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One might wonder why the electrical repulsion of the protons is able to overcome the strong nuclear force, since the nuclear force is known to be about a million times stronger than the electrical force. The reason is that the electrical force is normally observed (for example, in ionization energies and in chemical bonds) ''at distances comparable to an atom's electron cloud''. The electrical force has an energy inversely proportional to distance, so that, when a charged particle is ''inside a nucleus'' its electrical energy is about 100,000 times greater, almost as strong as the nuclear force.

One might wonder why the electrical repulsion of the protons is able to overcome the strong nuclear force, since the nuclear force is known to be about a million times stronger than the electrical force. The reason is that the electrical force is normally observed (for example, in ionization energies and in chemical bonds) ''at distances comparable to an atom's electron cloud''. The electrical force has an energy inversely proportional to distance, so that, when a charged particle is ''inside a nucleus'' its electrical energy is about 100,000 times greater, almost as strong as the nuclear force.

One might also wonder why such nuclei fall apart instantly. It happens that the nucleus has to pass through a temporary state of higher energy, which it can't do in classical mechanics, for the same reason water doesn't leak out of a glass by moving up over the edge. But under the rules of [[quantum mechanics]], an extremely tiny (on the atomic level) barrier can sometimes be breached. This is called [[quantum tunneling]]. It is a probabilistic phenomenon governed by the [[Heisenberg Uncertainty Principle]], so an unstable nucleus has a certain probability of disintegrating per second. This leads to the observed exponential decay and measured [[half-life]] of radioactive nuclei. Larger nuclei have a stronger tendency to disintegrate, so they can tunnel through the barrier more easily.  This is why [[Uranium]] has a half-life of 4.5 billion years, whereas heavier artificial elements have half lives in milliseconds.

One might also wonder why such nuclei don't fall apart instantly. It happens that the nucleus has to pass through a temporary state of higher energy, which it can't do in classical mechanics, for the same reason water doesn't leak out of a glass by moving up over the edge. But under the rules of [[quantum mechanics]], an extremely tiny (on the atomic level) barrier can sometimes be breached. This is called [[quantum tunneling]]. It is a probabilistic phenomenon governed by the [[Heisenberg Uncertainty Principle]], so an unstable nucleus has a certain probability of disintegrating per second. This leads to the observed exponential decay and measured [[half-life]] of radioactive nuclei. Larger nuclei have a stronger tendency to disintegrate, so they can tunnel through the barrier more easily.  This is why [[Uranium]] has a half-life of 4.5 billion years, whereas heavier artificial elements have half lives in milliseconds.

Heavy nuclei can actually disintegrate in many ways. They are most likely to disintegrate in ways that produce results ("daughter nuclei") that have the lowest energy. Helium (2 protons and 2 neutrons) has an extraordinarily low relative energy for reasons related to particle spin, so disintegration into a helium nucleus, plus whatever is left over, is by far the commonest form of decay. The "alpha particle" is, of course, a Helium nucleus. It was named an alpha particle long before it was discovered that this was a Helium nucleus, and even longer before it was known why this happens.

Heavy nuclei can actually disintegrate in many ways. They are most likely to disintegrate in ways that produce results ("daughter nuclei") that have the lowest energy. Helium (2 protons and 2 neutrons) has an extraordinarily low relative energy for reasons related to particle spin, so disintegration into a helium nucleus, plus whatever is left over, is by far the commonest form of decay. The "alpha particle" is, of course, a Helium nucleus. It was named an alpha particle long before it was discovered that this was a Helium nucleus, and even longer before it was known why this happens.