You gotta watch this, be careful. the binding energy does not always decrease whan a neutron is added.
When you would plot the binding energy per nucleus as a function of the mass number you will find that the most stable nuclei occur at the top of the curve because they require the largest energy to decompose the nucleus (this is the binding energy ofcourse). You can use E=mc² in order to calculate these binding energies (well, you gotta use intervals of energy and mass like in decay, but the formula remains the same for intervals . ).
A decrease in binding energy would occur when the nucleus becomes less stable when a neutron is added. For example when you go from He_4 to Li_6 you will get a decrease in binding-energy per nucleus of about 1.5MeV.
To be complete the 4 and 6 in the above elements are the mass numbers, ok ???