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Nuclei Class 12 Notes CBSE Physics Chapter 13 (Free PDF Download)

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Revision Notes for CBSE Class 12 Physics Chapter 13 (Nuclei) - Free PDF Download

In Class 12 CBSE Physics, Chapter 13 focuses on the topic of nuclei, which delves into the fundamental properties and behavior of atomic nuclei. This crucial chapter lays the foundation for a deeper understanding of nuclear physics and various nuclear phenomena. To assist you in your revision, this article provides comprehensive revision notes covering the essential concepts and key points of Nuclei Chapter 13. Let's dive in!


Comprehensive Revision Notes for CBSE Class 12 Physics Chapter 13: Nuclei

Structure of an Atom:

  • The atom consists of a central nucleus and extranuclear electrons.

  • The nucleus is positively charged and contains protons and neutrons.

  • Protons carry a positive charge, whereas neutrons are neutral.


Atomic Number (Z) and Mass Number (A):

  • Atomic number (Z) represents the number of protons in the nucleus.

  • Mass number (A) denotes the total number of protons and neutrons in the nucleus.

  • Number of neutrons (N) can be calculated as $N = A - Z$.


Nuclear Forces:

  • Nuclear forces are responsible for binding protons and neutrons together within the nucleus.

  • These forces are strong but short-range in nature.

  • They overcome the electrostatic repulsion between positively charged protons.


Nuclear Stability:

  • Stability of a nucleus depends on the ratio of protons to neutrons.

  • For light nuclei (Z<20), a roughly equal number of protons and neutrons ensures stability.

  • For heavier nuclei, the optimal ratio of neutrons to protons increases.


Radioactivity:

  • Radioactive decay is the spontaneous disintegration of an unstable nucleus.

  • Three common types of radioactive decay include alpha (α) decay, beta (β) decay, and gamma (γ) decay.

  • Alpha decay emits an alpha particle (helium nucleus), beta decay releases beta particles (electrons or positrons), and gamma decay emits high-energy photons.


Radioactive Decay Law:

  • Radioactive decay follows an exponential decay law: $N(t) = N_0 \times e^(-\lambda t)$.

  • N(t) represents the number of radioactive nuclei remaining at time t.

  • N₀ is the initial number of radioactive nuclei, and λ is the decay constant.


Half-Life:

  • Half-life $\left(T_{\frac{1}{2}}\right)$ is the time taken for half of the radioactive nuclei to decay.

  • It is related to the decay constant as $\left(T_{\frac{1}{2}}\right) = \dfrac{\ln(2)}{\lambda}$.


Nuclear Fission and Fusion:

  • Nuclear fission involves the splitting of a heavy nucleus into two lighter nuclei.

  • Nuclear fusion is the process of combining two light nuclei to form a heavier nucleus.

  • Both fission and fusion release an enormous amount of energy.


Nuclear Reactors:

  • Nuclear reactors utilize controlled fission reactions to generate electricity.

  • They consist of a nuclear fuel, moderators, control rods, and a coolant.

  • Control rods absorb excess neutrons to regulate the reaction.


Atoms Nucleus

  • The atomic nucleus is a small, dense region located at the center of an atom.

  • Ernest Rutherford discovered the atomic nucleus in 1911 through the Geiger-Marsden gold foil experiment conducted in 1909.

  • The nucleus is positively charged and consists of protons and neutrons.

  • Surrounding the nucleus is a cloud of negatively charged electrons.

  • The nucleus and electron cloud are bound together by electrostatic force.

  • The majority of the mass of an atom is concentrated in the nucleus.

  • The electron cloud contributes very little to the overall mass of the atom.

  • Protons and neutrons in the nucleus are held together by the nuclear force.


Download CBSE Class 12 Chapter-wise Physics Notes 2024-25 PDF

Also, check CBSE Class 12 Physics revision notes for all chapters:



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Nuclei Class 12 Notes Physics - Basic Subjective Questions


Section-A (1 Mark Questions)

1. What will be the ratio of the radii of two nuclei of mass numbers $A_{1}$ and $A_{2}$ ?  

Ans. The strong attractive nuclear force holds the nucleons together inside a nucleus which even overcomes the electrostatic repulsion between the protons. 


2. What holds nucleons together in a nucleus? 

Ans. The strong attractive nuclear force holds the nucleons together inside a nucleus which even overcomes the electrostatic repulsion between the protons. 


3. Define mass defect of a nucleus. How is it related to the binding energy of the nucleus? 

Ans. The difference between the sum of the rest masses of the nucleons constituting a nucleus and the rest mass of the nucleus is called mass defect. The binding energy of a nucleus is a measure of the energy equivalent to its mass defect $B\cdot E=\Delta m\times c^{2}$ 


4. In the nuclear decay reaction $_{1}^{1}\textrm{H}\rightarrow _{0}^{1}n+_{Q}^{P}\textrm{X}$ find P, Q and hence identify X. 

