O protactinium, symbol Pa, is element number 91 of the Periodic table. Rare and difficult to obtain, there are few applications of this element. It is known, however, that its most stable oxidation state is +5, with a chemical behavior that resembles tantalum and niobium. It is the first element in the actinide series to have electrons in the f sublevel.
This element has superconductivity at temperatures below 1.4 K, in addition to 29 known isotopes. Of these, only two are natural: the one with mass 231 and the one with mass 234. Most protactinium is obtained from uranium nuclear waste. Pa was discovered through work that took place in the 1910s.
Read too: Nobelium — another element belonging to the actinide group
Summary on protactinium
Protactinium is a metal belonging to block f of the Periodic Table.
In metallic form, it is ductile and malleable.
In solution, its main NOx is +5, like tantalum and niobium.
It has 29 known isotopes, only two of which are found in nature: mass 231 and 234.
It is difficult to obtain and extract. Its main natural source is uranium nuclear waste.
There are almost no known applications for protactinium, although it is known to be extremely dangerous.
Protactinium properties
Symbol: Shovel.
Atomic number: 91.
Atomic mass: 231.03588 c.u.s.
Electronegativity: 1,5.
Fusion point: 1572°C.
Boiling point: 4000°C.
Density: 15.37 g.cm-3 (calculated).
Electronic configuration: [Rn] 7s2 5f2 6d1.
Chemical series: actinides, block f, internal transition elements.
Characteristics of protactinium
the protactinium, atomic number 91 and symbol Pa, is one of the elements called actinides. although an element rare and difficult to obtain, it is known that Pa, in its metallic form, is ductile and malleable. It does not oxidize in contact with air at room temperature, which changes with increasing temperature.
Its main oxidation state is +5, which resembles the elements tantalum and niobium, in a way, with regard to the chemical behavior in aqueous solution. Protactinium is also the first in the actinide series to have a electron in the f sublevel (more specifically 5f), with intermediate properties between those of thorium It's from uranium.
is attacked by hydrochloric acid (8 mol. L-1), acid hydrofluoric acid (12 mol. L-1) and sulfuric acid (2.5 mol. L-1). Still on its reactional aspects, protactinium can react with O2, H2O or CO2 at a temperature between 300 and 500 °C, producing the oxide Pa2O5.
With ammonia (NH3), protactinium reacts to form PaN2, and with the gas hydrogen (H2), there is the formation of PaH3. Between the halogens, protactinium reacts with iodine (I2) at a temperature of about 400 °C to form PaI5.
the protactinium ifbecomes a superconductor at a temperature of 1.4 K. In addition, it was noticed that such properties were a consequence of the 5f sublevel in its structure, which made it evident that Pa would in fact be an actinide.
29 isotopes are known of protactinium, highlighting only the isotopes 231pa and 234Pa, which are natural, and the 233Pa, produced in nuclear reactors. Among these, the one with the longest half life and the 231Pa, with 3.28 x 104 years old.
Where can protactinium be found?
In geological terms, the half-life of protactinium (231Pa) is too small. Therefore, any and all protactinium found in nature comes from the radioactive decay of 235u.
The point is that, although uranium is well distributed throughout the Earth's crust (with an average content of 2.7 ppm), only 0.711% of this mass corresponds to the mass 235 isotope of uranium. In this way, it is estimated that the average protactinium content is 8.7 x 10-7 ppm.
Obtaining protactinium
Extraction of element 91 is one of the most difficult via natural sources.. Until then, protactinium has not been produced on a large scale, as there is no commercial interest. Measurable amounts of this element are generally obtained from uranium waste.
Furthermore, classic purification techniques, such as ion exchange resins, precipitation and crystallization, in addition to solvent extraction and chromatography, can be used to obtain a product richer in protactinium.
In 1959 and 1961, it was announced that the Atomic Energy Authority of Great Britain extracted, in a 12-year process, steps, 125 g of 99.9% pure protactinium from 60 tons of waste, at a cost of about US$ 500.000.
