The subatomic particles that can change in a nuclear reaction are protons and neutrons. When a proton and a neutron are combined, they form a deuteron, which is the nucleus of a heavy hydrogen atom. When two deuterons are combined, they form a helium-4 nucleus, which is the nucleus of an alpha particle. When a neutron is added to a helium-4 nucleus, it forms a tritium nucleus, which is the nucleus of a radioactive isotope of hydrogen. When a proton is added to a tritium nucleus, it forms a helium-3 nucleus, which is the nucleus of a stable isotope of helium.