Ans. By conservation of mass, P=1-1=0.

By conservation of charge, 

Q=1-0=1

$\therefore X=_{1}^{0}\textrm{e}$

i.e., X is a positron.


5. What is the difference between an electron and a $\beta -$ particle? 

Ans. An electron and a $\beta -$ particle are essentially the same. An electron of nuclear origin is called a $\beta -$ particle. 


6. Why do lighter nuclei tend to fuse together? 

Ans. When lighter nuclei fuse together, they from heavier nuclei having greater binding energy per nucleon and they tend to attain a stable structure. 


7. State the reason, why heavy water is generally used as a moderator in a nuclear reactor. 

Ans. The basic principle of mechanics is that momentum transfer is maximum when the mass of colliding particle and target particle are equal. Heavy water has negligible absorption cross-section for neutrons and its mass is small; so heavy water molecules do not absorb fast neutrons; but simply slow them.


8. The sun is constantly losing mass due to thermonuclear fusion. Comment.

Ans. In each fusion reaction, a small mass of the sun changes into thermal energy. So, sun is constantly losing mass due to thermonuclear fusion.


9. Why is nuclear fusion not possible in a laboratory? 

Ans. Nuclear fusion requires extremely high temperature of $10^{6}-10^{7}K$ . This temperature is attained by causing explosion due to the fission process. Moreover, no solid container can withstand such a high temperature.


10. Two nuclei have mass number in the ratio 1:3. What is the ratio of their nuclear densities?

Ans. Since nuclear density is independent of the mass number, the ratio of nuclear densities will be 1:1.


Section-B (2 Marks Questions)

11. Write any two characteristic properties of nuclear force.

Ans. Characteristic properties of nuclear forces are: 

1. Nuclear force are strongest forces in nature: magnitude of nuclear forces is 100 times that of electrostatic force and $10^{38}$ times the gravitational forces.

2. Nuclear forces are charge independent: Nuclear forces between a pair of protons, a pair of neutrons or a pair of neutron and proton act with same strength.


12. Calculate the energy released in MeV in the following nuclear reaction:

$_{92}^{238}\textrm{U}\rightarrow _{90}^{234}Th+_{2}^{4}He+Q\left [ Mass\;of\;_{92}^{238}\textrm{U}=238\cdot 05079u \right ]$

$Mass\;of\;_{92}^{238}\textrm{U}=238\cdot 05079u$

$Mass\;of\;_{2}^{4}\textrm{th}=234\cdot 043630u$

$Mass\;of\;_{2}^{4}\textrm{He}=4\cdot 002600u$

$1u=931\cdot 5MeV/c^{2}$

Ans. Given nuclear reaction is $_{92}^{238}\textrm{U}\rightarrow _{90}^{234}Th+_{2}^{4}He+Q$

Mass defect $=M_{u}-M_{Th}-M_{He}=238\cdot 05079-234\cdot 043630-4\cdot 002600=0.00456u$

Energy released $=(0\cdot 00456u)\times (931\cdot 5MeV/C^{2})=4\cdot 25MeV$


13. Two nuclei have mass numbers in the ratio 1: 8 what is the ratio of their nuclear radii?

Ans. $A_{1}:A_{2}=1:8$

Since, 

$\Rightarrow \dfrac{A_{1}}{A_{2}}=\dfrac{1}{8}$

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

$\therefore \dfrac{R_{1}}{R_{2}}=\left ( \dfrac{A_{1}}{A_{2}} \right )^{1/3}=\left ( \dfrac{1}{8} \right )^{1/3}=\dfrac{1}{2}$


14. Give one similarity and one dissimilarity between nuclear fission and nuclear fusion.

Ans. Similarity. Both nuclear fission and fusion are the source of enormous amount of energy. Dissimilarity. In nuclear fission, a heavy nucleus splits into two smaller nuclei. In nuclear fusion, two smaller nuclei fuse together to form a heavier nucleus.


15. If both the number of protons and neutrons in a nuclear reaction is conserved, in what way is mass converted into energy (or vice versa)? Explain giving one example.

