## NCERT Solutions for Class 12 Physics Chapter 13 â€“ Nuclei â€“ FREE PDF Download

CoolGyan provides All Chapter 13 â€“ Nuclei Exercise Questions with Solutions to help you to revise complete Syllabus and Score More marks.

**NCERT Solutions for Class 12 Physics Chapter 13 Nuclei**Â includes all the important topics with detailed explanation that aims to help students to understand the concepts better. Students who are preparing for their Class 12 exams must go through NCERT Solutions forÂ **Class 12 Physics Chapter 13 Nuclei**. Going through the solutions provided on this page will help you to know how to approach and solve the problems.

Revision Notes Class 12 Physics

NCERT Solutions Class 12 Physics

### Subtopics of class 12Â physics chapter 13 Nuclei

- Introduction
- Atomic Masses And Composition Of Nucleus
- Size Of The Nucleus
- Mass-energy And Nuclear Binding Energy
- Mass â€“ Energy
- Nuclear binding energy

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

- Nuclear Energy
- Fission
- Nuclear reactor
- Nuclear fusion â€“ energy generation in stars
- Controlled thermonuclear fusion.

## Class 12 Physics NCERT Solutions Nuclei Questions

**NCERT Exercises**

**Question 1.**

(a)Â Two stable isotopes of lithiumÂ Â andÂ Â have respective abundances of 7.5% and 92.5%. These isotopes have masses 6.01512 u and 7.01600 u respectively. Find the atomic mass of lithium.

(b)Â Boron has two stable isotope.s,Â Â andÂ Â Their respective masses are 10.01294 u and 11.00931 u, and the atomic mass of boron is

10.811 u. Find the abundances ofÂ Â andÂ .

(a)Â Two stable isotopes of lithiumÂ Â andÂ Â have respective abundances of 7.5% and 92.5%. These isotopes have masses 6.01512 u and 7.01600 u respectively. Find the atomic mass of lithium.

(b)Â Boron has two stable isotope.s,Â Â andÂ Â Their respective masses are 10.01294 u and 11.00931 u, and the atomic mass of boron is

10.811 u. Find the abundances ofÂ Â andÂ .

**Solution:**

Abundance ofÂ Â is 7.5% and abundance

**(b)**Â Let abundance ofÂ Â x% than abundance ofÂ Â will be (100 â€“ x)%.

**Question 2.**

The three stable isotopes of neon :Â ,Â Â and have respective abundances of 90.51%, 0.27% and 9.22%. 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.

The three stable isotopes of neon :Â ,Â Â and have respective abundances of 90.51%, 0.27% and 9.22%. 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.

**Solution:**

Average atomic mass of neon with the given abundances,

**Question 3.**

Obtain the binding energy (in MeV) of a nitrogen nucleusÂ , given mÂ Â = 14.00307 u

Obtain the binding energy (in MeV) of a nitrogen nucleusÂ , given mÂ Â = 14.00307 u

**Solution:**

The nucleus contains 7 protons andÂ

**Question 4.**

Obtain the binding energy of the nucleiÂ Â andÂ Â in units of MeV from the following data:

Â = 55.934939 u

Â = 208.980388 u

Obtain the binding energy of the nucleiÂ Â andÂ Â in units of MeV from the following data:

Â = 55.934939 u

Â = 208.980388 u

**Solution:**

Let us first find the binding energy of

**Question 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 ofÂ
Â atoms (of mass 62.92960 u). **

**Solution:**

Let us first find the B.E. of each copper nucleus and then we can find binding energy

**Question 6.**

Write nuclear reaction equations for

(i) a-decay ofÂ

(ii) a-decay ofÂ

(iii) p-decay ofÂ

(iv) p-decay ofÂ

(v) p+-decay ofÂ

(vi) p+-decay ofÂ

(vii) Electron capture ofÂ

Write nuclear reaction equations for

(i) a-decay ofÂ

(ii) a-decay ofÂ

(iii) p-decay ofÂ

(iv) p-decay ofÂ

(v) p+-decay ofÂ

(vi) p+-decay ofÂ

(vii) Electron capture ofÂ

**Solution:**

**Question 7.**

A radioactive isotope has a half-life of T years. How long will it take the activity to reduce to

(a)Â 3.125%

(b)Â 1% of its original value?

Required time, as cannot be solved by direct calculation as in part (a).

