## NCERT Solutions for Class 12 Physics Chapter 13 – Free PDF Download

The **NCERT Solutions for Class 12 Physics Chapter 13 Nuclei** is provided here to help students in their CBSE Class 12 exam preparations. These NCERT Solutions for Class 12 Physics are prepared by the subject experts at BYJU’S as per the latest CBSE Syllabus for 2023-24 and its guidelines. Students can prepare well and score good grades in the Class 12 board examinations by referring to these solutions.

The **NCERT Solutions for Class 12 Physics** of this chapter provided here has answers to the textbook questions along with Class 12 important questions, exemplary problems, worksheets and exercises, which will help students gain complete knowledge on the topics Nuclei. Students can avail free PDF of the NCERT Solutions for Class 12 Physics Chapter 13 by downloading it from the link given below.

## NCERT Solutions for Class 12 Physics Chapter 13 Nuclei

### NCERT Solutions Class 12 Physics Chapter 13 Nuclei Important Questions

**Q 13.1 (a) Lithium has two stable isotopes **

**\(\begin{array}{l}_{3}^{6}{Li}\end{array} \)**

**and\(\begin{array}{l}_{3}^{7}{Li}\end{array} \), which have abundances of 7.5% and 92.5%, and masses 6.01512 u and 7.01600 u, respectively. Find the atomic mass of lithium.**

**(b) Boron has two stable isotopes, **

**\(\begin{array}{l}_{5}^{10}{B}\end{array} \)**

**and\(\begin{array}{l}_{5}^{11}{B}\end{array} \). Their respective masses are 10.01294 u and 11.00931 u, and the atomic mass of boron is 10.811 u. Find the abundances of \(\begin{array}{l}_{5}^{10}{B}\end{array} \) and \(\begin{array}{l}_{5}^{11}{B}\end{array} \).**

**A1 :**

(a) Mass of

_{1}= 6.01512 u

Mass of

_{2}= 7.01600 u

Abundance of

_{1}= 7.5%

Abundance of

_{2}= 92.5%

The atomic mass of the lithium atom is:

= 6.940934 u

Therefore, The atomic mass of the lithium atom is 6.940934 u.

(b) Mass of boron isotope

_{1}= 10.01294 u

Mass of boron isotope

_{2}= 11.00931 u

Abundance of

_{1}= x%

Abundance of

_{2}= (100 − x)%

The atomic mass of boron, m = 10.811 u

The atomic mass of the boron atom is:

1081.11 = 10.01294x + 1100.931 – 11.00931 x

x = 19.821/0.99637 = 19.89 %

And 100 − x = 80.11%

Hence, the abundance of

** ****Q 13.2: The three stable isotopes of neon: **

**\(\begin{array}{l}_{10}^{20}{Ne}\end{array} \)**

**,\(\begin{array}{l}_{10}^{21}{Ne}\end{array} \) and \(\begin{array}{l}_{10}^{22}{Ne}\end{array} \) have abundances of 90.51%, 0.27% and 9.22%, respectively. The atomic masses of the three isotopes are 19.99 u, 20.99 u and 21.99 u, respectively. Obtain the average atomic mass of neon.**

**Answer**

Atomic mass of

_{1}= 19.99 u

Abundance of

_{1}= 90.51%

Atomic mass of

_{2}= 20.99 u

Abundance of

_{2}= 0.27%

Atomic mass of

_{3}= 21.99 u

Abundance of

_{3}= 9.22%

The average atomic mass of neon:

=

= 20.1771 u

Therefore, the average atomic mass of neon is 20.1771 u.

**Q 13.3: Obtain the binding energy in MeV of a nitrogen nucleus **

**\(\begin{array}{l}_{7}^{14}{N}\end{array} \)**

**, given m(\(\begin{array}{l}_{7}^{14}{N}\end{array} \))=14.00307 u.**

**Ans:**

Atomic mass of nitrogen

A nucleus of

∆m = 7m_{H} + 7m_{n} − m is the mass defect of the nucleus

Where,

Mass of a proton, m_{H} = 1.007825 u

Mass of a neutron, m_{n }= 1.008665 u

∆m = 7 × 1.007825 + 7 × 1.008665 − 14.00307

= 7.054775 + 7.06055 − 14.00307

= 0.11236 u

But, 1 u = 931.5 MeV/c^{2}

∆m = 0.11236 × 931.5 MeV/c^{2}

E_{b} = ∆mc^{2} is the binding energy of the nucleus

Where, c =Speed of light

= 104.66334 Mev

Therefore, 104.66334 MeV is the binding energy of the nitrogen nucleus.

**Q 13.4: Obtain the binding energy of the nuclei **

**\(\begin{array}{l}_{26}^{56}{Fe}\end{array} \)**

**and\(\begin{array}{l}_{83}^{209}{Bi}\end{array} \) in units of MeV from the following **

**data:**

**(a) m (**

**\(\begin{array}{l}_{23}^{56}{Fe}\end{array} \)**

**) = 55.934939 u**

**(b) m(**

**\(\begin{array}{l}_{83}^{209}{Bi}\end{array} \)**

**) = 208.980388 u**

*Ans: *

(a) Atomic mass of

_{1}= 55.934939 u

Hence, the mass defect of the nucleus, ∆m = 26 × m_{H} + 30 × m_{n} − m_{1}

Where,

Mass of a proton, m_{H} = 1.007825 u

Mass of a neutron, m_{n} = 1.008665 u

∆m = 26 × 1.007825 + 30 × 1.008665 − 55.934939

= 26.20345 + 30.25995 − 55.934939

= 0.528461 u

But, 1 u = 931.5 MeV/c^{2}

∆m = 0.528461 × 931.5 MeV/c^{2}

^{2 }is the binding energy of the nucleus.

