What is Nuclear Stability?
Nuclear stability refers to the tendency of an atomic nucleus to resist spontaneous decomposition or decay. A nucleus is considered stable when the attractive forces (strong nuclear force) within it are balanced by the repulsive forces (electromagnetic repulsion between protons), preventing it from undergoing radioactive decay. Only certain combinations of protons and neutrons result in stable nuclei, with all other combinations being unstable and radioactive.
Key Factors Influencing Nuclear Stability
The primary factors determining nuclear stability are the neutron-to-proton ratio and the total number of nucleons (protons + neutrons). For lighter elements, a neutron-to-proton ratio close to 1:1 is ideal for stability. As the atomic number increases, more neutrons are needed to overcome the increasing electrostatic repulsion between protons, so the stable ratio shifts to approximately 1.5:1 for heavier elements. Additionally, nuclei with 'magic numbers' of protons or neutrons (2, 8, 20, 28, 50, 82, 126) tend to be exceptionally stable.
The Band of Stability
When plotting the number of neutrons versus protons for all known isotopes, stable nuclei fall within a specific region known as the 'band of stability' or 'belt of stability.' Nuclei above this band have too many neutrons and typically undergo beta-minus decay to convert a neutron into a proton. Nuclei below the band have too few neutrons or too many protons and often undergo positron emission or electron capture. Very heavy nuclei, beyond atomic number 83 (bismuth), are all unstable and decay primarily via alpha emission.
Consequences of Nuclear Instability
Unstable nuclei, located outside the band of stability, undergo radioactive decay to transform into more stable configurations. This process involves emitting particles (like alpha or beta particles) or energy (gamma rays), changing the composition of the nucleus over time. This decay continues until a stable isotope is formed, which can sometimes be a different element entirely. The rate of decay is measured by an isotope's half-life.