A radioactive isotope has a half-life of T years. How long will it take the activity to reduce to

(a)Â 3.125%

(b)Â 1% of its original value?

Required time, as cannot be solved by direct calculation as in part (a).

**Solution:**

Required time, as cannot be solved by direct calculation as in part (a)

**Question 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 radioactiveÂ Â present with the stable carbon isotopeÂ . 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) ofÂ , and the measured activity, the age of the specimen can be approximately estimated. This is the principle ofÂ Â 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 civilization.

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 radioactiveÂ Â present with the stable carbon isotopeÂ . 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) ofÂ , and the measured activity, the age of the specimen can be approximately estimated. This is the principle ofÂ Â 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 civilization.

**Solution:**

In order to estimate age, let us first find the activity ratio in form of time â€˜tâ€™. Given normal activity, R0 = 15 decays min

^{-1}Â Present activity, R = 9 decays min

^{-1}, Tin = 5730 years Since activity is proportional to the number of radioactive atoms, therefore,

**Question 9.**

Obtain the amount ofÂ Â necessary to provide a radioactive source of 8.0 mCi strength. The half-life ofÂ Â is 5.3 years.

Obtain the amount ofÂ Â necessary to provide a radioactive source of 8.0 mCi strength. The half-life ofÂ Â is 5.3 years.

**Solution:**

Here rate of disintegration required

As 1 mole i.e., 60 g of cobalt contains 6.023 Ã— 10

^{23}Â atoms, so, the mass of cobalt required for given rate of disintegration

**Question 10.**

The half-life ofÂ Â is 28 years. What is the disintegration rate of 15 mg of this isotope?

The half-life ofÂ Â is 28 years. What is the disintegration rate of 15 mg of this isotope?

**Solution:**

**Question 11.**

Obtain approximately the ratio of the nuclear radii of the gold isotopeÂ Â and the silver isotopeÂ .

Obtain approximately the ratio of the nuclear radii of the gold isotopeÂ Â and the silver isotopeÂ .

**Solution:**

We know the radius of nucleus depend upon mass number â€˜Aâ€™

**Question 12.**

Find the Q-value and the kinetic energy of the emitted a-particle in the a-decay of

(a)Â Â and (b)Â .

GivenÂ Â = 226.02540 u,

Â = 222.01750 u,

Â = 220.01137 u,

Â = 216.00189 u, and

m

Find the Q-value and the kinetic energy of the emitted a-particle in the a-decay of

(a)Â Â and (b)Â .

GivenÂ Â = 226.02540 u,

Â = 222.01750 u,

Â = 220.01137 u,

Â = 216.00189 u, and

m

_{x}Â = 4.00260 u.**Solution:**

**Question 13.**

The radionuclide â€œC decays according toÂ Â + e

The maximum energy of the emitted positron is 0.960 MeV.

Given the mass values:

Â = 11.011434 u

Â = 11.009305 u

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

The radionuclide â€œC decays according toÂ Â + e

^{+}Â + v: T_{1/2}Â = 20.3 minThe maximum energy of the emitted positron is 0.960 MeV.

Given the mass values:

Â = 11.011434 u

Â = 11.009305 u

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

**Solution:**

The given equation

As we know that different positrons comes out with different possible energies shared between daughter nucleus and positron.

So, the Q value of reaction is almost same as the maximum energy of positron emitted.

**Question 14.**

The nucleusÂ Â decays by Î²

r the maximum kinetic energy of the electrons emitted. Given that:

Â = 22.994466 amu,

Â = 22.989770 amu.

The nucleusÂ Â decays by Î²

^{â€“}Â emission. Write t down the Î²^{â€“Â }decay equation and determiner the maximum kinetic energy of the electrons emitted. Given that:

Â = 22.994466 amu,

Â = 22.989770 amu.

**Solution:**

The Î²

^{â€“}Â decay ofÂ Â may be explained as

AsÂ Â is massive, the kinetic energy released is mainly shared by electron-positron pair. When the neutrino carries no energy, the electron has a maximum kinetic energy equal to 4.374 MeV.

**Question 15.**

The Q value of a nuclear reaction A + bâ€”>C+d is defined by Q = [m

(i)Â

(ii)Â

Atomic masses are given to be

The Q value of a nuclear reaction A + bâ€”>C+d is defined by Q = [m

_{A}Â + m_{b}-m_{c}â€“ 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)Â

(ii)Â

Atomic masses are given to be

**Solution:**

**(i)**Â Let us find the Q value in given first equation,

Negative Q value shows that reaction is endothermic.

