To determine which type of nuclear decay makes Thorium-231 (Th-231) more stable, we first need to understand the two decay processes it can undergo: alpha decay and beta decay (both beta plus and beta minus).
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Alpha Decay: In alpha decay, the nucleus emits an alpha particle (which consists of 2 protons and 2 neutrons). This results in a new nucleus with a atomic number decreased by 2 and mass number decreased by 4. Alpha decay tends to occur in heavier nuclei, which are often unstable.
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Beta Plus Decay (Positron Emission): In beta plus decay, a proton is converted into a neutron, and a positron is emitted. This results in a decrease in the atomic number by 1 but does not change the mass number. Beta plus decay typically occurs in nuclei that are proton-rich and unstable.
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Beta Minus Decay: In beta minus decay, a neutron is transformed into a proton while emitting an electron and an antineutrino. This results in an increase in the atomic number by 1, while the mass number remains unchanged. Beta minus decay usually occurs in neutron-rich nuclei.
In general, the stability of a nucleus is enhanced when it moves toward a more balanced ratio of protons to neutrons.
Th-231 (with 90 protons and 141 neutrons) has more neutrons compared to protons. It can undergo both beta plus (to Th-230) and alpha decay (to Radium-227).
Between these options, alpha decay typically leads to greater stability for heavy nuclei like thorium since it reduces both mass and proton count more significantly than beta decay processes. Therefore, in the case of Thorium-231:
Alpha decay will make the nucleus MORE stable.