Where, c = Speed of light

= 492.26 MeV

Therefore, the average binding energy per nucleon =

(b) Atomic mass of

_{2}= 208.980388 u

The mass defect of the nucleus is given as:

∆m’ = 83 × m_{H} + 126 × m_{n} − m_{2}

Where,

Mass of a proton, m_{H} = 1.007825 u

Mass of a neutron, m_{n} = 1.008665 u

∆m’ = 83 × 1.007825 + 126 × 1.008665 − 208.980388

= 83.649475 + 127.091790 − 208.980388

= 1.760877 u

But, 1 u = 931.5 MeV/c^{2}

∆m’ = 1.760877 × 931.5 MeV/c^{2}

E_{b2 }= ∆m’c^{2} is the binding energy of the nucleus.

= 1.760877 × 931.5

= 1640.26 MeV

Therefore, the average binding energy per nucleon = 1640.26/209 = 7.848 MeV.

**Q 13.5: A given coin has a mass of 3.0 g. Calculate the nuclear energy that would be required to separate all the neutrons and protons from each other. For simplicity, assume that the coin is entirely made of **

**\(\begin{array}{l}_{29}^{63}{Cu}\end{array} \)**

**with mass = 62.92960 u.**

**Ans :**

Mass of a copper coin, m’ = 3 g

Atomic mass of

The total number of

Where,

N_{A} = Avogadro’s number = 6.023 × 10^{23} atoms /g

Mass number = 63 g

^{22}atoms

Mass defect of this nucleus, ∆m’ = 29 × m_{H} + 34 × m_{n} − m

Where,

Mass of a proton, m_{H} = 1.007825 u

Mass of a neutron, m_{n} = 1.008665 u

∆m’ = 29 × 1.007825 + 34 × 1.008665 − 62.9296

= 0.591935 u

Mass defect of all the atoms present in the coin, ∆m = 0.591935 × 2.868 × 10^{22}

= 1.69766958 × 10^{22} u

But, 1 u = 931.5 MeV/c^{2}

∆m = 1.69766958 × 10^{22} × 931.5 MeV/c^{2}

E_{b} = ∆mc^{2} is the binding energy of the nuclei of the coin

= 1.69766958 × 10^{22} × 931.5 MeV/c^{2 }× c^{2}

= 1.581 × 10^{25} MeV

But 1 MeV = 1.6 × 10^{−13} J

E_{b} = 1.581 × 10^{25} × 1.6 × 10^{−13}

= 2.5296 × 10^{12} J

Therefore, the energy required to separate all the neutrons and protons from the given coin is 2.5296 × 10^{12} J.

**Q 13.6: Write nuclear reaction equations for the following:**

**(i) α-decay of**

**\(\begin{array}{l}_{88}^{226}{Ra}\end{array} \)**

**(ii) α-decay of\(\begin{array}{l}_{94}^{242}{Pu}\end{array} \)**

**(iii) β ^{−}-decay of **

**\(\begin{array}{l}_{15}^{32}{P}\end{array} \)**

**(iv) β\(\begin{array}{l}_{83}^{210}{Bi}\end{array} \)**

^{–}-decay of**(v) β ^{+}-decay of**

**\(\begin{array}{l}_{6}^{11}{C}\end{array} \)**

**(vi) β\(\begin{array}{l}_{43}^{97}{Tc}\end{array} \)**

^{+}-decay of**(vii) Electron capture of**

**\(\begin{array}{l}_{54}^{120}{Xe}\end{array} \)**

**Ans:**

In helium, α is a nucleus

^{−}for β

^{−}and e

^{+}for β

^{+}). 2 protons and 4 neutrons are lost in every α decay. Whereas 1 proton and a neutrino are emitted from the nucleus in every β

^{+}decay. In every β

^{–}decay, there is a gain of 1 proton, and an anti-neutrino is emitted from the nucleus.

For the given cases, the various nuclear reactions can be written as follows:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

**Q 13.7: A radioactive isotope has a half-life of T years. How long will it take the activity to reduce to (a) 3.125% and (b) 1% of its original value?**

**Ans:**

The half-life of the radioactive isotope = T years

N_{0 }is the actual amount of radioactive isotope.

(a) After decay, the amount of the radioactive isotope = N

It is given that only 3.125% of N_{0} remains after decay. Hence, we can write:

But,

Where, λ = Decay constant

t = Time

Since,

t =

Hence, the isotope will take about 5T years to reduce to 3.125% of its original value.

(b) After decay, the amount of the radioactive isotope = N

It is given that only 1% of N_{0} remains after decay. Hence, we can write:

t = 4.6052/

Since, λ = 0.693/T

Hence, the isotope will take about 6.645T years to reduce to 1% of its original value.

* ***Q 13.8: The normal activity of living carbon-containing matter is found to be about 15 decays per minute for every gram of carbon. This activity arises from the small proportion of radioactive**

**\(\begin{array}{l}_{6}^{14}{C}\end{array} \)**

**present with the stable carbon isotope\(\begin{array}{l}_{6}^{12}{C}\end{array} \) . When the organism is dead, its interaction with the atmosphere (which maintains the above equilibrium activity) ceases, and its activity begins to drop. From the known half-life (5730 years) of\(\begin{array}{l}_{6}^{14}{C}\end{array} \) and the measured activity, the age of the specimen can be approximately estimated. This is the principle of \(\begin{array}{l}_{6}^{14}{C}\end{array} \) dating used in archaeology. Suppose a specimen from Mohenjodaro gives an activity of 9 decays per minute per gram of carbon. Estimate the approximate age of the Indus-Valley civilisation. **

**Ans :**

The decay rate of living carbon-containing matter, R = 15 decay/min

Let N be the number of radioactive atoms present in a normal carbon-containing matter.