Ans. Since proton number and neutron number are conserved in a nuclear reaction, the total rest mass of neutrons and protons is the same on either side of the nuclear reaction. But total binding energy of nuclei on the left side need not be the same as that on the right hand side. The difference in binding energy causes a release of energy in the reaction. Examples:

(i) $_{1}^{2}\textrm{H}+_{1}^{2}\textrm{H}\rightarrow _{2}^{3}He+_{0}^{1}n+energy$

(ii) $_{92}^{235}\textrm{U}+_{0}^{1}N\rightarrow _{56}^{144}Ba+_{36}^{89}Kr+3_{0}^{1}n+energy$


16. Calculate the energy in fusion reaction:

$_{1}^{2}\textrm{H}+_{1}^{2}\textrm{H}\rightarrow _{2}^{3}He+n$ , Where B.E, of $_{1}^{2}\textrm{H}=2\cdot 23MeV$ and of $_{1}^{2}\textrm{He}=7\cdot 73MeV$

Ans. Total binding energy of initial system $(E_{i})$

$=_{1}^{2}\textrm{H}+_{1}^{2}\textrm{H}=(2\cdot 23+2\cdot 23)MeV=4\cdot 46MeV$

Binding energy of final system

i.e., $=_{1}^{2}\textrm{He}(E_{f})=7\cdot 73MeV$

Hence, energy released $=E_{f}-E_{i}$

$=7\cdot 73MeV-4\cdot 46MeV$

$=3\cdot 27MeV$


17. Distinguish between nuclear fission and fusion. Explain how the energy is released in both the processes.

Ans. In nuclear fission a heavy nucleus breaks up into smaller nuclei accompanied by release of energy; whereas in nuclear fusion two light nuclei combine to form a heavier nucleus accompanied by release of energy.

In both the cases, some mass (= mass defect) gets converted into energy as per the relation: $E=\Delta mc^{2}$


18. Show that the density of nucleus over a wide range of nuclei is constant- independent of mass number A. 

Ans. Let A be the mass number and R be the radius of a nucleus 

If m is the average mass of a nucleon, then

Mass of nucleus = mA

Volume of nucleus $=\dfrac{4}{3}\pi R^{3}$

$=\dfrac{4}{3}\pi (R_{0}A^{1/3})^{3}=\dfrac{4}{3}\pi R_{0}^{3}A$

∴ nuclear density $=\dfrac{mass\;of\;nucleus}{volume\;of\;nucleus}$

$=\dfrac{mA}{\dfrac{4}{3}\pi R_{0}^{3}A}=\dfrac{3m}{4\pi R_{0}^{3}}$

Clearly, nuclear density is independent of mass number A or the size of the nucleus.



19. Why is the binding energy per nucleon found to be constant for nuclei in the range of mass number (A) lying between 30 and 170? 

Ans. The binding energy is a very short order range force. So, beyond a certain amount of range the force has no influence beyond that. That is why after certain number the binding energy per nucleon do not change even if we add more nucleons.


PDF Summary - Class 12 Physics Nuclei Notes (Chapter 13)

By a factor of about 26,634 (uranium atomic radius is about 156 pm to 60250 (i.e. 156×10−12 m))  (the radius of hydrogen atomic is about 52.92 pm). We have seen and discussed in many of the subjects that there is a smallest unit which is often referred to as the cell and this whole cell was discussed in the biology of junior classes. Now if we see then we can notice that it's the same structure which we are discussing here about the cells is the atoms. The atom was also considered as the smallest unit on the planet until the quarks were discovered.


Forces 

  • Nuclei are bound together by the residual strong force, also known as the nuclear force.

  • The residual strong force binds quarks together to form protons and neutrons.

  • The nuclear force is highly attractive at typical nucleon separation distances, overcoming the electromagnetic repulsion between protons.

  • Nuclei exist because the attractive nuclear force overcomes the electromagnetic repulsion.

  • The residual strong force has a limited range and decays quickly with distance.

  • Only nuclei smaller than a certain size can be completely stable due to the limited range of the residual strong force.

  • The largest completely stable nucleus is lead-208, which contains 208 nucleons (126 neutrons and 82 protons).

  • Nuclei larger than lead-208 are unstable and tend to have shorter lifetimes.

  • Bismuth-209 is stable to beta decay and has the longest half-life to alpha decay among all known isotopes.

  • The residual strong force is effective over a very short range, typically only a few femtometers.

  • The residual strong force causes an attraction between any pair of nucleons.


Advances and Challenges 

  • Ongoing research efforts are focused on nuclei with up to eight nucleons to measure various properties of light.

  • The simplest nuclei provide the basis for quantitative comparisons between experimental and theoretical maps of their global and short-range structure.

  • Light nuclei are ideal for probing the microscopic aspects of nuclear structure, especially those related to gluons and quarks.

  • Light nuclei play important roles in astrophysics, elementary particle physics, and energy production.

  • The majority of matter in the visible universe exists in the form of light nuclei.

  • Reactions between light nuclei primarily govern the nuclear physics of conventional stars like the Sun and the Big Bang.

