Radioactivity: Alpha, Beta and Gamma Rays and Their Properties

RADIOACTIVITY is the spontaneous emission of particles or high-energy radiation from the nucleus of an unstable atom, first discovered by Henri Becquerel in uranium salts and studied in detail by Marie and Pierre Curie. A radioactive nucleus is unstable because its particular combination of protons and neutrons does not sit at the lowest possible energy for that number of nucleons; by emitting radiation, it transforms itself into a new, more stable nuclide, in a process that occurs entirely on its own, with no external trigger needed and no way to predict exactly WHEN any one particular nucleus will decay. Experiments (originally, observing how the emitted radiation bends in a magnetic field) show that a radioactive nucleus can emit three distinct kinds of radiation, named after the first three letters of the Greek alphabet.

Alpha ($\alpha$) decay occurs when a heavy nucleus emits an alpha PARTICLE, which is nothing but the nucleus of a helium atom, $^{4}_{2}\text{He}$, made of two protons and two neutrons bound tightly together. Emitting an alpha particle reduces the parent's mass number by $4$ and its atomic number by $2$:

Beta ($\beta$) decay occurs when a nucleus emits a fast-moving electron (called a beta-minus particle, $\beta^-$) or, less commonly for the nuclei met at this level, a positron (a beta-plus particle, $\beta^+$). In $\beta^-$ decay, a neutron inside the nucleus transforms into a proton, emitting an electron and an (anti)neutrino in the process, so the mass number $A$ stays UNCHANGED while the atomic number $Z$ INCREASES by $1$:

$^{A}_{Z}X \;\longrightarrow\; {}^{A}_{Z+1}Y \;+\; {}^{0}_{-1}e \;+\; \bar\nu$ …