CBSE Class 12 Physics Revision Notes Chapter 13

Class 12 Physics Chapter 13 Notes

Physics subject in Class 12 is regarded as one of the most crucial subjects. It is important for students to get  a thorough understanding of every chapter included in the NCERT syllabus to attain high scores in Class 12 board exams. 

Students are advised to refer to the Class 12 Physics Chapter 13 notes to solve and review their answers. Most students try to learn and memorise the concepts instead of understanding them. However, this will not work if the student is planning to pursue a career in engineering or other physics-related streams. Gaining in-depth theoretical knowledge and applying it to solve sums is essential for board exam preparation. Using the Class 12 Physics Chapter 13 notes will help students to plan their studies. For this, it is important to use the right reference materials from authentic, dependable and reliable sources, to make significant improvements in their academic performance. 

Extramarks provides the Chapter 13 Physics Class 12 notes to help students answer questions of any difficulty level effortlessly. Visit the link mentioned below to get access to the best academic notes from Extramarks.

Key Topics Covered In Class 12 Physics Chapter 13 Notes

The main topics covered in the CBSE Class 12 Physics Chapter 13 notes are:

  • Introduction
  • Atomic Masses and Composition of Nucleus
  • Size of the Nucleus
  • Mass Energy and Nuclear binding energy
  • Nuclear force
  • Radioactivity
  • Nuclear Energy

A brief of the notes of  Class 12 Physics Chapter 13 is below.


In this Class 12 Physics Chapter 13 notes, students will learn about various properties of the nuclei like their size, mass, density, stability, and phenomena such as radioactivity, Nuclear fission and Nuclear fusion.


Atomic Mass Unit (u):

It is defined as 112th of the mass of the carbon atom. Therefore, 1 u = 1.660539 x 10-27 kg 

For example, the mass of the Chlorine atom is 35.46 u.

Atomic Number:

The Atomic Number (Z) is the total number of protons in the nucleus. 

Mass Number: 

The Atomic Mass Number (A) is the sum of the number of protons and neutrons in a nucleus. Therefore, A = N + Z. 

Hence, the number of protons is given by Z

The number of electrons is given by Z

The number of nucleons is given by A

The number of neutrons is given by N = A – Z.


It is defined as the atoms of any element having the same atomic number (Z) but different atomic mass numbers (A). These lead to similar chemical properties, whereas the physical properties might differ. 


It is defined as the atoms of any element with the same atomic mass numbers (A) but different atomic numbers (Z). 


It is defined as atoms of any element having the same number of neutrons. 


It is defined as the atoms of any element with the same atomic mass numbers (A) and atomic numbers (Z) but in different energy states. 

Discovery of Neutron:

James Chadwick discovered Neutrons in 1932. The Beryllium nuclei, when bombarded with -particles led to the emission of highly penetrating radiations. These radiations consisted of neutral particles of mass similar to that of a proton. These particles were later named neutrons. 

Therefore the mass of a neutron is 1.6749×10-27 kg.


The scattering experiment was performed by Geiger and Marsden in which the fast electrons are bombarded on various elements. The sizes of nuclei of different elements are accurately measured. 

It has been experimentally found that any nucleus with atomic mass number A has a radius R=RoA1/3where Ro= 1.2 x 10-15m. It means that Volume is directly proportional to the atomic mass number, i.e., R3 is directly proportional to A. 

The density of the nucleus is given as 2.3 × 1017 kgm3. It is independent of the atomic mass number(A). 

Students can practice the problems based on Nuclear density given in the Class 12 Physics Chapter 13 notes. 


Mass – Energy:

Through his experiments, Einstein proved that mass should be treated as another form of energy. It can be converted to various forms of energy, such as kinetic energy. He has stated the mass-energy equivalence relation as E = mc2, where m is mass, and c = 3×10 m/s is the velocity of light in a vacuum. 

This reaction states the conservation of energy, which means that the initial and final energy remains the same, provided that the energy is associated with mass. 

Nuclear binding energy:

Mass defect is defined as the difference in mass of a nucleus and its constituents. It is denoted by M and is given by

M=[Zmp+(A-Z)mn]-M, where

Z is the Atomic Number,

A is Atomic Mass Number,

mp is the mass of protons,

mn is mass of neutrons,

M is the atomic mass. 

The binding energy is given as Eb=Mc2 is defined as the required energy to break the nucleus into its constituent (protons and neutrons). The energy is used to separate these constituents to a large distance so that they will not be able to interact with each other. 

Binding energy can also be defined as the energy of the nucleus by virtue of which their attractions become stronger to form a nucleus together.

