Problem 77 - Chemistry and Chemical Reactivity

Question

The last unknown element between bismuth and uranium was discovered by Lise Meitner \((1878-1968)\) and Otto Hahn \((1879-1968)\) in 1918. They obtained \(^{231}\) Pa by chemical extraction of pitchblende, in which its concentration is about 1 ppm (part per million). This isotope, an \(\alpha\) emitter, has a half-life of \(3.27 \times 10^{4}\) years. (a) Which radioactive decay series (the uranium-235, uranium-238, or thorium-232 series) contains \(^{231} \mathrm{Pa}\) as a member? (b) Suggest a possible sequence of nuclear reactions starting with the long- lived isotope that eventually forms this isotope. (c) What quantity of ore would be required to isolate \(1.0 \mathrm{g}\) of \(^{231} \mathrm{Pa},\) assuming \(100 \%\) yield? (d) Write an equation for the radioactive decay process for \(^{231} \mathrm{Pa}\)

Step-by-Step Solution

Answer

(a) Uranium-235 series; (b) \(^{235}\)U: \( \alpha \) decay \(\rightarrow \) \(^{231}\)Th: \( \beta^{-} \rightarrow \) \(^{231}\)Pa; (c) 1 ton of ore; (d) \(^{231}\)Pa \(\rightarrow \) \(^{227}\)Ac + \(\alpha\).

Step 1: Determine Radioactive Decay Series

To find out which decay series contains \(^{231}\)Pa, we look at the atomic number and mass number. \(^{231}\)Pa has an atomic number of 91 and is a member of the decay series starting from \(^{235}\)U (Uranium-235), which eventually decays to \(^{207}\)Pb (Lead-207). The series with odd mass numbers, such as 231, typically belong to the Uranium-235 series.

Step 2: Suggest Nuclear Reaction Sequence

To obtain \(^{231}\)Pa, one possible sequence is starting from \(^{235}\)U. \(^{235}\)U can decay by alpha decay to \(^{231}\)Th (Thorium-231), and \(^{231}\)Th further undergoes beta decay \(\beta^{-}\) to form \(^{231}\)Pa. This sequence can be represented as: 1. \(^{235}_{92}\text{U} \rightarrow \;^{231}_{90}\text{Th} + \alpha \text{(particle)}\)2. \(^{231}_{90}\text{Th} \;\rightarrow \;^{231}_{91}\text{Pa} + \beta^{-} + \bar{u}\), where \(\beta^{-}\) is a beta particle and \(\bar{u}\) is an anti-neutrino.

Step 3: Calculate Quantity of Ore

The concentration of \(^{231}\)Pa in pitchblende is about 1 ppm. To isolate 1.0 g of \(^{231}\)Pa, calculate the total mass of ore needed:Given: Concentration \( = 1 \text{ part per } 10^{6} \text{ parts of ore }\text{ (1 ppm) } \)Therefore, 1 g of \(^{231}\)Pa is contained in \(10^{6}\) g of ore. Hence, the quantity of ore required is:\(\boxed{10^{6}}\) g or 1 ton.

Step 4: Write Decay Equation for 231 Pa

The isotope \(^{231}\)Pa undergoes alpha decay. This process can be represented by the equation:\[^{231}_{91}\text{Pa} \rightarrow ^{227}_{89}\text{Ac} + \alpha\]This shows that when \(^{231}\)Pa decays, it emits an alpha particle \((\alpha, ^{4}_{2}\text{He})\) and transforms into \(^{227}\text{Ac} \text{ (Actinium-227)}\).

Key Concepts

Alpha Decay

Alpha decay is a type of radioactive decay where an unstable nucleus releases an alpha particle, which comprises two protons and two neutrons. This emission decreases the atomic number by 2 and the mass number by 4.
For example, in the decay of the isotope \(^{231}_{91}\text{Pa}\), the process involves releasing an alpha particle \((\alpha, ^{4}_{2}\text{He})\).
- This transforms \(^{231}_{91}\text{Pa}\) into \(^{227}_{89}\text{Ac}\) (Actinium-227).
- The equation for this transformation is: \[^{231}_{91}\text{Pa} \rightarrow ^{227}_{89}\text{Ac} + \alpha\]Alpha decay is commonly found in heavier elements such as uranium and thorium. It's a significant part of the natural radioactive decay processes that power the earth's heat engine and shape the composition of elements found in the earth's crust.

Beta Decay

Beta decay is another form of radioactive decay where a beta particle (electron or positron) and a neutrino/antineutrino is emitted from an atomic nucleus.
In this process, a neutron can turn into a proton (beta-minus decay), or a proton can be converted into a neutron (beta-plus decay):

• In beta-minus decay, a neutron is transformed into a proton, emitting an electron \((\beta^{-})\) and an antineutrino.
• For \(^{231}_{90}\text{Th}\), the decay equation can be: \[^{231}_{90}\text{Th} \rightarrow ^{231}_{91}\text{Pa} + \beta^{-} + \bar{u}\]

This change in the nuclear structure increases the atomic number by 1, while the mass number remains unchanged.
Beta decay is critical in balancing the neutron-to-proton ratio, especially in nuclei that are neutron-heavy or proton-heavy, thus consequently stabilizing the atom.

Pitchblende Extraction

Pitchblende, or uraninite, is a mineral rich in uranium and a primary source of this element. Its extraction involves several chemical processes to separate uranium from other elements.
- The process usually involves crushing the pitchblende ore and utilizing chemical reactions to dissolve uranium compounds.
- Uranium can then be precipitated out or extracted using ion-exchange techniques.
During these extraction processes, small amounts of isotopes like \(^{231}\text{Pa}\) may be isolated as byproducts.
Given that \(^{231}\text{Pa}\) is present in pitchblende at a concentration of about 1 part per million, significant amounts of ore are required to extract even tiny quantities of \(^{231}\text{Pa}\). For instance, to isolate 1 gram of \(^{231}\text{Pa}\), you would need to process approximately 1 million grams (1 ton) of pitchblende.

Uranium Series

The uranium series, also known as the uranium-radium series, is one of three naturally occurring radioactive decay chains. It begins with Uranium-238 (or Uranium-235 in its specific decay path) and ends with stable isotopes of lead.
In the context of the exercise, the uranium-235 series suits \(^{231}\text{Pa}\) as a member:

• Starting from \(^{235}\text{U}\), the series involves a sequence of alpha and beta decays.
• It includes isotopes such as \(^{231}\text{Th}\) and \(^{231}\text{Pa}\), before culminating in lead \(^{207}\text{Pb}\).

This series notably incorporates both alpha and beta decays, reflecting a range of changes which impacts both mass and atomic numbers at each step. Such sequences play important roles in nuclear chemistry, providing deep insights into natural radioactive processes and the transmutation of elements over geological timescales.