Know more: Antimony — element considered rare used since ancient times
Precautions with Protactinium
The protactinium is very dangerous and toxic. This makes it necessary to adopt plutonium-like handling precautions. It is estimated that protactinium dispersed in the air in the form of an aerosol can be up to 250 million times more toxic than hydrocyanic acid at the same concentrations.
Applications of protactinium
All the toxicity of protactinium, added to the fact that it is an element that is difficult to extract, limits its applications. Among the few known applications, protactinium has already been used in scintillators for x-ray detection. It has also been used for dating of old objects, through the relationship 231Shovel/235U.
history of protactinium
Mendeleev predicted element 91 in the vacant space between thorium and uranium of your Periodic Table. He called it "eka-tantalum", giving it an approximate atomic mass of 235 and predicting that its chemical properties would be close to those of niobium and tantalum.
However, It was not until 1913 that Kasimir Fajans and his student Oswald Göhring identified element 91, based on experiments and previous work by Ernest Rutherford and Frederick Soddy.
The new element, which was actually the 234mPa (a metastable isomer of protactinium-234), was given the name “brevius” (symbol Bv), because of its brief existence: only one minute of half-life.
At the same time, there was another problem at the time: the origin of the actinium (Ac), element 89. It was already known that Ac could not be the primary radioactive element, since its half-life was about 30 years, but it was not known which decay series produced it.
From there, Frederick Soddy suggested that the element that would give rise to actinium would be an alpha particle emitter, positioned in group 5 of the Periodic Table, after tantalum. The name “eka-tantalus” was then used to designate this element.
Until in March 1918, overcoming Soddy, Lise Meitner and Otto Hahn discovered the isotope 231Shovel, which received the codename “abracadabra” in his correspondence. In fact, this new element generated actinium by alpha particle emission and received the name protactinium from both, meaning “relative of actinium”. This nomenclature for element 91 ended up overlapping with Fajans and Göhring’s “brief”, since the half-life of 231Pa is about 32 thousand years old.
Solved exercises on protactinium
question 1
Although an actinide, protactinium, symbol Pa, has the same oxidation state as niobium and tantalum (+5). Perhaps that is why, at the time of its discovery, it was called “eka-tantalum”. In which of the following compounds does protactinium exhibit the aforementioned oxidation state?
A) PaBr2
B) PaH3
C) PaCl4
D) Pa2O5
And father
Resolution:
Alternative D
Halogens, in the absence of atom in oxygen in the formula, have a charge equal to -1. Hydrogen has a charge equal to +1. Oxygen has a charge of -2. So, the calculation of NOx of protactinium in each substance is given as follows:
paBr2: x + 2(–1) = 0 → x = +2; so wrong answer.
PaH3: x + 3(+1) = 0 → x + 3 = 0 → x = -3; so wrong answer.
PaCl4: x + 4(–1) = 0 → x – 4 = 0 → x = +4; so wrong answer.
Shovel2O5: 2x + 5(–2) = 0 → 2x – 10 = 0 → x = +5; so correct answer.
PaI: x + (–1) = 0 → x – 1 = 0 → x = +1; so wrong answer.
question 2
Initially, protactinium, element 91, was called “brevius”, symbol Bv, since its first isotope, 234, had about a minute of half-life. What is the resulting mass percentage of the “brevial” isotope after five minutes of its preparation?
A) 50%
B) 25%
C) 12.5%
D) 6.25%
E) 3.125%
Resolution:
Alternative E
The half-life is characterized by the time required for the mass of the radioactive sample to halve. If the half-life is one minute, it means that every minute the mass drops by half.
Thus, in five minutes, the mass dropped from 25, the same as 1/32 of the initial mass. Thus, the remaining mass is 3.125%.
By Stefano Araújo Novais
Chemistry teacher
Source: Brazil School - https://brasilescola.uol.com.br/quimica/protactinio-pa.htm