Binding Energy per Nucleon: 

It is said to be the average energy required to remove or extract one nucleon from the nucleus of an atom.

It is denoted by Ebnand is obtained as the ratio of binding energy (Eb) of a nucleus by the mass number (A).


  1. The attractive force is sufficiently strong to produce binding energy of a few MeV for every nucleon.
  2. The binding energy of every nucleus is constant. Therefore we can say that the nuclear force is short-ranged. The saturation property of nuclear force states that any given nucleon will influence only other nucleons close to it. 
  3. A heavy nucleus having a higher mass number(A) has lower binding energy per nucleon. Therefore, when separation takes place, the nucleons get more tightly bound. This implies that the energy is released in the process.
  4. Consider two light nuclei joining together and a heavier nucleus. Then we can say that the binding energy per nucleon of the lighter nuclei is less than that of the binding energy per nucleon of the heavier nuclei. Therefore, energy would be released in the process of fusion.


Nuclear forces are the strong, attractive forces that hold protons and neutrons together in the nucleus. These forces operate over short distances of separation between any two given nucleons. It also overcomes the repulsion between two protons in the nucleus. 

Some features of the nuclear force are mentioned below: 

  1. The nuclear binding force is stronger than the Coulomb force, which acts between the charges or the gravitational forces between masses. It dominates over the repulsive Coulomb force between the protons present in the nucleus. The gravitational force is very weak as compared to the Coulomb force.
  2. The nuclear binding force between two nucleons reduces rapidly to zero as their distance is a few femtometers. Because of this, there is a saturation of forces that takes place in a medium or large-sized nucleus. This leads to constancy in the binding energy per nucleon.
  3. The nuclear binding force between two neutrons, two protons or a proton and a neutron is almost the same. It is independent of the electric charge. 


In 1896, Radioactivity was discovered purely by accident by A. H. Becquerel. It is defined as a nuclear phenomenon in which the unstable nucleus undergoes a process of radioactive decay. There are three different types of radioactive decay that occur in nature. They are

(i) α-decay: A helium nucleus is emitted; 

(ii) β-decay: Electrons or positrons are emitted; 

(iii) γ-decay: High energy photons are emitted.

Law of radioactive decay:

The nuclei which undergo the radioactive decay per unit of time are directly proportional to the total number of the nucleus. Let N be the number of nuclei and ∆N undergo the decay in time ∆t, then we say that Nt N or Nt = N, where is the constant of the radioactive decay or disintegration constant. The change in the total number of nuclei is given as dN = – ∆N with respect to time ∆t. Therefore, the rate of change of N is given as dNdt =-N.

The law of radioactive decay can be represented by the following equation, N(t) = Noe − λ t. 

The total rate of decay (R) of a sample is defined as the number of nuclei differentiated with respect to per unit time and is given as R = dNdt.

Also, R = λN gives the relation between the decay rate R at time t and the number of undecayed nuclei (N). 

The SI unit for radioactivity is Becquerel. It is named Henry Becquerel. 

Curie is also known as the SI unit of decay, where one curie is given as the decay rate of 3.7 X 1010Bq disintegrations in one second.

The half-life of a radio nucleus is denoted by T1/2. It is time the nucleus for a sample takes to reduce to, say No/2 from initially No radio nuclei. The mean-life() gives the average lifetime of all the nuclei.

All these results can be summarized as T1/2 = ln (2). 

Refer to the Class 12 Physics Chapter 13 notes provided from Extramarks to understand the concept of radioactivity better. 

Alpha decay:

When any radioactive nucleus emits an -particle, the mass number of the product nucleus decreases by four while the atomic number decreases by two.


  1. The -particles are the helium nuclei having two units of positive charge (protons) and a mass of 4 atomic mass units (amu). 
  2. They can be deflected by electric fields and magnetic fields as these particles are charged. 
  3. They possess ionizing power. Hence, the -particles ionize the medium through which they pass. The ionizing power is much higher than -particles and -rays. 
  4. The -particles are scattered when it passes through very thin metal foils. 
  5. -particles have comparatively less penetrating power than -particles and -rays. 
  6. -emitters are not safe and might produce harmful effects on humans. 


