Which statement best describes stability for heavier elements (beyond the first 20)?

Which statement best describes stability for heavier elements (beyond the first 20)?

• A. Stability requires N > Z by a large margin
• B. Stability is achieved only when N = Z
• C. Stability often requires N > Z, i.e., more neutrons than protons
• D. Stability is independent of neutron count

Answer: (C)

As nuclei get heavier, the balance of forces changes: protons repel each other electromagnetically, and the strong force that holds the nucleus together acts over short ranges and benefits from having enough neutrons to contribute binding without adding charge. Adding neutrons increases the binding energy and helps stabilize the nucleus without increasing Coulomb repulsion. That’s why stable heavy nuclei typically have more neutrons than protons; the neutron-to-proton ratio must be greater than one to counteract the growing proton repulsion and to fill nuclear energy levels in a way that lowers the overall energy. For example, lead-208 (N = 126, Z = 82) has a notably high N/Z ratio, reflecting this trend. So stability for heavier elements often requires N > Z.

As nuclei get heavier, the balance of forces changes: protons repel each other electromagnetically, and the strong force that holds the nucleus together acts over short ranges and benefits from having enough neutrons to contribute binding without adding charge. Adding neutrons increases the binding energy and helps stabilize the nucleus without increasing Coulomb repulsion. That’s why stable heavy nuclei typically have more neutrons than protons; the neutron-to-proton ratio must be greater than one to counteract the growing proton repulsion and to fill nuclear energy levels in a way that lowers the overall energy. For example, lead-208 (N = 126, Z = 82) has a notably high N/Z ratio, reflecting this trend. So stability for heavier elements often requires N > Z.