Question

why binding energy for heavier nuclei is low

Answer 1

So average binding is weak. hence binding energy is low. Binding energy is a measure of the strength or energy required to break a nucleus into its constituent particles, namely neutrons and protons. The binding energy vs mass curve shows that the BE of heavy nuclides is lower than expected

i hope this is the answer

Answer 2

Binding energy is the energy required to knock electrons from the outer orbit commonly called as ionisation energy.
In the same large nucleus, if we increase the mass number (A) by adding nucleons, it will not change the binding energy . Because, in a large nucleus, most of the nucleons lie inside it and not on the surface. Hence, the change in binding energy, if any, would be negligibly small.