Half-life of

The decay rate of the specimen obtained from the Mohenjo-Daro site:

R’ = 9 decays/min

Let N’ be the number of radioactive atoms present in the specimen during the Mohenjo-Daro period.

Therefore, the relation between the decay constant, λ and time, t, is:

But

So, the approximate age of the Indus-Valley civilisation is 4223.5 years.

**Q 13.9: Obtain the amount of **

**\(\begin{array}{l}_{27}^{60}{Co}\end{array} \)**

**necessary to provide a radioactive source of 8.0 mCi strength. The half-life of**

**\(\begin{array}{l}_{27}^{60}{Co}\end{array} \) is 5.3 years.**

**Ans:**

The strength of the radioactive source is given as:

= 8 x 10^{-3} x 3.7 x 10^{10}

= 29.6 x 10^{7} decay/s

Where,

N = Required number of atoms

Half-life of

= 5.3 × 365 × 24 × 60 × 60

= 1.67 × 10^{8} s

The rate of decay for decay constant λ is:

Where,

= 7.133 x 10^{16 }atoms

For

Mass of 6.023 × 10^{23} (Avogadro’s number) atoms = 60 g

Mass of 7.133 x 10^{16} atoms =

= 7.106 x 10^{-6} g

Hence, the amount of

^{−6}g.

**Q 13.10: The half-life of **

**\(\begin{array}{l}_{ 38 }^{ 90 }{ Sr }\end{array} \)**

**is 28 years. What is the disintegration rate of 15 mg of this isotope?**

**Ans:**

Half-life of

= 28 × 365 × 24 × 60 × 60

= 8.83 × 10^{8} s

Mass of the isotope, m = 15 mg

90 g of

^{23}(Avogadro’s number) atoms.

Therefore, 15 mg of

=

i.e., 1.0038 x 10^{20 }number of atoms

Rate of disintegration,

Where,

= 7.878 x 10^{10} atoms/s

Hence, the disintegration rate of 15 mg of

^{10}atoms/s.

** ****Q 13.11: Obtain approximately the ratio of the nuclear radii of the gold isotope **

**\(\begin{array}{l}_{ 79 }^{ 197 }{Au}\end{array} \)**

**and the**

**\(\begin{array}{l}_{ 47 }^{ 107 }{Ag}\end{array} \) silver isotope.**

**Ans:**

Nuclear radius of the gold isotope

_{Au}

Nuclear radius of the silver isotope

_{Ag}

The mass number of gold, A_{Au} = 197

The mass number of silver, A_{Ag} = 107

Following is the relationship between the radii of the two nuclei and their mass number:

= 1.2256

Hence, 1.23 is the ratio of the nuclear radii of the gold and silver isotopes.

** ****Q 13.12: Find the Q-value and the kinetic energy of the emitted α-particle in the α-decay of **

**(a)**

**\(\begin{array}{l}_{88}^{226}{Ra}\end{array} \)**

**(b)**

**\(\begin{array}{l}_{86}^{220}{Rn}\end{array} \)**

**Given,**

**\(\begin{array}{l}m ( _{ 88 }^{ 226 }{ Ra })\end{array} \)**

**= 226.02540 u,**

**\(\begin{array}{l}m ( _{ 86 }^{ 222 }{ Rn })\end{array} \) = 222.01750 u,**

**\(\begin{array}{l}m ( _{ 86 }^{ 220 }{ Rn })\end{array} \)**

**= 220.01137 u,**

**\(\begin{array}{l}m ( _{ 84 }^{ 216 }{ Po })\end{array} \) = 216.00189 u.**

**Answer**

(a) Alpha particle decay of

Q-value of emitted α-particle = (Sum of initial mass − Sum of final mass) c^{2}

Where, c = Speed of light

It is given that:

Q-value = [226.02540 − (222.01750 + 4.002603)] u c^{2}

= 0.005297 u c^{2}

But, 1 u = 931.5 MeV/c^{2}

Q = 0.005297 × 931.5 ≈ 4.94 MeV

Kinetic energy of the α-particle =

(b) Alpha particle decay of

It is given that:

Mass of

Mass of

Q-value = [ 220.01137 – ( 216.00189 + 4.00260 ) ] x 931.5

≈ 641 MeV.

Kinetic energy of the α-particle =

= 6.29 MeV.

**Q 13.13: The radionuclide ^{11}C decays according to**

**\(\begin{array}{l}{ }_{6}^{11} \mathrm{C} \rightarrow_{5}^{11} \mathrm{~B}+e^{+}+v: \quad T_{1 / 2}=20.3 \mathrm{~min}\end{array} \)**

**The maximum energy of the emitted positron is 0.960 MeV.****Given the mass values:****m ( ^{11}_{6} C) = 11.011434 u and **

**m ( ^{11}_{6}B ) = 11.009305 u,**

**calculate Q and compare it with the maximum energy of the positron emitted.**

Ans:

Mass defect in the reaction is Δm = m (_{6}C^{11}) – m(_{5}B^{11}) – m_{e}

This is given in terms of atomic masses. To express in terms of nuclear mass, we should subtract 6m_{e} from carbon, and 5m_{e} from boron.

