Final answer:
Heavier nuclei have a greater ratio of neutrons to protons to decrease electrostatic repulsion between protons and enhance nuclear stability through the short-ranged strong nuclear force.
Step-by-step explanation:
The reason why heavier nuclei have a greater ratio of neutrons to protons than lighter nuclei is rooted in the balance of forces within the nucleus. In atomic nuclei, there are two primary forces at work: the electrostatic force (Coulomb force), which causes like-charged protons to repel one another, and the strong nuclear force, which binds both protons and neutrons (nucleons) together. For heavy nuclei, having more neutrons than protons helps to mitigate the electrostatic repulsion between the many protons present because the strong nuclear force is short-ranged while the Coulomb force is long-ranged.
Nuclei with more neutrons can space protons further apart, reducing the electrostatic repulsion and contributing to nuclear stability. This is especially important in heavy elements, where electrostatic forces are very strong due to the larger number of protons. Moreover, nuclear stability is enhanced when neutrons and protons exist in pairs, and certain 'magic numbers' of both correspond to more stable, closed nuclear shells. The stability trend and the emphasis on excess neutrons become more pronounced as the mass number (A) increases, particularly for elements heavier than those with an atomic mass of around 40.