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1. CHAPTER 13 CLASS 12TH PHYSICS
NUCLEI IN ONE SHOT
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2. Proton Neutron Hypothesis-
According to this hypothesis-
• Protons and neutrons are the main building blocks of the
nuclei of all atoms.
• Nucleus of mass number A and atomic number Z contains
Z protons and (A - Z) neutrons.
• The protons give positive charge to the nucleus, while
protons and neutrons together give it mass.
• To neutralize the positive charge of the nucleus,there should be Z elelctron
it to make the atom electrically neutral.

3. • Proton- It is a fundamental particle which may be called the nucleus of
hydrogen.
It has a positive charge of 1.6 x 10-19C.
It has a rest mass 1.6726x 10-27kg, which is about 1836 times the mass of
an electron.
A proton has an intrinsic (spin angular momentum equal to 1/2. It also
possesses magnetic moment much smaller than that of electron.
• Neutron- It is a fundamental particle having mass slightly
greater than proton.
• Neutrons do not have any charge.
• It has spin angular momentum equal to proton.
• Despite being neutral it has small magnetic moment.

4. •Basic Terms-
• Nucleons- Protons and neutrons which are present inside a
nucleus collectively known as Nucleons.
• Atomic number- It is the number of proton inside nucleus. It
is denoted by Z.
• Mass number- It is the total number of proton and neutron
inside nucleus.
• Nuclear mass- It is the total mass of proton and neutron
inside nucleus.
• Nuclide- It is specific nucleus of an atom characterised by
atomic number Z mass number A. Symbolically written as-

5. • Isotope- Atoms of an element having same number of
proton but different number of neutron are called
Isotopes.
• Ex-
•Isobar- Atoms having same mass number but different
atomic number are called Isobars.
• Ex-

6. • Isotone- Atoms having same number of neutrons are called isotopes.
Isomer- These are nuclei with same atomic number and same mass
number but exists in different energy state for ex- A nucleus in its
ground state and another nucleus in meta stable state.

7. Atomic mass Unit- It is defined as the 1/12th of actual
mass of Carbon-12 atom. It is denoted by amu or just u.
Acc to mass energy equivalence, (1amu = 931 MeV energy )
1 amu = 1.660565x10-27kg
Mass of electron me = 0.00055 amu = 9.1 x 10-31 kg
Mass of Proton mp = 1.0073 amu = 1.672x 10-27 kg
Mass of neutron mn = 1.0086 amu = 1.674x 10-27 kg
Mass of Hydrogen atom = 1.0078 amu =
(Mp + Mn)
Atomic masses are measured using Spectrometer.

8. • Nuclear size- Assuming nuclei to be spherical their
volumes can be estimated.
• Experimental observation shows that volume of nucleus is
directly proportional to the mass number.
• Volume ἀ A
•

9. • Nuclear Density-
•

10. Discovery of Neutron- Chadwick discovered the neutron during an
experiment where he hit Beryllium with alpha particles, and when he
looked at the rays produced he realized that they hadn't discharged
electrons. The rays that were emitted had a neutral charge, which meant
that neutrons had a neutral charge.
2He4 + 4Be9
0n1 + 6C12

11. • Properties of neutron-
• Neutrons are slightly heavier than protons and balance the
electron and hold the electron in the atom.
• The number of protons is equal to the number of neutrons
in the nucleus or number of a nucleus is greater than one
from the number of protons.
• If the number of neutrons gets more than the number of
protons in excess in the nucleus then the particle becomes
radioactive and breaks up to release alpha or beta rays.
• Neutron has penetrating power greater than gamma rays.

12. • Nuclear force- The strong attraction force between
nucleons is called nuclear force.
• Properties of Nuclear force-
• Strongest interaction- It is the strongest force in
nature.
Fg: Fe : Fn = 1:1036:1038
• Short range force- It is a short range force (2-3fm).
• Charge independent- Nuclear force does not depend on
charge of particle.
• Saturation effect- Nuclear force shows saturation
effect, It only interacts with its neighboring nucleons.

13. • Non central force- Nuclear force does not act along
the line joining the centers of nucleons.
• Spin dependent- It is stronger between nucleons of
parallel spins.

15. • Einstein mass energy equivalence- E=mc2
This equation suggest that even a particle is at rest, it
possesses an enormous amount of energy because of its
mass. Thus mass and energy are interconvertible.
For ex- Energy produced by converting 1g mass.

16. Mass defect- The difference in rest mass of nucleus
and sum of rest mass of individual nucleons is called
mass defect.
∆m = Zmp + (A-Z) mn – m
Mp, mn, m are the rest mass of proton, neutron and the
given nucleus respectively.