Δm = m (_{6}C^{11}) – 6m_{e} – (m(_{5}B^{11}) – 5m_{e} )- m_{e}

= m (_{6}C^{11}) – 6m_{e} – m(_{5}B^{11}) + 5m_{e} – m_{e}

= m (_{6}C^{11}) – m(_{5}B^{11}) – 2 m_{e}

Δm = [11.011434 – 11.009305 – 2 x 0.000548] u

= 0.002129 – 0.001096 = 0.001033

Q = Δm x 931 MeV

= 0.001033 x 931

Q= 0.9617 MeV

Therefore, the Q-factor of the reaction is equal to the maximum energy of the emitted positron.

**Q 13.14: The nucleus **

**\(\begin{array}{l}_{10}^{23}{Ne}\end{array} \)**

**decays\(\begin{array}{l}\beta ^-\end{array} \) by emission. Write down the\(\begin{array}{l}\beta \end{array} \) decay equation and determine the maximum kinetic energy of the electrons emitted. Given that:**

**\(\begin{array}{l}m(_{10}^{23}{Ne})\end{array} \)**

**= 22.994466 u**

**\(\begin{array}{l}m(_{11}^{23}{Na})\end{array} \)**

**= 22.989770 u.**

**Ans: **

In

It is given that:

Atomic mass of

Atomic mass of

Mass of an electron, m_{e} = 0.000548 u

The Q-value of the given reaction is given as:

There are 10 electrons and 11 electrons in

Q = [ 22.994466 – 22.989770 ] c^{2}

= (0.004696 c^{2}) u

But, 1 u = 931.5 MeV/c^{2}

Q = 0.004696 x 931.5 = 4.374 MeV

The daughter nucleus is too heavy as compared to e^{–} and

** ****Q 13.15: The Q value of a nuclear reaction A + b → C + d is defined by**

**Q = [ m _{A}+ m_{b}− m_{C}− m_{d}]c^{2,} where the masses refer to the respective nuclei. Determine **

**from the given data the Q-value of the following reactions and state whether the reactions**

**are exothermic or endothermic.**

**(i) **

**\(\begin{array}{l}_{ 1 }^{ 1 }{H} + _{ 1 }^{ 3 }{H}\rightarrow _{ 1 }^{ 2 }{ H } + _{ 1 }^{ 2 }{H}\end{array} \)**

**(ii) **

**\(\begin{array}{l}_{ 6 }^{ 12 }{C} + _{ 6 }^{ 12 }{C}\rightarrow _{ 10 }^{ 20 }{ Ne } + _{ 2 }^{ 4 }{He}\end{array} \)**

**Atomic masses are given to be**

**\(\begin{array}{l}m( _{ 1 }^{ 2 }{H} ) = \; 2.014102 \; u\end{array} \)**

**\(\begin{array}{l}m( _{ 1 }^{ 3 }{H} ) = \; 3.016049 \; u\end{array} \)**

**\(\begin{array}{l}m( _{ 6 }^{ 12 }{C} ) = \; 12.000000 \; u\end{array} \)**

**\(\begin{array}{l}m( _{ 10 }^{ 20 }{Ne} ) = \; 19.992439 \; u\end{array} \)**

**Ans:**

(i) The given nuclear reaction is:

It is given that:

Atomic mass

Atomic mass

Atomic mass

According to the question, the Q-value of the reaction can be written as:

Q = [

^{2}

= [ 1.007825 + 3.016049 – 2 x 2.014102] c^{2}

Q = ( – 0.00433 c^{2}) u

But 1 u = 931.5 MeV/c^{2}

Q = -0.00433 x 931.5 = – 4.0334 MeV

The negative Q-value of the reaction shows that the reaction is endothermic.

(ii) The given nuclear reaction is:

It is given that:

Atomic mass of

Atomic mass of

Atomic mass of

The Q-value of this reaction is given as:

Q = [

^{2}

= [ 2 x 12.000000 – 19.992439 – 4.002603 ] c^{2}

= [ 0.004958 c^{2}] u

= 0.004958 x 931.5 = 4.618377 MeV

Since we obtained a positive Q-value, it can be concluded that the reaction is exothermic.

* ***Q 13.16: Suppose, we think of fission of a**

**\(\begin{array}{l}_{ 26 }^{ 56 }{Fe}\end{array} \)**

**nucleus into two equal fragments,**

**\(\begin{array}{l}_{ 13 }^{ 28 }{Al}\end{array} \). Is fission energetically possible? Argue by working out the Q of the process. Given,**

**\(\begin{array}{l}m( _{ 26 }^{ 56 }{Fe} ) = \; 55.93494 \; u\end{array} \)**

**\(\begin{array}{l}m( _{ 13 }^{ 28 }{Al} ) = \; 27.98191 \; u\end{array} \)**

**Ans:**

The fission of

It is given that:

Atomic mass of

Atomic mass of

The Q-value of this nuclear reaction is given as:

Q = [

^{2}

= [ 55.93494 – 2 x 27.98191 ]c^{2}

= ( -0.02888 c^{2}) u

But, 1 u = 931.5 MeV/c^{2}

Q = – 0.02888 x 931.5 = -26.902 MeV

Since the Q-value is negative for fission, it is energetically not possible.

**Q 13.17: The fission properties**

**\(\begin{array}{l}_{ 94 }^{ 239 }{ Pu }\end{array} \)**

**of are very similar to those of\(\begin{array}{l}_{ 92 }^{ 235 }{ U }\end{array} \). The average energy released per fission is 180 MeV. How much energy is released if all the atoms in 1 kg of pure \(\begin{array}{l}_{ 94 }^{ 239 }{ Pu }\end{array} \) undergo fission? **

**Ans:**

Average energy released per fission of

_{av }= 180 MeV

Amount of pure

N_{A}= Avogadro number = 6.023 × 10^{23}

Mass number of

1 mole of

_{A}atoms.

