Changes

Alpha decay can be thought of simply as the disintegration of an atomic nucleus because it is too big.  Large nuclei have a greater mutual electrical repulsion from the protons that they contain. This is offset by the nuclear [[strong force]] that makes protons and neutrons stick together. As nuclei get larger, the repulsion overtakes the attraction, so a disintegrated nucleus has lower energy than a complete one, and the nucleus moves toward a state of lower energy.

Alpha decay can be thought of simply as the disintegration of an atomic nucleus because it is too big.  Large nuclei have a greater mutual electrical repulsion from the protons that they contain. This is offset by the nuclear [[strong force]] that makes protons and neutrons stick together. As nuclei get larger, the repulsion overtakes the attraction, so a disintegrated nucleus has lower energy than a complete one, and the nucleus moves toward a state of lower energy.

Why don't 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.

Why don't 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.

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.)