Alpha Decay

Very heavy nuclei, like uranium or radium, carry too many protons packed together, and the electric repulsion between all that positive charge starts to overwhelm the strong force holding the nucleus together. Spitting out a single proton barely helps, but the alpha particle, two protons and two neutrons, is an unusually stable, tightly bound package. Ejecting it all at once removes charge and mass efficiently and costs the nucleus less energy than emitting the same four nucleons one at a time, so it's the favored escape route for the heaviest elements.

Every alpha decay follows the same bookkeeping rule: mass number drops by 4 and atomic number drops by 2, since the nucleus loses 2 protons and 2 neutrons together. On a chart of nuclides, with protons on one axis and neutrons on the other, this moves the daughter nucleus two steps down and two steps left of the parent. That pattern lets you predict exactly which element and isotope you end up with, without needing any other data.

Alpha decay sits next to beta and gamma decay as one of the three main ways unstable nuclei reach a lower-energy, more stable configuration, and like the others it releases energy from the strong nuclear force rather than from chemistry. That released energy becomes kinetic energy shared between the alpha particle and the recoiling daughter nucleus, and because momentum must balance with the parent starting at rest, the lighter alpha particle always carries away the larger share of the energy.