The document discusses nuclear models and nuclear forces. It describes three nuclear models: the liquid drop model, shell model, and collective model. The liquid drop model treats the nucleus as a liquid drop and examines its global properties. The shell model arranges nucleons into energy shells like electrons in an atom and explains magic numbers. The collective model incorporates aspects of the liquid drop and shell models. Nuclear forces operate very strongly within the nucleus, are attractive between protons and neutrons, and are responsible for nuclear stability.

1. NUCLEAR MODELS AND
NUCLEAR FORCES
RABIA AZIZ,
PHYSICAL CHEMISTRY,
BS-III CHEMISTRY, VI SEMESTER,
JINNAH UNIVERSITY FOR WOMEN

2. Nuclear Models:
• Nucleus is a complicating structure to
understand. There are three basic models for
discribing the stucture of nucleus.
The liquid-drop
model
Shell model
Collective model
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3. • The starting point for theoretical models of
the nucleus treats the problem from two
divergent perspectives:
•At the macroscopic extreme is the Liquid Drop
Model, which examines the global properties of
nuclei, such as energetics, binding energies, sizes,
shapes and nucleon distributions. This model
assumes that all nucleons are alike (other than
charge).
Global
Properties
•In contrast the Shell Model is designed to account
for the quantal properties of nuclei such as spins,
quantum states, magnetic moments and magic
numbers. The basic assumption of the Shell
Model is that all nucleons are different, i.e.
nucleons are fermions and must occupy different
quantum states, as is the case for atoms.
Quantal
Properties
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4. Liquid Drop Model
• This model introduced by Niels Bohr is based
on the aggregate of nucleons has the same
properties of a liquid drop, such as surface
tension, cohesion, and deformation.
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5. Liquid Drop Model Assumptions
Volume
•The volume of the nucleus is directly proportional to the total number of
nucleons present in it.
•The volume of a drop is also proportional to the total number of atoms or
molecules contained in it.
Density
•The density of a nucleus is nearly constant, and so that of a liquid drop.
Binding Energy
•The binding energy of the nucleus increases with increasing number of
nucleons.
•The net chemical binding energy within a liquid drop also varies with the
number of atoms or molecules within it.
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6. Nuclear Forces
• This is analogous to the cohesive force of surface tension
within a liquid drop. Under the influence of surface
tension, the liquid drop attains spherical shape.
• Nuclear forces are short range forces. The atomic
nucleus is also spherical.
Disintegration
• Very large drops of a liquid are not stable for long, since
the force of surface tension progressively becomes
weak. The large drop dissoicates into smaller droplets.
• In case of atomic nuclei, the size of the nucleus increases
with mass number. For very heavy elements the size of
the nuclei reaches a critical value and gets deformed. A
slight provocation is enough to cause its disintegration.
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7. Advantages and Disadvantages of
Liquid Drop Model
Advantages
• Liquid Drop Model examines the macro properties of nuclei, such as
energetics, binding energies, sizes, shapes and nucleon
distributions.
• The binding energy equation derives by this model. The formula is
called the Weizsaecker Formula (or the semi-empirical mass
formula). The nuclear binding energy as a function of the mass
number A and the number of protons Z based on the liquid drop
model can be written as:
Disadvantage:
• This model do not gives the idea of magic number.
• It does not explain the micro properties of nucleus.
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8. Shell Model
The Shell Model developed by
Maria Goeppert-Mayer and
Hans Jensen.
The Nuclear Shell Model is
similar to the atomic model
where electrons arrange
themselves into shells around
the nucleus.
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9. Shell Model Assumptions
The nucleons are arranged
in different energy levels
within the nucleus.
There are two sets of
energy shells in the
nucleus, one for the
occupation of neutrons and
the other for protons.
Each energy level can be
occupied by two neutrons or
two protons, provided their
spins are opposite. When
two neutrons or protons
spin in opposite directions,
the magnetic field of each
cancels that of other. This
leads to stability.
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10. Advantages of Shell Model
Properties
• This model has been very successful in
explaining the basic nuclear properties
(angular momentum, magnetic moment,
shape, etc.)
Magic
Number
• The nuclear shell model explains the
existence of "magic numbers".
Nuclear
Stability
• It explains the Relationship between the
Number of Protons and the Number of
Neutrons and Nuclear Stability.
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11. The Collective Model
Collective model is the sum of the
liquid drop model and shell model.
It explains both global properties
and quantal properties.
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12. Nuclear Force
 There are particles neutrons and
protons in the nucleus. These
particles are binded together by
strong nuclear forces.
 Nuclear force is the force that
exists between neutron-neutron,
neutron-proton or proton-proton.
These nucleus particles are known
as nucleons. This nuclear force is
responsible for stability of nucleus.
The force between these particles
are attractive even the protons
have same charge. It means that
these nuclear forces are attractive
by nature.
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13. Nuclear Force Properties
1. Nuclear forces are the strongest forces in nature. They
are 1038 times stronger than gravitational forces, 102
times stronger than electrostatic forces and 1013 times
stronger than the weak forces.
2. Nuclear forces have the shortest range. They operate
within the nucleus only i.e. upto distances of the order
of 10^ -14 meter.
3. Nuclear forces do not depend on charge on the
nucleon.
4. They are basically attractive forces. Only when distance
between nucleons is less than 0.5 Fermi, nuclear
forces become repulsive.
5. Nuclear forces are non central forces.
6. They are also non-conservative forces
7. The field particle of strong nuclear force is Pi – Meson.
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