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The chapter **“Nuclei”** of Class 12 Physics consists of topics like
radioactivity, atomic masses, size of the nucleus, nuclear binding energy, nuclear
energy, the composition of nucleus, mass-energy, nuclear force, mass defect, nuclear
fusion, nuclear fission, mass-energy relation, etc. Students can note down important
points and formulas to efficiently prepare for the board exams.

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##### Question 2:

these isotopes are 19.99 u and 21.99 u respectively. Obtain the average atomic mass of neon.

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The average atomic mass of neon is

##### Question 3:

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##### Question 4:

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##### Question 5:

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Here Z = 29, A = 63, so A – Z = 34

Mass defect

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##### Question 7:

A radioactive isotope has a half-life of T years. After how much time is its activity reduced to (a) 3.125% (b) 1% of its original value ?

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##### Question 8:

carbon. Estimate the approximate age of Indus-Valley civilization.

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##### Question 9:

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The rate of decay of radioactive material is given by

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##### Question 11:

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##### Question 12:

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Given

energy released in the decay process. The Q of this decay process is given by

Now, if we express the Q value in terms of atomic masses we have to subtract 6m _{e}
from the atomic mass of carbon and 5m _{e} from that of boron atomic to get the
corresponding nuclear masses. Therefore, we have

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##### Question 14:

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= 0.004958 × 931.5

= + 4.62 MeV ; reaction is exothermic.

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##### Question 17:

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##### Question 18:

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##### Question 19:

Calculate the height of the potential barrier for head-on collision of two deuterons, (Hint : The height of the potential barrier if given by the Coulomb repulsion between the two deuterons when they just touch each other.) Assume that they can be taken as hard spheres of radius 2.0 fm.

##### Answer:

Radius of deuteron r = 2.0 fm

= 2 × 10^{–15} m

Initial energy of two deuterons = 2KE

This energy is converted into PE, after the headon collision, when deuteron stops and is
given by

##### Question 20:

From the relation R = R_{0} A^{1/3} , where R_{0} is a constant and A
is the mass number of a nucleus. Show that the nuclear matter density is nearly constant
(i.e. independent of A).

##### Answer:

Density of nucleus ($\mathrm{\rho}$). It is defined as the nuclear mass per unit volume.

Thus the nuclear density is of the order of 10^{17} kgm^{–3} and is
independent of its mass number. Therefore, all nuclei have the same approximate density.

##### Question 21:

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Consider the two competing processes :

Case of positron emission