text: The analogous calculation for electron capture must take into account the binding energy of the electrons. This is because the atom will be left in an excited state after capturing the electron, and the binding energy of the captured innermost electron is significant. Using the generic equation for electron capture
  we have
  which simplifies to
  where  is the binding energy of the captured electron.
  Because the binding energy of the electron is much less than the mass of the electron, nuclei that can undergo β decay can always also undergo electron capture, but the reverse is not true.