why do very large atoms decay (break down) more often than smaller atoms?

Brief Explanations:

Large atoms have more protons and neutrons in their nucleus. The strong - nuclear force that holds the nucleus together has a limited range. As the nucleus gets larger, the electrostatic repulsion between protons overcomes the strong - nuclear force more easily, making the atom unstable and more likely to decay.

Answer:

Large atoms have more protons and neutrons. The electrostatic repulsion between protons overcomes the strong - nuclear force more easily in large atoms due to the limited range of the strong - nuclear force, making them more unstable and prone to decay.