NCERT Solutions
Class 12 Physics
Nuclei

Q.4
Obtain the binding energy of the nuclei 5626Fe and 20983 Bi in units of MeV from the following data: m (5626Fe) = 55.934939 u m (20983Bi) = 208.980388 u
Given:
Atomic mass of (5626Fe), mFe = 55.934939 u
(5626Fe) nucleus has 26 protons and (56 − 26) = 30 neutrons
Hence, the mass defect of the nucleus, ∆m = ((26 × mH) + (30 × mn) – mFe)
where,
Mass of a proton, mH = 1.007825 u
Mass of a neutron, mn = 1.008665 u
∆m = ((26 × 1.007825 + 30 × 1.008665) − 55.934939)
= (26.20345 + 30.25995 − 55.934939)
= 0.528461 u
But 1 u = 931.5
Therefore, ∆m = (0.528461 × 931.5)
The binding energy of this nucleus is given as:
Eb1 = ∆mc2
Where, c =Speed of light
Eb1 = (0.528461 × 931.5 c2)
= 492.26 MeV
Average binding energy per nucleon= (492.26/56) =8.76 MeV
Atomic mass of (20983Bi), m2 = 208.980388 u
(20983Bi) nucleus has 83 protons and (209 − 83) = 126 neutrons.
Hence, the mass defect of this nucleus is given as:
∆m' = (83 × mH + 126 × mn) − m2
where,
Mass of a proton, mH = 1.007825 u
Mass of a neutron, mn = 1.008665 u
∆m' = ((83 × 1.007825) + (126 × 1.008665) – 208.980388)
= 83.649475 + 127.091790 − 208.980388
= 1.760877 u
But 1 u = 931.5
Therefore, ∆m' = (1.760877 × 931.5) x c2
Hence, the binding energy of this nucleus is given as:
Eb2 = ∆m'c2
= (1.760877 × 931.5 x c2)
= 1640.26 MeV
Average binding energy per nucleon = =7.848MeV