Atoms Crash course 12 th

Atoms Crash course 12 th 

 

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1. NUCLEUS
(i) Central core of every atom.
(ii) Discovered by Rutherford in a-scattering experiment.
(iii) The order of nuclear size = 10–15 m or fm while the order of atomic size = 10–10 m or Å
(iv) Protons and neutrons, together referred as nucleons.
(v) A nuclide is represented by A
ZX
Z = atomic number = p (no. of protons)
A = mass number = total no. of nucleons = n + p
(vi) Atomic masses are generally represented by atomic mass unit (u) 

1.1 Types of Nuclei
(i) Isotope : same Z
(ii) Isobar : same A
(iii) Isotone : same (A – Z)

Forces acting inside the nucleus
There are three forces interacting between nucleons, these are
(i) Gravitational force - weakest force of nature
(ii) Electrostatic repulsive (coulombian) force ® only works between proton proton. This is stronger than gravitational
force.
(iii) Nuclear force ® strongest intraction that holds nucleons together to form nuclei and it is powerful enough to
overcome the electric repulsion of proton and proton

Features of Nuclear Force (Fn ) :–
1. The strongest force in the universe.
2. Works only between the nucleons.
3. Very short range : only upto size of nucleus (3 or 4 fermi). More than this distance, nuclear force is almost
zero.
4. Very much depends upon distance :– Small variation in distance may cause of large change in nuclear
force while electrostatic force remains almost unaffected.
5. Independent of charge :– Interacts between n–n as well as between p–p and also between n–p.
6. Spin dependent :– It is stronger between nucleons having same sense of spin than between nucleons having
opposite sense of spin.
7. It is not a central force :– Definition of central force (Fc) : Whose line of action always passes through a fixed
point and its magnitude depends only on distance, if medium is same.

Electrostatic and gravitational forces are central forces.
8. Nature :– (i) Attractive – If distance is greater than 0.8 fm or above. (ii) Repulsive – If distance is lesser
than 0.8 fm

EINSTEIN'S MASS ENERGY EQUIVALENCE
According to Einstein, mass can be converted into energy and energy into mass. This relation is given by -
E = mc2
Here E = total energy associated with mass m ; c2 = used as a conversion coefficient

Mass defect
(i) Mass of a nucleus is always less than the sum of masses of its constituent nucleons. This difference is called
mass defect.
(ii) If observed mass of nucleus ZXA be M, mass of proton is Mp and mass of neutron is Mn then
mass defect = Dm = [ZMp + (A – Z)Mn] – M.
(iii) If M is taken as mass atom of ZXA instead of mass of nucleus then
Dm = [Z(Mp + Me) + (A – Z)Mn] – Matom

Binding energy (Eb)
(i) Binding energy of a nucleus is the energy required to split it into its nucleons (free).
(ii) DEb = Dm.c2
(iii) It is always positive and numerically equal to the energy equivalent of mass defect (or equal to the energy
liberated when it was formed)

(i) The value of binding energy per nucleon decides the stability of a nucleus. It is obtained by dividing binding
energy by the mass number of given nucleus.
(ii) The following figure shows the binding energy per nucleon plotted against the mass number of various
atoms nuclei
Greater the binding energy per nucleon, the more stable the nucleus.
(iii) It is maximum for isotope of iron – 26
56Fe and is 8.8 MeV/nucleon. It is the most stable nucleus.
(iv) For Uranium, binding energy per nucleon is about 7.7 MeV/nucleon and it is unstable.
(v) The medium size nuclei are more stable than light or heavy nuclei

NUCLEAR FISSION
Splitting of a heavy nucleus (A > 230) into two or more lighter nuclei when struck by a neutron.
In this process certain mass disapears which is obtained in the form of energy (enormous amount)
A + n ® excited nucleus ® B + C + Q
Hahn and Strassmann done the first fission (fission of nucleus of U235).
When U235 is bombarded by a neutron it splits into two fragments and 2 or 3 secondary neutrons and releases
about 200 MeV energy per fission (or from single nucleus)
Fragments are uncertain but each time energy released is almost same.
Possible reactions are -
U235 + 0n1 ® Ba + Kr + 30n1 + 200 MeV
or U235 + 0n1 ® Xe + Sr + 20n1 + 200 MeV
and many other reactions are possible.
(i) The average number of secondary neutrons is 2.5.
(ii) Nuclear fission can be explained by using " liquid drop model" also.
(iii) The mass defect Dm is about 0.1% of mass of fissioned nucleus
(iv) About 93% of released energy (Q) is appear in the form of kinetic energies of products and about 7% part in
the form of g - rays.

NUCLEAR CHAIN REACTION
The equation of fission of U235 is 92U235 + 0n1 ® 56Ba144 + 36Kr89 + 30n1 + Q
These three secondary neutrons produced in the reaction may cause of fission of three more U235 and give 9
neutrons, which in turn, may cause of nine more fission of U235 and so on.
Thus a continuous 'Nuclear Chain reacion' would start.
If there is no control on chain reaction then in a short time ( ~- 10–6 sec.) a huge amount of energy will be
released. (This is the principle of 'Atom bomb')
If chain reaction is controlled then produced energy can be used for peaceful purposes. For example nuclear
reactor (Based on fission) generats electricity