Atomic nucleus
If a nucleus has too few or too many neutrons it may be unstable, and will decay after some period of time. For example, nitrogen-16 atoms (7 protons, 9 neutrons) beta decay to oxygen-16 atoms (8 protons, 8 neutrons) within a few seconds of being created.  In this decay a neutron in the nitrogen nucleus is turned into a proton and an electron by the weak nuclear force.  The element of the atom changes because while it previously had seven protons (which makes it nitrogen) it now has eight (which makes it oxygen).  Many elements have multiple isotopes which are stable for weeks, years, or even billions of years.