NUCLEUS
1. RUTHERFORD postulated the existence of nucleus inside an atom.
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2. Nuclear size: order of 10 m.
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3. Composition: (a) neutrons- discovered by CHADWICK. Neutral particles. Rest mass = 1.6749 × 10 Kg. neutron is
unstable outside the nucleus. They have low ionizing powers (do not interact with electrons).
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(b) protons – positive charge particle (1.6 × 10 C). rest mass = 1.6726 × 10 Kg. discovered by
Rutherford.
Isotopes: H1 protium, H2 dueterium, H3 tritium. Li6 and Li7. Cl35 and Cl37. Gold has 32 isotopes.
Isobars: 1H3 and 2He3. 17Cl37 and 16S37. 20Ca40 and 18Ar40.
Isotones: 17Cl37 and 19K39. 80Hg198 and 79Pu197.
Atomic Mass Unit (amu): amu is (1 / 12)th of mass of 1 atom of
carbon – 12.
The energy equivalent to 1 amu is 931 MeV.
The nuclear density does not depend upon mass number. So, all the nuclei possess nearly the same density.
The nuclear density is not uniform throughout the nucleus. It has maximum value at the centre and decreases
gradually as we move away from the centre of the nucleus.
The nuclear radius is the distance from the centre of the nucleus at which the density of nuclear matter decreases to
one-half of its maximum value at the centre.
The nuclear density has extremely large value. Such high densities are found in white dwarf stars which contain mainly
nuclear matter.
NUCLEAR FORCE
The strong attractive forces that binds the protons and neutrons together inside the nucleus is called nuclear force.
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Fg : Fe : Fn = 1 : 10 : 10
The graph of P.E. of a pair of nucleons as a
function of their separation: the P.E. is
minimum at distance r0 = 0.8 fm.
a) For r<r0 the P.E. increases rapidly. Indicates
strong repulsive force.
b) For r>r0 the P.E. gradually decreases to
zero. Indicates attractive nuclear force.
c) For r = 4 fm the nuclear force becomes
zero. Indicates nuclear force is short range
force.
Nuclear force does not depend on the charge of
the particles.
Nucleon interacts only with neighbouring
nucleons (saturation effect).
According to YUKAWA theory nuclear force
between two nucleons is due to constant
exchange of mesons between them.
Non central force. (does not act along the line joining the centers of nucleons.
Mass Defect: It is the difference between the rest mass of the nucleus and the sum of the
masses of the nucleons constituting a nucleus is known as mass defect.
Mass defect per nucleon is called packing fraction.
Binding Energy: It is the energy required to break up a nucleus into its constituent particles and place them at
an infinite distance from one another.
Binding Energy per Nucleon: B.E / Nucleon = Δm c2 / A
1. Binding energy per nucleon of very light
2
nuclides such as 1H is very small.
2. Initially, there is a rapid rise in the value of
binding energy per nucleon.
3. Between mass numbers 4 and 20, the
curve shows cyclic recurrence of peaks
4 8 12 16
corresponding to 2He , 4Be , 6C , 8O and
20
10Ne . This shows that the B.E. per
nucleon of these nuclides is greater than
those of their immediate neighbor’s.
4. After A = 20, there is a gradual increase in
B.E. per nucleon. The maximum value of
8.8 MeV is reached at A = 56. Therefore,
Iron nucleus is the most stable.
5. Binding energy per nucleon of nuclides
having mass numbers ranging from 40 to
120 are close to the maximum value. So,
these elements are highly stable and non-radioactive.
6. Beyond A = 128, the value shows a rapid decrease. This makes elements beyond Uranium (trans – uranium elements) quite
unstable and radioactive.
7. The drooping of the curve at high mass number indicates that the nucleons are more tightly bound and they can undergo
fission to become stable.
8. The drooping of the curve at low mass numbers indicates that the nucleons can undergo fusion to become stable.
