In which isotope is the strong nuclear force not strong enough to balance electrostatic forces between protons?

As nuclei get larger, the electrostatic repulsion among their many protons grows, while the strong nuclear force acts only over a very short range. Uranium-238 has a much larger nucleus and more protons than the other choices, so the strong force cannot fully overcome proton repulsion; it is radioactive. The correct choice is uranium-238.

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