Therefore, mg of

The total energy released during the fission of 1 kg of

E = E_{av }x 2.52 x 10^{24}

= 180 x 2.52 x 10^{24}

= 4.536 x 10^{26} MeV

Hence, 4.536 x 10^{26} MeV is released if all the atoms in 1 kg of pure

**Q 13.18: A 1000 MW fission reactor consumes half of its fuel in 5.00 y. How much _{92}^{235} U did it contain initially? Assume that the reactor operates 80% of the time, that all the energy generated arises from the fission **

**of**

_{92}^{235}U and that this nuclide is consumed only by the fission process.Ans:

The reactor consumes half of its fuel in 5 years. Therefore, the half-life of the fuel of the fission reactor, t_{1/2} = 5 x 365 x 24 x 60 x 60 s

200 MeV is released during the fission of 1 g of _{92}^{235}U.

1 mole, i.e., 235 g of _{92}^{235}U, contains 6.023 x 10^{23} atoms.

The number of atoms in 1 g of _{92}^{235}U is (6.023 x 10^{23}/235) atoms.

The total energy generated per gram of _{92}^{235}U is (6.023 x 10^{23}/235) x 200 MeV/g

= 8.20 x 10^{10} J/g

The reactor operates only 80% of the time.

Therefore, the amount of _{92}^{235}U consumed in 5 years by the 1000 MW fission reactor is

= 12614400/8.20

= 1538341 g

= 1538 Kg

Therefore, the initial amount of _{92}^{235}U is 2 x 1538 = 3076 Kg.

**Q 13.19: How long can an electric lamp of 100 W be kept glowing by fusion of 2.0 kg of deuterium? Take the fusion reaction as**

**\(\begin{array}{l}{ }_{1}^{2} \mathrm{H}+{ }_{1}^{2} \mathrm{H} \rightarrow{ }_{2}^{3} \mathrm{He}+\mathrm{n}+3.27 \mathrm{MeV}\end{array} \)**

Ans:

The fusion reaction given is

Amount of deuterium, m = 2 kg

1 mole, i.e., 2 g of deuterium, contains 6.023 x 10^{23} atoms.

Therefore, 2 Kg of deuterium contains (6.023 x 10^{23}/2) x 2000 = 6.023 x 10^{26} atoms

From the reaction given, it can be understood that 2 atoms of deuterium fuse, 3.27 MeV energy are released. The total energy per nucleus released during the fusion reaction is

E = (3.27/2) x 6.023 x 10^{26 }MeV

= (3.27/2) x 6.023 x 10^{26 }x 1.6 x 10^{ -19} x 10^{6}

= 1.576 x 10^{14} J

Power of the electric lamp, P = 100 W = 100 J/s

The energy consumed by the lamp per second = 100 J

The total time for which the electric lamp will glow is 1.576 x 10^{14} /100 = 1.576 x 10^{12}s

= (1.576 x 10^{12})/ (365 x 24 x 60 x 60)

= (1.576 x 10^{12})/3.1536 x10^{7}

= 4.99 x 10^{4 }years.

**Q 13.20: Calculate the height of the potential barrier for a head-on collision of two deuterons. (Hint: The height of the potential barrier is 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.)**

**Ans :**

When two deuterons collide head-on, the distance between their centres, d, is given as:

Radius of 1st deuteron + Radius of 2nd deuteron

Radius of a deuteron nucleus = 2 f m = 2 × 10^{−15} m

d = 2 × 10^{−15} + 2 × 10^{−15} = 4 × 10^{−15} m

Charge on a deuteron nucleus = Charge on an electron = e = 1.6 × 10^{−19} C

The potential energy of the two-deuteron system:

Where,

Therefore,

= 360 k eV

Hence, the height of the potential barrier of the two-deuteron system is 360 k eV.

**Q 13.21: 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).**

Ans:

The nuclear radius is given as

R = R_{0}A^{1/3}

Here,

R_{0} is Constant

A is the mass number of the nucleus

Nuclear matter density, ρ = Mass of the nucleus/Volume of the nucleus

Mass of the nucleus = mA

Density of the nucleus = (4/3)πR^{3}

= (4/3)π(R_{0}A^{1/3})^{3}

= (4/3)πR_{0}^{3}A

ρ = mA/[(4/3)πR_{0}^{3}A]

= 3mA/(4πR_{0}^{3}A)

ρ = 3m/(4πR_{0}^{3})

Hence, nuclear matter density is nearly a constant and is independent of A.

**Q 13.22: For the **

**\(\begin{array}{l}\beta^{+}\end{array} \)**

**(positron) emission from a nucleus, there is another competing process known as electron capture (Electron from an inner orbit, say, the K−shell, is captured by the nucleus and a neutrino is emitted).**

**\(\begin{array}{l}e^+ + _{Z}^{A}{X} \rightarrow _{ Z-1 }^{A}{Y}+v\end{array} \)**

**Show that if**

**\(\begin{array}{l}\beta^{+}\end{array} \)**

**emission is energetically allowed, electron capture is necessarily allowed but not vice−versa.**

**Ans :**

The chemical equations of the two competing processes are written as follows:

The Q-value of the positron emission reaction is determined as follows :

The Q-value of the electron capture reaction is determined as follows:

In the equation, m_{N }is the mass of the nucleus, and m denotes the mass of the atom.

From equations (1) and (2), we understand that,

Q_{1} = Q_{2} – 2m_{e}c^{2}

From the above relation, we can infer that if Q1 > 0, then Q2 > 0; Also, if Q2> 0, it does not necessarily mean that Q1 > 0.