When any radioactive nucleus emits an -particle, the mass number of the product nucleus remains the same while the atomic number increases by one. It is further Classified into three categories, namely 

  1. i) Electron emission
  2. ii) Positron emission

iii) Electron capture


  1. -particles are negatively charged particles whose mass and charges are the same as the electrons.
  2. The penetrating power of -particles is approximately 100 times that of -particles but only about 1% of that of -rays. 
  3. They possess ionizing power. Hence, the -particles ionize the medium through which they pass. However, the ionizing power is very small than -particles.
  4. These charged particles can be deflected by electric fields and magnetic fields. 
  5. They may be dangerous for the human body. Therefore, any contact must be avoided.

Gamma decay:

When any radioactive nucleus emits an -particle, neither the mass number of the product nor the atomic number change.


  1. These electromagnetic waves are not deflected in an electric field or magnetic field as they are of extremely short wavelengths.
  2. -rays can travel at the speed of light. 
  3. -rays carry a very large amount of energy. 
  4. The penetrating power of -rays is very large, approximately 100 times that of -particle and 10000 times that of -particle. 
  5. They, too, possess ionizing power. Hence, the -rays also ionize the medium through which they pass. However, the ionizing power is 1% and 0.01% that of -particle and -particle, respectively.


  1. Nuclear Fission: 

Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei. The extra neutrons are released during this process. This is very well explained using the example of Uranium. 

  1. Nuclear Fusion:

The process by which two lighter nuclei are combined to form a heavy nucleus is known as nuclear fusion. Immense energy is released in this process. The Hydrogen bomb and source of the Sun’s energy is based on nuclear fusion. 

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Class 12 Physics Chapter 13 Notes: Exercises &  Solutions

Extramarks aims to  assist students in their exam preparation by providing the best academic notes, such as the Class 12 Physics Chapter 13 notes. It contains every minute detail included in the NCERT books. The CBSE revision notes also include detailed solutions, formulas, and important questions. 

The solutions in the CBSE Class 12 Physics Chapter 13 notes are prepared in a concise manner and in easy language. The experts at Extramarks have analyzed various CBSE previous year question papers before making the final draft. With the help of these solutions, students will gain confidence to face the Class 12 board exams. Based on the latest guidelines, the Class 12 Physics Chapter 13 notes help students to get an overview of the chapter and master all concepts. 

Visit the link given below to get access to the solutions to all exercise problems, important questions, and more:

In addition, students may also access the following.

CBSE Revision Notes

CBSE Extra Questions

CBSE Past  Year Question Papers

CBSE Syllabus

CBSE Sample Papers

Key Features of Class 12 Physics Chapter 13 notes

The key features of Extramarks Class 12 Physics Chapter 13 notes are as follows:

  • Class 12 Physics Chapter 13 notes help students to prepare efficiently for the final exams and  get  high scores. It consolidates basic knowledge and focuses on providing a deeper understanding of complex concepts. 
  • Students can acquire these notes from any device such as tablets, mobiles and desktop computers.
  • Using the notes of Class 12 Physics Chapter 13, students can also prepare for different competitive exams such as  , JEE main,JEE Advanced, NEET, and other entrance exams. 
  • Solving unlimited problems included in the Class 12 Physics Chapter 13 notes will help students develop analytical  and problem-solving skills besides teaching them about time management.
  • The NCERT solutions follow the latest norms of the CBSE syllabus. 
  • Extramarks notes are 100% dependable, reliable and trustworthy. It helps you to prepare even the most challenging topics with ease.

FAQs (Frequently Asked Questions)

1. Which topics are included in the Class 12 Physics Chapter 13 notes?

 Chapter 13 Physics Class 12 notes explain concepts such as the composition of the nucleus, mass-energy, nuclear force and energy, atomic masses, nucleus size, nuclear binding energy, and radioactive decay. Every topic from the chapter is important , as questions in the board exams, as well as the competitive exam, will be asked from NCERT books. The questions are often tweaked and asked by CBSE in the question paper. The step-by-step explanation of each concept, numerical problems and derivations are covered in the notes of Class 12 Physics Chapter 13.

2. What are the best reference books for Physics Class 12?

Reference books play an important role in exam preparation. Use Class 12 physics Chapter 13 notes to master concepts from this chapter. The ideal way of studying is to give first preference to NCERT books.

Students are advised to use the following books: 

  • NCERT Books:
  • Physics, Class XII, Part -I and II, 
  • Laboratory Manual of Physics for Class XII

Best Reference Books are:

  • Class 12 Concepts of Physics (2 Volumes) – H C Verma
  • Class 12 Chapter-wise Solutions – S. K. Singh
  • CBSE Chapter-wise Solutions Physics for Class 12 – Singh S
  • Class 12 CBSE Sample Papers 
  • New Simplified Physics for Class 12 (2 Volumes) – Dhanpat Rai