mostly contained the nuclides 234 Th and 234 Pa.

234 m Pa was discovered in 1913 by Kasimir Fajans and Oswald Helmuth Göhring , who gave it the name Brevium ( Latin brevis 'short') because of its short half-life (1.17 minutes ).

The long-lived 231 Pa (t ½ = 32,760 years) was found in 1917 by Otto Hahn and Lise Meitner (published in 1918), they called it Protoactinium (from Greek πρῶτος = protos : the first , the preceding , the chemical element that is in the decay series of uranium -235 before the actinium is). Independently, the long-lived isotope was discovered in England by Frederick Soddy and John Arnold Cranston , although the latter could not publish it because he was a soldier in the First World War in 1915.

In 1921 Otto Hahn made the further discovery that there is a second beta-emitting isotope with the same mass number 234 for the brevium 234 found by Fajans, which differs from the brevium only in its longer half-life of 6.7 hours; this is the rare case of nuclear isomerism .

Protactinium was first isolated in 1934 by Aristid von Grosse .

The official name for all three isotopes as well as all artificially produced isotopes with the atomic number 91 was determined by the IUPAC to Protactinium in 1949 , instead of the more difficult to pronounce Protoactinium by Hahn and Meitner.

## Occurrence

Protactinium is a radioactive decay product of uranium and is found in nature in the form of the two isotopes 231 Pa and 234 Pa, whereby the isotope 234 Pa can occur in two different energy states. Protactinium 231 Pa, an alpha emitter, is created when 235 U decays (see uranium actinium series ), the beta-emitting protactinium 234 Pa when uranium 238 U decays (see uranium radium series ).

## Extraction and presentation

In 1927, Aristid von Grosse isolated 2 milligrams of protactinium (V) oxide  (Pa 2 O 5 ) from waste from the production of radium . In 1934 he isolated elemental protactinium for the first time from 0.1 milligrams Pa 2 O 5 . To do this, he used two different methods: On the one hand, protactinium oxide was irradiated with 35 keV electrons in a vacuum. On the other hand, the oxide was converted to halides ( chloride , bromide or iodide ) and these were then reduced in a vacuum on an electrically heated wire.

Later he also made metallic protactinium from protactinium (V) iodide  (PaI 5 ).

${\ displaystyle {\ ce {2 PaI5 -> 2 Pa + 5 I2}}}$

In 1959 and 1961, the United Kingdom Atomic Energy Authority (UKAEA) extracted 125 g of protactinium with a purity of 99.9% from 60 t of spent nuclear fuel in a 12-stage process; the cost was about US \$ 500,000. For many years this was the only source of Protactinium available worldwide, from which various laboratories were supplied for scientific research.

## properties

In the periodic table , protactinium with atomic number 91 is in the series of actinides , its predecessor is thorium , the next element is uranium . Its analogue in the lanthanide series is praseodymium .

### Physical Properties

Protactinium is silvery metallic and becomes superconducting below 1.4 K.

### Chemical properties

Protactinium occurs mainly in two oxidation states , +4 and +5, both in solids and in solution.

## use

Because of its rarity, high level of radioactivity and toxicity, Protactinium has no practical application other than research.

In protactinium 231 Pa, which is formed when uranium 235 U decays and is also formed in nuclear reactors through the reaction 232 Th + n →  231 Th + 2n and subsequent beta decay , a nuclear chain reaction can possibly take place, which in principle also leads to the construction of nuclear weapons could be used. The critical mass is as specified by Walter Seifritz 750 ± 180 kg. Other authors come to the conclusion that a chain reaction is not possible even with any large mass in Protactinium 231 Pa.

Protactinium 233 Pa is an intermediate product in the breeding process from thorium 232 Th to uranium 233 U in thorium high-temperature reactors .

${\ displaystyle \ mathrm {^ {232} _ {\ 90} Th \ + \ _ {0} ^ {1} n \ \ longrightarrow \ _ {\ 90} ^ {233} Th \ {\ xrightarrow [{22, 3 \ min}] {\ beta ^ {-}}} \ _ {\ 91} ^ {233} Pa \ {\ xrightarrow [{26,967 \ d}] {\ beta ^ {-}}} \ _ {\ 92 } ^ {233} U}}$
The times given are half-lives .

Since the availability of modern, very sensitive mass spectrometers , it has become possible to use the 231 Pa as a tracer in paleoceanography, for example .

→ Category: Protactinium compound

Protactinium (IV) oxide (PaO 2 ) is a black, crystalline powder. Protactinium (V) oxide (Pa 2 O 5 ) is a white, crystalline powder. Both have a cubic crystal system.

Protactinium (V) chloride (PaCl 5 ) forms yellow monoclinic crystals and has a chain structure consisting of 7-fold coordinated pentagonal bipyramids.

## safety instructions

Classifications according to the GHS regulation are not available because they only include chemical hazards which play a completely subordinate role compared to the hazards based on radioactivity . The latter also only applies if the amount of substance involved is relevant.

## literature

• Harold W. Kirby: The Radiochemistry of Protactinium. National Academies, 1959 (PDF)
• Boris F. Myasoedov, Harold W. Kirby, Ivan G. Tananaev: Protactinium. In: Lester R. Morss, Norman M. Edelstein, Jean Fuger (Eds.): The Chemistry of the Actinide and Transactinide Elements. Springer, Dordrecht 2006, ISBN 1-4020-3555-1 , pp. 161-252 ( doi: 10.1007 / 1-4020-3598-5_4 ).
• Eric Scerri : A tale of seven elements , Oxford University Press, Oxford, 2013

Wiktionary: Protactinium  - explanations of meanings, word origins, synonyms, translations
Commons : Protactinium  - collection of images, videos and audio files

## Individual evidence

1. ^ Harry H. Binder: Lexicon of the chemical elements. S. Hirzel Verlag, Stuttgart 1999, ISBN 3-7776-0736-3 .
2. The values ​​for the properties (info box) are taken from www.webelements.com (Protactinium) , unless otherwise stated .
3. Entry on protactinium in Kramida, A., Ralchenko, Yu., Reader, J. and NIST ASD Team (2019): NIST Atomic Spectra Database (ver. 5.7.1) . Ed .: NIST , Gaithersburg, MD. doi : 10.18434 / T4W30F ( https://physics.nist.gov/asd ). Retrieved June 13, 2020.
4. entry on protactinium at WebElements, https://www.webelements.com , accessed on June 13, 2020.
5. The hazards emanating from radioactivity do not belong to the properties to be classified according to the GHS labeling. With regard to other hazards, this element has either not yet been classified or a reliable and citable source has not yet been found.
6. ^ Siegfried Niese : The discovery of the element 91 by Kasimir Fajans and Oswald Göhring in 1913 and the naming by Otto Hahn and Lise Meitner in 1918 (digitized version) .
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17. Lise Meitner, Otto Hahn: About the Protactinium and the question of the possibility of its production as a chemical element. In: The natural sciences . 1919, 7 (33), pp. 611-612 ( doi: 10.1007 / BF01498184 ).
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