In other words, this means that if $β$^{ }emission is energetically allowed, then the electron capture process is necessarily allowed, but not vice-versa. This is because the Q-value must be positive for an energetically allowed nuclear reaction.

**Q 13.23: In a periodic table, the average atomic mass of magnesium is given as 24.312 u. The average value is based on their relative natural abundance on earth. The three isotopes and their masses are ^{24}_{12}Mg (23.98504u), ^{25}_{12}Mg(24.98584u) and ^{26}_{12}Mg (25.98259u). The natural abundance of ^{24}_{12}Mg is 78.99% by mass. Calculate the abundances of the other two isotopes.**

Ans:

Let the abundance of ^{25}_{12}Mg be x%

The abundance of ^{26}_{12}Mg = (100 – 78.99 -x)%

= (21.01 – x)%

The average atomic mass of magnetic

24.312 x 100 = 1894.57 + 24.98584x + 545. 894 – 25.98259x

2431.2 = 2440.46 – 0.99675x

0.99675x = 2440.46 – 2431.2

0.99675x = 9.26

x = 9.26/0.99675

x = 9.290%

The abundance of ^{25}_{12}Mg is 9.290%

The abundance of ^{26}_{12}Mg = (21.01 – x)%

= (21.01 -9.290 )% = 11.71%.

**Q 13.24: The neutron separation energy is defined as the energy required to remove a neutron from the nucleus. Obtain the neutron separation energies of the nuclei ^{41}_{20}Ca and ^{27}_{13}Al from the following data:**

**m(**

^{40}_{20}Ca ) = 39.962591 u**m(**

^{41}_{20}Ca ) = 40.962278 u**m(**

^{26}_{13}Al ) = 25.986895 u**m(**

^{27}_{13}Al ) = 26.981541 uAns:

When the nucleons are separated from_{ 20}Ca^{41} → _{20} Ca^{40} + _{0}n^{1}

Mass defect, Δm = m (_{20} Ca^{40}) + m_{n} – m(_{20} Ca^{41})

= 39.962591 + 1.008665 – 40.962278

= 0.008978 amu

Neutron separation energy = 0.008978 x 931 MeV = 8.362 MeV

Similarly, _{13}Al^{27} → _{13} Al^{26} + _{0}n^{1}

Mass defect, Δm = m (_{13} Al^{26}) + m_{n} – m(_{13}Al^{27})

= 25.986895 + 1.008665 – 26.981541

= 26.99556 – 26.981541 = 0.014019 amu

Neutron separation energy = 0.014019 x 931 MeV

= 13.051 MeV.

**Q 13.25: A source contains two phosphorous radionuclides ^{32}_{15}P (T_{1/2} = 14.3d) and ^{33}_{15}P(T_{1/2} = 25.3d). Initially, 10% of the decays come from ^{33}_{15} P. How long one must wait until 90% do so?**

Ans:

The rate of disintegration is

– (dN/dt) ∝ N

Initially, 10% of the decay is due to ^{33}_{15} P, and 90% of the decay comes from ^{32}_{15}P.

We have to find the time at which 90% of the decay is due to ^{33}_{15} P and 10% of the decay comes from ^{32}_{15}P.

Initially, if the amount of ^{33}_{15} P is N, then the amount of ^{32}_{15}P is 9N.

Finally, if the amount of ^{33}_{15} P is 9N’, then the amount of ^{32}_{15}P is N’.

For ^{32}_{15}P,

For ^{33}_{15} P

On dividing (1) by (2),

Hence, it will take about 208.5 days for a 90% decay of _{15}P^{33}.

**Q 13.26: Under certain circumstances, a nucleus can decay by emitting a particle more massive than an α-particle. Consider the following decay processes: **

**\(\begin{array}{l}{ }_{88}^{223} \mathrm{Ra} \rightarrow{ }_{82}^{209} \mathrm{~Pb}+{ }_{6}^{14} \mathrm{C}\end{array} \)**

**\(\begin{array}{l}{ }_{88}^{223} \mathrm{Ra} \rightarrow{ }_{86}^{219} \mathrm{Rn}+{ }_{2}^{4} \mathrm{He}\end{array} \)**

**Calculate the Q-values for these decays and determine that both ****are energetically allowed.**

Ans:

Δm = m (_{88}Ra^{223}) – m (_{82}Pb^{209}) – m(_{6}C^{14})

= 223.01850 – 208.9817 – 14.00324

= 0.03419 u

The amount of energy released is given as

Q = Δm x 931 MeV

= 0.03419 x 931 MeV = 31.83 MeV

Δm = m (_{88}Ra^{223}) – m (_{86}Rn^{219}) – m(_{2}He^{4})

= 223.01850 – 219.00948 – 4.00260

= 0.00642 u

Q = 0.00642 x 931 MeV = 5.98 MeV

As both the Q-factor values are positive, the reaction is energetically allowed.

