Question

Describe, with appropriate equations, nuclear processes that lead to the formation of the noble gases He, Ne, Ar, Kr, Xe, and Rn. (Hint: Helium is formed from radioactive decay, neon is formed from the positron emission of \({ }^{22} \mathrm{Na}\), the formation of \(\mathrm{Ar}\), Xe, and \(\mathrm{Rn}\) are discussed in the chapter, and \(\mathrm{Kr}\) is produced from the fission of \(\left.{ }^{235} \mathrm{U} .\right)\)

Short answer

Helium is formed through alpha decay. Neon is formed from positron emission by \(^{22}Na\). Argon is formed through beta decay of Potassium-40. Krypton is formed from the fission of \(^{235}U\). Xenon is formed by neutron capture, followed by beta decay. Radon is formed by the natural radioactive decay of Uranium and Thorium.

Step-by-step solution

Formation of Helium

Helium is formed from the radioactive decay of heavier elements. The alpha decay of heavier radioactive elements such as Uranium and Thorium emits alpha particles. An alpha particle is identical to a helium-4 atom, hence leads to the formation of Helium.

Formation of Neon

Neon is formed from the positron emission of \(^{22}Na\). In this process, a proton in the sodium atom converts into a neutron, a positron, and a neutrino. The positron then quickly annihilates with an electron, and their combined energy results in two gamma photons. Then, the neutrinos are able to escape, leading to the transformation of Sodium into Neon.

Formation of Argon

Argon is primarily formed from the decay of Potassium-40, a process discussed in the chapter. The Potassium-40 undergoes beta decay and transforms into Argon-40.

Formation of Krypton

Krypton isotopes are produced from the fission of Uranium-235. When a neutron strikes the nucleus of Uranium-235 atom, it causes the nucleus to split into two smaller fragments along with the release of few neutrons. One of those fragments can be a Krypton isotope.

Formation of Xenon

Xenon, discussed in the chapter, is primarily formed through a process called neutron capture, followed by a beta decay in fission product precursors.

Formation of Radon

Radon is formed by the natural radioactive decay of Uranium and Thorium. Uranium and Thorium decay by alpha emission to form Radon.

Key concepts

Nuclear Decay

Nuclear decay is a fascinating natural process where unstable atomic nuclei lose energy by emitting radiation. This process leads to the transformation of elements into different elements or isotopes over time. Decay types, such as alpha and beta decay, are responsible for the formation of various noble gases. For instance, radioactive decay of heavier elements like Uranium and Thorium gradually forms Helium through alpha decay.
Each type of decay involves specific changes in the nucleus, releasing particles like alpha particles or electrons (beta particles), contributing to the stability of the nucleus.
Nuclear decay is not only crucial for forming elements but also plays a significant role in applications like dating ancient artifacts and understanding nuclear energy.

Alpha Decay

Alpha decay is a type of nuclear decay where an unstable nucleus emits an alpha particle. An alpha particle consists of two protons and two neutrons. Essentially, this particle is identical to a helium-4 nucleus.
During alpha decay, the parent atom loses these two protons and two neutrons, effectively reducing its atomic number by two and its mass number by four.
For instance, an element such as Uranium-238 decays via alpha emission to form Thorium-234:\[ {}^{238}_{92}U \rightarrow {}^{234}_{90}Th + {}^{4}_{2}He \]
This process is instrumental in the formation of Helium, as the emitted alpha particles are indeed helium nuclei that eventually capture electrons to form helium atoms. Alpha decay is a slow process, contributing to the slow but steady transformation of matter in nature.

Beta Decay

Beta decay is another form of nuclear decay, where an unstable nucleus converts a neutron into a proton (or vice versa). Alongside this transformation, the nucleus emits a beta particle, which is a high-speed electron or positron.
This decay increases or decreases the atomic number by one, without changing the mass number significantly.
An example from our exercise is the transformation of Potassium-40 into Argon-40 through beta decay:\[ {}^{40}_{19}K \rightarrow {}^{40}_{18}Ar + eta^+ + u \]
Here, the emission of a beta particle leads to the conversion of one element into another. Beta decay is essential for forming stable isotopes out of unstable ones and for contributing specific noble gases to the atmosphere through cosmic processes.

Neutron Capture

Neutron capture is a vital nuclear reaction in which a nucleus absorbs a free neutron. This process increases the mass number by one and sometimes leads to another type of decay, such as beta decay, if the resulting nucleus is unstable.
In the context of Xenon formation, neutron capture plays a significant role. Through this process followed by a beta decay, fission products can transform into Xenon.
Neutron capture is a critical process in stars, leading to the synthesis of heavier elements than iron. It's a key player in what we call the "s-process" (slow neutron capture process) in stellar environments. It demonstrates how our universe continuously forms complex elements through seemingly simple reactions.

Positron Emission

Positron emission is a rare but interesting form of beta decay. During this process, a proton in an unstable nucleus converts into a neutron.
This conversion results in the emission of a positron, a particle similar to an electron but with a positive charge.
An example noted in the exercise is the transformation of Sodium-22 into Neon-22:\[ {}^{22}_{11}Na \rightarrow {}^{22}_{10}Ne + eta^+ + u \]
It's noteworthy that the emitted positron quickly encounters an electron, leading to their annihilation and the production of gamma rays. This mechanism helps eliminate excess protons, balancing and stabilizing the nucleus. Positron emission plays a significant role not only in nuclear processes but also in medical imaging techniques such as PET scans.