**(ii)**Â Q value in the given second equation

Positive Q shows that the reaction is exothermic.

**Question 16.**

Suppose, we think of fission of aÂ Â nucleus into two equal fragments,Â . IS the fission energetically possible? Argue by working out Q of the process.

Given,Â Â = 55.93494 u

andÂ Â = 27.98191 u.

Suppose, we think of fission of aÂ Â nucleus into two equal fragments,Â . IS the fission energetically possible? Argue by working out Q of the process.

Given,Â Â = 55.93494 u

andÂ Â = 27.98191 u.

**Solution:**

The fission of Fe-56 into two fragments of

As the Q-value is negative, the fission is not possible energycally.

**Question 17.**

The fission properties ofÂ Â are very similar to those ofÂ . The average energy released per fission is 180 MeV. How much energy, in MeV, is released if all the atoms in 1 kg of pureÂ Â undergo fission?

The fission properties ofÂ Â are very similar to those ofÂ . The average energy released per fission is 180 MeV. How much energy, in MeV, is released if all the atoms in 1 kg of pureÂ Â undergo fission?

**Solution:**

**Question 18.**

A 1000 MW fission reactor consumes half of its fuel in 5.00 y. How muchÂ Â did it contain initially? Assume that the reactor operates 80% of the time and that all the energy generated arises from the fission ofÂ and that this nuclide is consumed by the fission process.

A 1000 MW fission reactor consumes half of its fuel in 5.00 y. How muchÂ Â did it contain initially? Assume that the reactor operates 80% of the time and that all the energy generated arises from the fission ofÂ and that this nuclide is consumed by the fission process.

**Solution:**

In the fission of one nucleus ofÂ , energy generated is 200 MeV.

**Question 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

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

**Solution:**

Number of atoms present in 2 g of deuterium = 6.023 Ã— 10

^{23}Â Total number of atoms present in 2000 g of deuterium

Energy released in the fusion of 2 deuterium atoms = 3.27 MeV

**Question 20.**

Calculate the height of potential barrier for a head-on collision of two deuterons. The effective radius of deuteron can be taken to be 2fm.

Calculate the height of potential barrier for a head-on collision of two deuterons. The effective radius of deuteron can be taken to be 2fm.

**Solution:**

For head on collision, distance between centers of two deuterons

This is a measure of height of coulomb barrier.

Question 21.

From the relation R = R

Question 21.

From the relation R = R

_{0}Â A^{1/3}, where R_{2}Â 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).**Solution:**

As R is constant, p is contact so, nuclear density is constant irrespective of mass number or size.

**Question 22.**

For the Î²

Show that if Î²

For the Î²

^{+}Â (positron) emission from a nucleus, there is another competing process known as electron capture (electron from an inner orbit, say, the /(-shell, is captured by the nucleus and a neutrino is emitted).Show that if Î²

^{+}Â emission is energetically allowed, electron capture is necessarily allowed but not vice-versa.**Solution:**

Let us first consider positron emission.

This mean if Q

_{1}Â > 0 then Q

_{2}Â > 0 but vice vesa is not necessarily allowed. So, electron capture is not necessary for positron emission.

**Question 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 areÂ Â (23.98504 u),Â Â (24.98584 u) andÂ Â (25.98259 u).The natural abundance ofÂ Â 78.99% by mass. Calculate the abundances of the other two isotopes.

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 areÂ Â (23.98504 u),Â Â (24.98584 u) andÂ Â (25.98259 u).The natural abundance ofÂ Â 78.99% by mass. Calculate the abundances of the other two isotopes.