**Q 13.27: Consider the fission of ^{238}_{92}U by fast neutrons. In one fission event, no neutrons are emitted, and the final end products, after the beta decay of the primary fragments, are ^{140}_{58}Ce and ^{99}**

_{44}Ru. Calculate Q-value for this fission process. The relevant atomic and particle masses are as follows:**m( ^{238}_{92}U ) = 238.05079 u**

**m(**

^{140}_{58}Ce ) = 139.90543 u**m(**

^{99}_{44}Ru ) = 98.90594 uAns:

In the fission of ^{238}_{92}U, 10 β^{–} particles decay from the parent nucleus. The nuclear reaction can be written as:

Given:

m_{1} = (^{238}_{92}U ) =238.05079 u

m_{2} = (^{140}_{58}Ce ) =139.90543 u

m_{3} = (^{99}_{44}Ru ) = 98.90594 u

m_{4} = ^{1}_{0}n = 1.008665 u

The Q value is given as

Q = [ m'(^{238}_{92}U ) + m (^{1}_{0}n) – m'(^{140}_{58}Ce ) – m'(^{99}_{44}Ru ) – 10m_{e} ] c^{2}

Here,

m’ is the atomic masses of the nuclei

m'(^{238}_{92}U ) = m_{1} – 92 m_{e}

m'(^{140}_{58}Ce ) = m_{2} – 58m_{e}

m'(^{99}_{44}Ru ) = m_{3} – 44m_{e}

Therefore, Q = [m_{1} – 92 m_{e}+ m_{4} – m_{2} + 58m_{e }– m_{3} + 44m_{e}– 10m_{e} ] c^{2}

= [m_{1} + m_{4} – m_{2} – m_{3}]c^{2}

= [238.0507 + 1.008665 – 139.90543 – 98.90594]c^{2}

= 0.247995 c^{2 } u

^{2}]

Q = 0.247995 x 931.5 = 231.007 MeV

Hence, the Q-value of the fission process is 231.007 MeV.

**Q 13.28: Consider the D-T reaction (deuterium-tritium fusion)**

**\(\begin{array}{l}{ }_{1}^{2} \mathrm{H}+{ }_{1}^{3} \mathrm{H} \rightarrow{ }_{2}^{4} \mathrm{He}+\mathrm{n}\end{array} \)**

**(a) Calculate the energy released in MeV in this reaction from the following ****data:****m( ^{2}_{1}H ) = 2.014102 u**

**m(**

^{3}_{1}H ) = 3.016049 u**(b) Consider the radius of both deuterium and tritium to be approximately 2.0 fm. What is the kinetic energy needed to overcome the coulomb repulsion between the two nuclei? To what temperature must the gas be heated to initiate the reaction? (Hint: Kinetic energy required for one fusion event =average thermal kinetic energy available with the interacting particles = 2(3kT/2); k = Boltzman’s constant, T = absolute temperature.)**

Ans:

The Q-value is given as

Q = Δm x 931 MeV

= (m (_{1}H^{2}) + m(_{1}H^{3}) – m(_{2}He^{4}) – m_{n}) x 931

= (2.014102 + 3.016049- 4.002603-1.00867) x 931

Q = 0.0188 x 931 = 17.58 MeV

Therefore, the energy released in the given reaction is 17.58 MeV.

(b) Repulsive potential energy of two nuclei when they touch each other is

Where ε_{0} = Permittivity of free space

^{9}Nm

^{2}C

^{-2}

= (23.04 x 10^{-29})/(4 x 10^{-15})

= 5.76 x 10^{-14} J

The kinetic energy needed to overcome the **c**oulomb repulsion between the two nuclei is

K.E = 5. 76 x 10^{-14} J

K.E= 2 x (3/2) KT

K is the Boltzmann constant = 1.38 x 10^{-23}

T = K.E/3K = 5. 76 x 10^{-14}/(3 x 1.38 x 10^{-23})

= 5. 76 x 10^{-14}/(4.14 x 10^{-23})

= 1.3913 x 10^{9} K

Therefore, the kinetic energy needed to overcome the **c**oulomb repulsion between the two nuclei is 1.3913 x 10^{9} K.

**Q 13.29: Obtain the maximum kinetic energy of β-particles and the radiation frequencies of γ decay in the decay scheme shown in Fig. 13.6. You are given that **

**m( ^{198}Au) = 197.968233 u**

**m(**

^{198}Hg) =197.966760 uAns:

From the γ decay diagram, it can be seen that γ_{1} decays from the 1.088 MeV energy level to the 0 MeV energy level.

The energy corresponding to γ_{1} decay is given as

E_{1} = 1.088−0=1.088MeV

hv_{1} = 1.088×1.6×10^{−19} ×10^{6}J

Where,

h = Planck’s constant = 6.6 x 10^{-34} Js

v_{1} = frequency of radiation radiated by γ_{1} decay.

From the diagram, it can be observed that γ_{2} decays from the 0.412 MeV energy level to the 0 MeV energy level.

The energy corresponding to γ_{2} decay is given as

E_{2} = 0.412−0=0.412MeV

hv_{2} = 0.412×1.6×10^{−19} × 10^{6} J

Where v_{2} is the frequency of the radiation radiated by γ_{2} decay.

Therefore, v_{2} =E_{2}/h

^{19}Hz

From the gamma decay diagram, it can be γ

_{3}decays from the 1.088 MeV energy level to the 0.412 MeV energy level.

The energy corresponding to γ_{3} is given as

E_{3} = 1.088−0.412=0.676 MeV

hv_{3} = 0.676×10^{−19} ×10^{6} J

Where v_{3} = frequency of the radiation radiated by γ_{3} decay

Therefore, v_{3} =E_{3}/h

=1.639×10^{20} Hz

The mass of Au is 197.968233 u

Mass of Hg = 197.966760 u

[1u=931.5MeV/c^{2}]

The energy of the highest level is given as:

E=[m(^{198}_{78}Au−m(^{190}_{80}Hg)]

=197.968233−197.96676=0.001473u

=0.001473×931.5=1.3720995MeV

β_{1} decays from the 1.3720995 MeV level to the 1.088 MeV level

Therefore, the maximum kinetic energy of the β_{1} particle =1.3720995−1.088=0.2840995MeV

β_{2} decays from the 1.3720995 MeV level to the 0.412 MeV level

Therefore, the maximum kinetic energy of the β_{2} particle = 1.3720995−0.412 = 0.9600995.

