Physical reactions change the states of materials: melting, crystallization, dissolving, sublimation etc.
Chemical reactions change the identities of substances: oxidation, neutralization, decomposition, combination, exchange, etc.
Nuclear reactions change the identities of nuclides: capture of sub-atomic particles or photons, release of subatomic particles or photons (radioactive decay), exchange subatomic particles or photons, nuclear fission, and nuclear fusion.
Subatomic particles: electrons, protons, neutrons, muons, mesons, hadrons, and (fragments of) nuclei.
A decay process is a nuclear reaction with no incidant particle, and it can be represented by
When the reactants contain more energy or mass than the products, the reaction releases energy and the reaction is exothermic. Considering only the particles, the reaction may require energy, and the reaction is endothermic.