  • Free neutrons are unstable and undergo radioactive decay.

  • Helium-3 (3He) and Deuterium (2H) are useful surrogates for neutron targets in comparative measurements of the internal structure of neutrons to protons.

  • A detailed understanding of the structure of these nuclei is necessary for interpreting experimental results.

  • Electron scattering is a direct method for probing the structure of nuclei.

  • The nucleus, located at the core of an atom, is a quantal many-body system governed by the strong interaction.

  • Neutrons and protons, as the most stable hadrons, compose the nucleus, just like gluons and quarks compose hadrons.

  • The binding of nucleons together in nuclei through the strong force is a fundamental question related to the existence of the universe.

  • Mutual interactions between nucleons shortly after the Big Bang led to the formation of light nuclei.


Related Physics Class 12 Links


Conclusion:

Understanding the concepts and key points of Nuclei Chapter 13 is crucial for a comprehensive understanding of atomic nuclei and nuclear physics. This set of revision notes covers the fundamental aspects of the chapter, including the structure of an atom, nuclear stability, radioactivity, radioactive decay law, half-life, nuclear fission and fusion, and nuclear reactors. By reviewing these concepts, you'll be well-prepared for your Class 12 CBSE Physics exam and have a solid foundation for further studies in nuclear physics. Best of luck with your revision!

FAQs on Nuclei Class 12 Notes CBSE Physics Chapter 13 (Free PDF Download)

1. What is a Nuclei?

It is defined as the collection of particles of protons which are positively charged and electrically neutral neutrons. Neutrons and protons are intern made up of quarks particles.

2. Which topics are included in Nuclear Physics?

The topics includes the nuclear weapons, nuclear power, medicine, magnetic imaging resonance, ions implantation, engineering, cultural and industrial isotope, in geology radiocarbon dating, in the nuclear engineering field such are used.

3. What is the function of the Nuclei?

The function of the nuclei is defined as to control gene expression and during the cell cycle they do the replication of DNA. The energy of nuclei is very huge and it can be used in nuclear power plants.

4. What do we mean by Nuclei?

Nuclei is the latin word which is described as the seed inside a fruit. To the atomic nucleus it is referred to and is the dense central region of the atom.

5. Explain the order of Radioactivity Decay of the Unstable Nuclei.

The loss of elementary particles from the unstable nucleus which changes the unstable elements into more stable ones is defined as the radioactivity decay. They are of five types namely: beta, alpha, gamma, piston electron and electron capture. 

6. If both the number of protons and neutrons in a nuclear reaction is conserved, in what way is mass converted into energy (or vice versa)? 

Since the number of protons and neutrons are unchanged in this nuclear reaction, the whole rest mass of protons and neutrons remains the same on both sides of this nuclear reaction. However, the total binding energy in the nuclei on the left-hand side does not necessarily have to be the same as on the right side. This difference in the binding energy results in the release of energy through the reaction. For a thorough understanding of the subject, visit the page NCERT Notes for Class 12 Physics.

7. Why is heavy water generally used as a moderator in a nuclear reactor?

Neutrons produced while fission will slow down if they strike a nucleus of equal mass. Because ordinary water is made of hydrogen atoms with a mass very close to the neutrons, so it may be used as the moderator. However, it will absorb neutrons at a very fast rate with the reaction; 

n+p —» d+y

Here, d refers to the deuteron. In order to overcome such a difficulty, heavy water has to be used as the moderator that has little to no cross-section for absorption of neutrons.  For a complete explanation of the chapter, visit the page  NCERT Notes for Class 12 Physics.

8. The mass of a nucleus in its ground state is always less than the total mass of its constituents – neutrons and protons. Explain.

If nucleons approach one another to form a nucleus, they strongly attract each other. Their potential energy decreases and becomes negative. This potential energy holds together the nucleons in the nucleus. A decrease in the potential energy thus results in a decrease in the mass of the nucleon inside the nucleus as well. For more explanation and study materials regarding the chapter, students can download the vedantu app.

9. Can it be concluded from beta decay that electrons exist inside the nucleus?

No, although an electron, the beta particle is in fact created in the moment of beta decay, and also ejected right then. It will not be able to exist within the nucleus because of the de-Broglie wavelength that is significantly larger than the size of the nucleus. To know more students can visit the page NCERT Notes for Class 12 Physics and download the revision notes free of cost.

10. Why do stable nuclei never have more protons than neutrons? 

Protons are parts of the atom that are positively charged, which means they electrically repel one another. This repulsion increases too much in the nuclei in case of greater than 10 protons. Therefore a greater amount of neutrons, that produce just the attractive forces, is mandatory for stability.