**Q 13.30. Calculate and compare the energy released by a) the fusion of 1.0 kg of hydrogen deep within the sun and b) the fission of 1.0 kg of ^{235}U in a fission reactor.**

Ans:

Amount of hydrogen, m = 1 kg = 1000 g

(a) 1 mole, i.e., 1 g of hydrogen, contains 6.023 x 10^{23} atoms. Therefore, 1000 g of hydrogen contains 6.023 x 10^{23 }x 1000 atoms.

In the sun, four hydrogen nuclei fuse to form a helium nucleus and will release the energy of 26 MeV

The energy released by the fusion of 1 kg of hydrogen,

E_{1} = (6.023 x 10^{23 }x 1000 x 26)/4

E_{1}= 39.16 x 10^{26} MeV.

(b) Amount of ^{235}U = 1 kg = 1000 g

1 mole, i.e., 235 g of uranium, contains 6.023 x 10^{23} atoms. Therefore, 1000 g of uranium contains (6.023 x 10^{23 }x 1000)/235 atoms.

The energy released during the fission of one atom of ^{235}U = 200 MeV

The energy released by the fission of 1 Kg of ^{235}U,

E_{2 }= (6.023 x 10^{23 }x 1000 x 200)/235

E_{2} = 5.1 x 10^{26} MeV

(E_{1}/E_{2}) = (39.16 x 10^{26}/5.1 x 10^{26})

= 7.67

The energy released in the fusion of hydrogen is 7.65 times more than the energy released during the fission of Uranium.

**Q 13.31. Suppose India had a target of producing, by 2020 AD, 200,000 MW of electric power, ten per cent of which was to be obtained from nuclear power plants. Suppose we are given that, on average, the efficiency of utilisation (i.e., conversion to electric energy) of thermal energy produced in a reactor was 25%. How much amount of fissionable uranium would our country need per year by 2020? Take the heat energy per fission of ^{235}U to be about 200MeV.**

Ans:

Electric power to be generated = 2 x 10^{5} MW = 2 x 10^{5 }x 10^{6} J/s = 2 x 10^{11} J/s

10 % of the amount is obtained from the nuclear power plant

P_{1} = (10/100) x 2 x 10^{11 }x 60 x 60 x 24 x 365 J/year

The heat energy released during per fission of a ^{235}U nucleus, E = 200 MeV

Efficiency of the reactor = 25%

The amount of energy converted to electrical energy in fission is (25/100) x 200 = 50 MeV

= 50 x 1.6 x 10^{-19} x 10^{6 } = 80 x 10^{-13} J

Required number of atoms for fission per year

= [(10/100) x 2 x 10^{11 }x 60 x 60 x 24 x 365]/80 x 10^{-13}

= 788400 x 10^{23 }atoms

1 mole, i.e., 235 g of ^{235}U, contains 6.023 x 10^{23} atoms

Mass of 6.023 x 10^{23} atoms of ^{235}U = 235 x 10^{-3} kg

Mass of 788400 x 10^{23 }atoms of ^{235}U

=[ (235 x 10^{-3})/(6.023 x 10^{23}) ] x 78840 x 10^{24 }

= 3.076 x 10^{4} Kg

Hence, the mass of uranium needed per year is 3.076 x 10^{4} Kg.

## Class 12 Physics NCERT Solutions for Chapter 13 Nuclei

**Chapter 13 Nuclei of Class 12 Physics** is well-structured as per the latest CBSE Syllabus 2023-24. One of the best methods to prepare for Physics is by taking notes and writing down all the important formulas and points in a notebook using the **NCERT Solutions for Class 12**. The notes will also help students during the time of revision. Once you get an idea about the type of questions asked in the examination, it is easy for you to collect important points and prepare Class 12 Physics Chapter 13 notes.

### Subtopics of Class 12 Physics Chapter 13 Nuclei

- Introduction
- Atomic Masses and Composition of Nucleus
- Size of the Nucleus
- Mass energy and Nuclear Binding Energy
- Mass energy
- Nuclear binding energy

- Nuclear Force
- Radioactivity
- Law of radioactive decay
- Alpha decay
- Beta-decay
- Gamma decay

- Nuclear Energy
- Fission
- Nuclear reactor
- Nuclear fusion – energy generation in stars
- Controlled thermonuclear fusion.

Also Access |

NCERT Exemplar for Class 12 Physics Chapter 13 |

CBSE Notes for Class 12 Physics Chapter 13 |

Physics is one of the most interesting and scoring subjects in the Class 12 board examination. But, students must learn the concepts in-depth to score well in the board exams. Students can practise these NCERT Solutions to prepare well for the board exams. Class 12 Physics Chapter 13 Nuclei are one of the most interesting topics in Physics Class 12. Students must prepare this topic effectively to score well in Physics subject, in the board examination.

Students should have a detailed knowledge of all the subtopics and important points of the chapter during their preparation. In order to make students thorough with the concepts covered in chapter Nuclei, apart from **NCERT Solutions**, BYJU’S introduces an interactive method of learning with videos and animations.

BYJU’S interactive teaching methods will assist students in getting complete knowledge of the topic. Students also can download study materials, sample papers, exercises, previous years’ question papers, prepared notes and textbooks.

**Disclaimer –**

**Dropped Topics – **

13.6.1 Law of Radioactive Decay

13.6.2 Alpha Decay

13.6.3 Beta Decay

13.6.4 Gamma Decay

13.7.2 Nuclear Reactor

Exercises 13.1, 13.2, 13.6–13.10, 13.12–13.14, 13.18, 13.22–13.31

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