In heavy nuclei, protons and neutrons do not spontaneously change into each other under normal conditions. Protons and neutrons are distinct particles with different properties and behaviors. Protons carry a positive charge, while neutrons are neutral.
The stability of atomic nuclei depends on the balance between the forces that hold the nucleus together, namely the strong nuclear force, and the electrostatic repulsion between positively charged protons. The ratio of protons to neutrons in a nucleus determines its stability.
However, in certain nuclear reactions, such as beta decay, a neutron can be converted into a proton or vice versa. Beta decay involves the transformation of a neutron into a proton along with the emission of an electron (beta minus decay) or the transformation of a proton into a neutron with the emission of a positron (beta plus decay). These processes occur in unstable or radioactive isotopes as they strive to reach a more stable configuration.
It's important to note that these nuclear transformations occur at the level of individual particles and are not continuous or spontaneous processes within stable heavy nuclei. The majority of heavy nuclei remain relatively stable and do not undergo constant interconversion of protons and neutrons.