We can draw the following conclusions from the plot-
1. The force is attractive and sufficiently strong to produce a binding energy of a few MeV per nucleon.
2. The constancy of the binding energy in the range 30 < A < 170 is a consequence of the fact that the nuclear force
is short-ranged. Consider a particular nucleon inside a sufficiently large nucleus. It will be under the influence of
only some of its neighbors, which come within the range of the nuclear force. If any other nucleon is at a
distance more than the range of the nuclear force from the particular nucleon it will have no influence on the
binding energy of the nucleon under consideration. If a nucleon can have a maximum of p neighbors within the
range of nuclear force, its binding energy would be proportional to p. Let the binding energy of the nucleus be
pk, where k is a constant having the dimensions of energy. If we increase A by adding nucleons, they will not
change the binding energy of a nucleon inside. Since most of the nucleons in a large nucleus reside inside it and
not on the surface, the change in binding energy per nucleon would be small. The binding energy per nucleon is
a constant and is approximately equal to pk. This is called saturation property of nuclear force.
3. A very heavy nucleus, say A = 240, has lower binding energy per nucleon compared to that of a nucleus with A =
120. Thus, if a nucleus A = 240 breaks into two A = 120 nuclei, nucleons get more tightly bound. This implies
energy would be released in the process. This is called nuclear fission.
4. Consider two very light nuclei (A ≤ 10) joining to form a heavier nucleus. The binding energy per nucleon of the
fused heavier nuclei is more than the binding energy per nucleon of the lighter nuclei. This means that the final
system is more tightly bound than the initial system. Therefore, energy would be released in such a process of
fusion.
Important PYQs
Ques: A nucleus with mass number A = 240 and B.E./A = 7.6 MeV and breaks into two fragments of each A = 120
with B.E./A = 8.5 MeV. Calculate the released energy (PYQ 2016) [2M]
Ans:
Nuclear Force (PYQ 2018, 2011)
1. The nuclear force is much stronger than the Coulomb force acting between
charges or the gravitational forces between masses.
2. The nuclear force between two nucleons falls rapidly to zero as their distance
is more than a few femtometers. This leads to saturation of forces in a
medium or a large-sized nucleus, which is the reason for the constancy of
the binding energy per nucleon.
3. The nuclear force between neutron-neutron, proton-neutron and
proton-proton is approximately the same. The nuclear force does not depend
on the electric charge.
Radioactivity (PYQ 2020)
Radioactivity is a nuclear phenomenon in which an unstable nucleus undergoes a decay. This is referred to as
radioactive decay. Three types of radioactive decay occur in nature:
1. α-decay in which a helium nucleus 42He is emitted
2. β-decay in which electrons or positrons (particles with the same mass as electrons, but with a charge exactly
opposite to that of electron) are emitted
3. ɣ-decay in which high energy (hundreds of keV or more) photons are emitted.
Ini5al binding energy =
Final Binding energy =
Released energy =
iEf

o .. NUCLEI. Atomic mass unit (u) An atomic mass unit i

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