State what is meant by the mass defect of a nucleus.
Some masses are shown in the table below.
Show that:
the energy equivalence of 
is 
.
the binding energy per nucleon of a helium-4 nucleus is 
.
Isotopes of hydrogen have binding energies per nucleon of less than 
. Suggest why a nucleus of helium-4 does not spontaneously break down to become nuclei of hydrogen.
