Answer to Question #5227 Submitted to "Ask the Experts"

Alpha particles are made up of two protons and two neutrons emitted from the nucleus of an unstable atom. Atoms tend to be unstable if they have an excess number of neutrons, especially when compared to the number of protons in the nucleus. So why is an alpha particle made up of protons AND neutrons, when surely it would be more productive to reach stability by simply emitting neutrons?

Since I am unsure of what your background is I will try to keep this answer reasonably free of detailed mathematics and physics but attempt to provide an acceptable qualitative response. It is the case that given any known nuclear species it is possible to do calculations to evaluate whether it is energetically possible for specified decay events to take place. The calculation is done by determining the mass difference between the products following the decay event and the atom that initially underwent the decay process. If the products have less combined mass than the initial species, we say that the decay event has a positive Q-value and is at least energetically possible. If we use the usual symbol A (called the mass number) to represent the combined number of protons plus neutrons in a nucleus and carry out such calculations for neutron emission for any or all atoms we would find that, regardless of A-value, the calculated Q-value would be negative, thus implying that neutron emission is energetically impossible.

We should also observe that, among all known atomic species, the heaviest stable atom is bismuth, with an atomic number of 83 and an A-value of 209. All atoms beyond bismuth in the periodic table are radioactive. Additionally, any nucleus with a neutron number greater than 126 is unstable (radioactive), and any nucleus with a neutron to proton ratio greater than about 1.5 is also radioactive. It turns out that if you were to calculate the Q-values for alpha-particle emission for all nuclei with realistic neutron and proton numbers, you would find that the values are positive for A-values larger than about 150. This does not mean that a nucleus with a mass number as low as 150 will be able to decay by alpha emission because the Q-value must not only be positive but, realistically, must be high enough to allow the alpha particle to penetrate the nuclear Coulomb barrier, but it does mean that, for all practical purposes, alpha-particle emission is restricted to heavy nuclei. In a quite heavy atom, such as uranium—let us assume a natural uranium isotope with an A-value of 238—the nucleus will have to shed a significant amount of mass before it reaches a configuration that is stable. Emission of alpha particles is a reasonably effective manner to accomplish this. Emission of groups of nucleons heavier than alpha particles tends to be precluded by the large Coulomb barrier requirements that prevails. Commonly, heavy atoms and their progeny may undergo numerous alpha transitions, often accompanied by interspersed beta decay events, before reaching stability. The beta-minus emission serves to get rid of excess neutrons and increases the proton number, thus improving the neutron-to-proton ratio, which actually increases following alpha emission. The ²³⁸U decays ultimately to stable lead, ²⁰⁶Pb, along its major decay pathway, it and its progeny emitting some eight alpha particles and six beta-minus particles. Hope this is helpful.

George Chabot, PhD, CHP