Answer
See explanation.
Work Step by Step
The mass defect is the total mass of the constituent particles, minus the atom’s mass.
a. Calculate the mass defect first: $7m_n+7m_H-m_N=0.112u$.
The binding energy is $(0.112u)\frac{931.5 MeV}{u}=104.3 MeV$
Binding energy per nucleon: divide the exact binding energy by 14 to obtain 7.45 MeV/nucleon.
b. Calculate the mass defect first: $2m_n+2m_H-m_He=0.0304u$.
The binding energy is $(0.0304u)\frac{931.5 MeV}{u}=28.3 MeV$
Binding energy per nucleon: divide the exact binding energy by 4 to obtain 7.08 MeV/nucleon.
c. The binding energy per nucleon is a little less for He than for N. This agrees with Figure 43.2.