• Tags: stability, N-Z-plot, valley
• Difficulty: Advanced

The N-Z plot (neutron number vs proton number) reveals stability patterns. For light stable nuclei (Z < 20), N approximately equals Z (the stability line follows N = Z). For heavier nuclei, N > Z is required for stability because additional neutrons provide nuclear force without adding Coulomb repulsion. The heaviest stable nucleus is Bi-209 (Z=83, N=126). The "valley of stability" is a narrow band on the N-Z plot. Nuclei above the valley (neutron-rich) undergo beta-minus decay (converting n to p). Nuclei below the valley (proton-rich) undergo beta-plus decay or electron capture (converting p to n). Very heavy nuclei (Z > 83) undergo alpha decay to reduce both Z and A. Magic numbers (2, 8, 20, 28, 50, 82, 126) represent closed nuclear shells — nuclei with magic N or Z (or both, "doubly magic") are exceptionally stable. Examples: He-4 (Z=2, N=2), O-16 (Z=8, N=8), Ca-40 (Z=20, N=20), Pb-208 (Z=82, N=126).