19.3
Stabilità nucleare
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Q1: What is the strong nuclear force and why is it necessary in the nucleus?
The strong nuclear force is the attractive force holding protons and neutrons together in the nucleus. Over distances less than 10−15 meters, this force is much stronger than electrostatic repulsions between protons. Without it, positively charged protons would repel each other and the nucleus would disintegrate immediately.
Q2: Why do heavier stable nuclei have more neutrons than protons?
Heavier nuclei have more proton-proton repulsions, requiring additional neutrons to provide compensating strong nuclear forces. Neutrons attract each other and protons through nuclear forces without experiencing repulsive interactions among themselves. This increased neutron count stabilizes larger nuclei against electrostatic repulsion and keeps them within the stability band.
Q3: What is the band of stability and where do stable nuclei fall on a neutron-proton plot?
The band of stability is a narrow region on a plot of neutrons versus protons where stable nuclei are located. Lighter stable nuclei have neutron-to-proton ratios near one, while heavier stable nuclei have increasingly higher ratios. Nuclei outside this band are unstable and undergo radioactive decay and decay series to reach stability.
Q4: How do radionuclides move toward the band of stability?
Radionuclides with higher neutron-to-proton ratios undergo beta-minus decay, converting neutrons into protons. Those with lower ratios emit positrons or undergo electron capture, converting protons into neutrons. Both processes shift the nucleus closer to the stability band until reaching a stable nuclide.
Q5: What are magic numbers and why do they affect nuclear stability?
Magic numbers are specific numbers of protons or neutrons (2, 8, 20, 28, 50, 82, 126) that form complete nuclear shells, making nuclei remarkably stable. Nuclei with magic numbers of both protons and neutrons, called doubly magic nuclei, are particularly stable. This concept parallels the stability of noble gases with complete electron shells.
Q6: How does nucleon pairing contribute to nuclear stability?
Proton-proton and neutron-neutron pairing occurs in nuclei, similar to electron pairing in orbitals. Nuclei with even numbers of both protons and neutrons are remarkably stable because all nucleons can pair. Only five nuclides with odd numbers of both protons and neutrons are stable, demonstrating pairing's importance.
Q7: Why are all nuclei with atomic numbers higher than 82 radioactive?
Nuclei with atomic numbers above 82 have excessive proton-proton repulsions that overwhelm the strong nuclear force, making them inherently unstable. Even bismuth-209, with atomic number 83, is radioactive despite its exceptionally long half-life. These unstable nuclei undergo decay series to reach stable configurations.