17. • Packing fraction- It is the mass defect per nucleon.
P.f = Mass defect = ∆m
Mass number A
If P.F is positive (for mass number less than 20 and for
more than 200) then nucleus is unstable.
If P.F is negative (for mass number between 20 to 200)
then nucleus is stable.
Thus it indicates some mass has been converted into
energy which binds the nucleon togeather and so the
nucleus is stable. Thus PF is directly related to the
availability of Nuclear energy.

18. • Nuclear reaction- A reaction which involves change of
stable nucleus of one element into nucleus of another
element is called Nuclear reaction.
• Types of Nuclear reaction- In nuclear reaction bombarding
particles are generally used as alpha particle(2He4), proton
(1H1), neutron (0n1) and gamma ray photon (Ỵ). Depending
on the nature of projectile and outgoing particle, the
various nuclear reactions are classified.

20. • Nuclear Fission-The process in nuclear physics in which
the nucleus of an atom splits into two or more daughter
nuclei.

21. • Nuclear fission on the basis of liquid drop model-
• According to the liquid drop model, the nucleus is assumed to be like a drop of a
liquid with certain analogies.
Then if a thermal neutron hits the nucleus, a compound nucleus is formed with
certain exciting energy due to the extra neutron.
This energy sets up the compound liquid drop nucleus into rapid oscillations
As a result, the nucleus takes the dumbbell like shape as is shown, and due to
positive charge repulsion on the two ends splits into two fragments hence,
forming daughter two nuclei.

22. • Nuclear Chain reaction- UNControlled chain reaction-

23. • Controlled chain reaction-

24. Thermal Neutron
• The neutron produced in fission of 235 U nuclei have average kinetic energy nearly 2MeV. Such
neutrons are called fast neutrons.
• The slow moving neutrons of energy 0.0235eV are called thermal neutrons. These neutrons
have velocity nearly 2200m/s which are the random velocities of atoms and molecules at room
temperature.
 Moderator-
• Any substance that is used to slow down the fast moving neutrons to thermal energy
(0.0235eV) is called moderator. The commonly used moderator are water, heavy water(D2O),
graphite and Beryllium.
• It Slows down neutron by elastic collisions also by removing neutron from system by absorbing
them.

25. For sustained chain reaction it is necessary to have sufficient amount of 235U but out of different isotopes of
Uranium, U235 is only 0.7% Although it can be enriched upto 3% by special techniques.
1. Neutron leakage- Some neutrons produced by fission may not interact with other nuclei and escape from the
system. This loss can be reduced by making system sufficiently large.
2. Neutron energy- Neutrons produced in the fission are fast neutrons having KE of about (2MeV). Such neutron
can easily escape from fissionable material without causing fission. This can be reduced by adding moderator
with uranium fuel.
3. Neutron Capture- If neutrons are slowed down to thermal energy then there are chances of getting it
absorbed by U238 nuclei which are present in large number to remove this problem distance between fuel rods
are so adjusted that neutrons coming from one rod is slowed down to thermal energy before it enters
neighboring rod. This eliminates the possibility of non fission capture of neutrons

26. • Multiplication factor
(K) = No. of neutrons present at the beginning of one generation
No. of neutrons present at beginning of previous generation
It gives the measure of growth rate of neutrons in fissionable
material.
K>1 = chain reaction grows
K<1 = Chain reaction gradually dies out
K=1 = Chain reaction remains steady

27. Critical size & mass-
The size of fissionable material for which the multiplication factor
k=1, is called critical size and mass is called critical mass. The chain
reaction in this case remains stationary.
For K>1 the neutron population increases rapidly and the size of
material is said to be Super Critical. The chain reaction build up at
faster rate and results in explosion.
For K<1 the neutron population decreases rapidly and the size is said
to be Subcritical.

28. • Nuclear Reactor- A device or space where nuclear reaction
takes place in a maintained and controlled way.

29. • Breeder reactor- The nuclear reactor that produces more fissionable
material than it consumes to generate energy. Whereas a
conventional nuclear reactor can use only the readily fissionable but
more scarce isotope uranium-235 for fuel, a breeder reactor employs
either uranium-238 or thorium, of which sizable quantities are
available.

30. Nuclear fusion- Nuclear fusion is the process where the nuclei of
two light atoms combine to form a new nucleus.
Necessary condition- High temperature to overcome Coluombic
force of repulsion.
High density or pressure to increase the frequency of collision.
Reaction-