Question

Alpha decay of \({ }^{238} \mathrm{U}\) forms \({ }^{234} \mathrm{Th}\). What kind of decay from \({ }^{234} \mathrm{Th}\) produces \({ }^{234} \mathrm{Ac}\) ? (Contributed by Prof. Mark Benvenuto, University of Detroit-Mercy.)

Short answer

Thorium-234 ({ }^{234} {Th}) undergoes beta-minus decay to produce Actinium-234 ({ }^{234} {Ac}).

Step-by-step solution

Understand Alpha Decay

Alpha decay involves the emission of an alpha particle from a nucleus. An alpha particle consists of two protons and two neutrons, which is the equivalent of a helium-4 nucleus. When a nucleus undergoes alpha decay, it loses two protons and two neutrons. The atomic number decreases by 2 and the mass number decreases by 4.

Identify Alpha Decay of Uranium-238

The alpha decay of Uranium-238 ({ }^{238} {U}) leads to the formation of Thorium-234 ({ }^{234} {Th}). Through this process, Uranium-238 loses two protons and two neutrons. This changes the atomic number from 92 (Uranium) to 90 (Thorium), and the mass number from 238 to 234.

Determine the Decay Needed from Thorium-234 to Actinium-234

To determine the type of decay that turns Thorium-234 into Actinium-234, we need to see the difference in their atomic numbers and mass numbers. Actinium-234 has an atomic number of 89, which is one less than that of Thorium-234 (90), and they have the same mass number (234). This indicates that Thorium-234 must undergo beta decay, specifically, beta-minus decay, which involves the conversion of a neutron into a proton with the emission of an electron (beta particle) and an electron antineutrino. This increases the atomic number by 1 but the mass number remains the same.

Key concepts

Beta Decay

Understanding beta decay is crucial when delving into the transformations that occur within an atomic nucleus. Beta decay is a type of radioactive decay where a beta particle, which can be an electron or a positron, is emitted. In the specific case of beta-minus decay, a neutron transforms into a proton, an electron (the beta particle), and an electron antineutrino. This transformation leads to an increase in the atomic number of the element by one, while the mass number remains unchanged, reflecting the nucleus's shift One section of the periodic table to the right.

For example, when Thorium-234 ({ }^{234} {Th}) undergoes beta-minus decay to become Actinium-234 ({ }^{234} {Ac}), the atomic number increases from 90 to 91, as one of its neutrons becomes a proton. The mass number stays at 234 because the total number of particles within the nucleus (protons and neutrons) remains constant. The emitted electron and antineutrino escape from the nucleus, carrying away the excess energy. This decay process is fundamental to the fields of nuclear physics and chemistry, impacting the stability and transformation of elements.

Radioactive Decay

Radioactive decay is a spontaneous process by which an unstable atomic nucleus loses energy by emitting radiation. This natural phenomenon can occur through various mechanisms, such as alpha decay, beta decay, gamma emission, or even spontaneous fission. Each type of decay helps transform an unstable nucleus into a more stable one, often resulting in the production of a different element or isotope.

During this process, the identity of the atom changes because the number of protons, which defines an element, may be altered. In the context of the exercise, we see that after Uranium-238 ({ }^{238} {U}) undergoes alpha decay, it becomes a different element, Thorium-234 ({ }^{234} {Th}). Then, Thorium-234 further decays via beta decay, causing a shift to Actinium-234 ({ }^{234} {Ac}). These transformations play an essential role in determining the isotopic composition of matter in the universe, the dating of archaeological finds via techniques like radiocarbon dating, and many applications in medical diagnostics and treatments.

Nuclear Chemistry

Nuclear chemistry is the branch of chemistry that focuses on the reactions and changes that occur in the nucleus of an atom. It encompasses a broad array of topics such as nuclear transmutations, fission, fusion, radioactive decay, and the production and use of radioactive isotopes.

Nuclear chemistry has practical applications in various fields. For example, it is vital in energy production, especially in nuclear power plants where nuclear fission is harnessed to generate electricity. In medicine, radioactive isotopes are useful in diagnosing and treating certain conditions. For instance, iodine-131 is widely used to treat thyroid disorders. When exploring alpha and beta decay processes, nuclear chemistry provides us with a deeper understanding of how these changes at the nuclear level can have significant effects on an element's properties and behavior, as well as on its place within the periodic table.