**Solution:**

Let the abundance of isotopeÂ Â is

**Question 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 nucleiÂ Â andÂ Â from

the following data:

The neutron separation energy is defined as the energy required to remove a neutron from the nucleus. Obtain the neutron separation energies of the nucleiÂ Â andÂ Â from

the following data:

**Solution:**

Neutron separation ofÂ Â can be obtained as E = Energy equivalent of total mass afterward â€“ Energy equivalent of nucleus before

**Question 25.**

A source contains two phosphorus radio -nuclidesÂ Â (T

A source contains two phosphorus radio -nuclidesÂ Â (T

_{1/2}Â = 14.3 days) andÂ Â (Tv2 = 25.3 days). Initially, 10% of the decays come fromÂ Â How long one must wait until 90% do so?**Solution:**

In the mixture of P-32 and P-33 initially 10% decay came from P-33. Hence initially 90% of the mixture is P-32 and 10% of the mixture is P-33. Let after timeâ€™tâ€™ the mixture is left with 10% of P-32 and 90% of P-33. Half life of both P-32 and P-33 are given as 14.3 days and 25.3 days respectively. Let V be total mass undecayed initially and â€˜yâ€™ be total mass undecayed finally. Let initial number of P-32 nuclides = 0.9 x Final number of P-32 nuclides = 0.1 y Similarly, initial number of P-33 nuclides = 0.l x Final number of P-33 nuclides = 0.9 y For isotope P-32

**Question 26.**

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

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

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

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

**Solution:**

Let us calculate Q value for the given decay process. For first decay process

Since, Q value is positive in both the cases, hence decay process in both ways are possible

**Question 27.**

Consider the fission ofÂ Â by fast neutrons. In one fission event, no neutrons are emitted and the final end products, after the beta decay of the primary fragments areÂ Â andÂ . Calculate Qfor this fission process. The relevant atomic and particle masses are:

Consider the fission ofÂ Â by fast neutrons. In one fission event, no neutrons are emitted and the final end products, after the beta decay of the primary fragments areÂ Â andÂ . Calculate Qfor this fission process. The relevant atomic and particle masses are:

**Solution:**

The fission of U-238 by fast neutrons into fragments Ce-140 and Ru-99 with energy released Q can be written as

**Question 28.**

Consider the D-T reaction (deuterium â€“ tritium fusion)

(a)Â Calculate the energy released in MeV in this reaction from the data

mÂ Â = 2.014102 u, mÂ Â = 3.016049 u

(b)Â 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 gases be heated to initiate the reaction?

Consider the D-T reaction (deuterium â€“ tritium fusion)

(a)Â Calculate the energy released in MeV in this reaction from the data

mÂ Â = 2.014102 u, mÂ Â = 3.016049 u

(b)Â 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 gases be heated to initiate the reaction?

**Solution:**

Classically, K.E. atleast equal to this amount is required to overcome Coulomb repulsion. Using the relation

**Question 29.**

Obtain the maximum kinetic energy of Î²-particles and the radiation frequencies of y-decays in the decay scheme shown in figure. You are given that

Â = 197.968233 u,Â Â = 197.966760 u

Obtain the maximum kinetic energy of Î²-particles and the radiation frequencies of y-decays in the decay scheme shown in figure. You are given that

Â = 197.968233 u,Â Â = 197.966760 u

**Solution:**

Energy corresponding to y

_{1}

**Question 30.**

Calculate and compare the energy released by

(a)Â fusion of 1.0 kg of hydrogen deep within the Sun and

(b)Â the fission of 1.0 kg ofÂ

Calculate and compare the energy released by

(a)Â fusion of 1.0 kg of hydrogen deep within the Sun and

(b)Â the fission of 1.0 kg ofÂ

^{235}U in a fission reactor.**Solution:**

**(a)**Â In the fusion reactions taking place within core of sun, 4 hydrogen nuclei combines to form a helium nucleus with the release of 26 MeV of energy.

**(b)**Â Energy released per fission of U-235 is 200 MeV.

So the energy released in fusion of 1 kg of Hydrogen is nearly 8 times the energy released in fission of 1 kg of uranium-235.

**Question 31.**

Suppose India has a target of producing by 2020 AD, 200,000 MW of electric power, ten percent of which is to be obtained from nuclear power plants. Suppose we are given that, on an average, the efficiency of utilization (/.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? Take the heat energy per fission ofÂ

Suppose India has a target of producing by 2020 AD, 200,000 MW of electric power, ten percent of which is to be obtained from nuclear power plants. Suppose we are given that, on an average, the efficiency of utilization (/.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? Take the heat energy per fission ofÂ

^{235}U to be about 200 MeV.**Solution:**

10% of total power 200,000 MW to be obtained from nuclear power plant by 2020 AD.

Hence mass of uranium needed per year = 3.08 Ã— 10

^{4}Â kg