If the ratio of N/2 is not at an optimum value, then b decay will occur.  Within b decay a neutron changes to a proton, or vice versa, leaving the mass number A unchanged.

• A neutron-rich nucleus will change a neutron into a proton via b^- decay.
  
  
  
• A proton-rich nucleus will change a proton into a neutron via b⁺ decay.