Class 12 Physics Chapter 13 Nuclei Notes

Understanding the nucleus - the tiny, dense centre of the atom is a vital part of modern Physics. Chapter 13 Nuclei in Class 12 Physics deals with nuclear properties, radioactivity, nuclear reactions, and the concept of binding energy, which has practical implications in nuclear energy and medical technology.

These CBSE Class 12 Physics Notes are designed to help students quickly grasp and revise the important concepts, derivations, and formulas before the board exams. They are simple, accurate, and follow the latest CBSE syllabus Class 12 .

Nuclei Class 12 Notes Material PDF Download

This study material for Class 12 explains the basics of Nuclei in an easy-to-understand way. Download the PDF to learn key concepts and prepare well for your exams.

Below we have provided the links to downloadable PDFs of class 12 science Ch 13 notes and get an in-depth explanation and understanding of the chapter.

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Important Notes from Nuclei Class 12

1. Atomic Nucleus and Its Composition
The nucleus is the small, dense central core of an atom. It contains:

• Protons (positively charged)
• Neutrons (neutral)
  Together, protons and neutrons are known as nucleons.
  

2. Atomic Number and Mass Number

• Atomic Number (Z): Number of protons in the nucleus.
• Mass Number (A): Total number of protons + neutrons.
  Neutrons (N) = A – Z.
  

3. Size of the Nucleus
Nuclei are incredibly small — typically a few femtometers (1 fm = 10⁻¹⁵ m) in radius.
Nuclear size increases with mass number, but not proportionally. A larger mass number means a slightly bigger nucleus.

4. Nuclear Density
Despite varying sizes, nuclei have nearly the same density, which is extremely high — about 10¹⁷ kg/m³. This means nuclear matter is one of the densest forms of matter in the universe.

5. Mass-Energy Equivalence (Einstein's Idea)
Mass and energy are related. Even a small loss in mass leads to a large amount of energy release. This is captured by E = mc², where "m" is mass loss and "c" is the speed of light.

6. Mass Defect
The actual mass of a nucleus is less than the total mass of its separate protons and neutrons. This difference is called mass defect and is due to the binding energy that holds the nucleus together.

7. Binding Energy
It is the energy required to break a nucleus into its individual protons and neutrons.

• A higher binding energy per nucleon means a more stable nucleus.
• Iron-56 has the highest binding energy per nucleon, making it one of the most stable nuclei.
  

8. Radioactivity
Some nuclei are unstable and break down spontaneously, emitting radiation. This is called radioactive decay, and it happens in three main forms:

• Alpha (α) Decay: Emits a helium nucleus (2 protons + 2 neutrons).
• Beta (β) Decay: A neutron changes into a proton (or vice versa), releasing an electron or positron.
• Gamma (γ) Decay: Emission of high-energy photons after alpha or beta decay.
  

9. Half-Life (T½)
The time taken for half of a radioactive sample to decay is called its half-life. It is constant for each substance and is independent of conditions like temperature or pressure.

10. Nuclear Fission
A heavy nucleus splits into two lighter nuclei, releasing a huge amount of energy.

• Used in nuclear power plants and atomic bombs.
• Example: Uranium-235 fission.
  

11. Nuclear Fusion
Two light nuclei combine to form a heavier nucleus.

• Releases even more energy than fission.
• Occurs naturally in stars like the Sun.

Important Questions from Ch 13 Nuclei Class 12

1. Very Short Answer Questions (1 Mark Each)

Q1. What are nucleons?
Ans: Nucleons are the particles present in the nucleus, i.e., protons and neutrons.

Q2. Define mass number.
Ans: Mass number is the total number of protons and neutrons in a nucleus.

Q3. What is the unit of nuclear radius?
Ans: The nuclear radius is measured in femtometers (fm), where 1 fm = 10⁻¹⁵ meters.

Q4. Which type of decay releases the most energy per event?
Ans: Nuclear fusion releases the most energy per event compared to fission or radioactive decay.

Q5. What is radioactivity?
Ans: Radioactivity is the spontaneous emission of particles or radiation from unstable nuclei.

2. Short Answer Questions (2–3 Marks Each)

Q1. What is meant by mass defect and why does it occur?
Ans:
Mass defect is the difference between the actual mass of a nucleus and the sum of the masses of its individual nucleons.
It occurs because some mass is converted into binding energy, which holds the nucleus together.

Q2. Define binding energy. How is it related to stability?
Ans:
Binding energy is the energy required to disassemble a nucleus into its protons and neutrons.
The higher the binding energy per nucleon, the more stable the nucleus is, as it is harder to break.

Q3. What is a half-life? Give an example.
Ans:
Half-life is the time taken for half of the radioactive nuclei in a sample to decay.
Example: The half-life of Carbon-14 is about 5730 years, used in dating archaeological samples.

Q4. What are the key differences between nuclear fission and fusion?
Ans:

• Fission: Splitting of a heavy nucleus; used in reactors; controlled process.
• Fusion: Combining of light nuclei; happens in stars; uncontrolled on Earth (for now).
  Fusion releases more energy but is harder to achieve.

3. Long Answer Questions (4–5 Marks Each)

Q1. Explain the process of nuclear fission with an example.
Ans:
In nuclear fission, a heavy nucleus like Uranium-235 absorbs a neutron and splits into two lighter nuclei (e.g., Barium and Krypton), releasing 2-3 more neutrons and a huge amount of energy.
These neutrons can trigger further fission in nearby nuclei, creating a chain reaction.
This process is harnessed in nuclear reactors for electricity and in nuclear weapons for destruction.

Q2. What is nuclear binding energy per nucleon? How does it vary with mass number?
Ans:
It is the total binding energy divided by the number of nucleons in a nucleus.
As mass number increases:

• Binding energy per nucleon increases up to Iron (A ≈ 56), then
• Decreases for heavier nuclei. This explains why fusion of light nuclei and fission of heavy nuclei both release energy.

Q3. Describe radioactive decay and its types.
Ans:
Radioactive decay is a natural process in which unstable nuclei become stable by emitting particles or radiation.
Types include:

• Alpha decay: Releases a helium nucleus, reduces mass and atomic number.
• Beta decay: A neutron changes to a proton or vice versa, releasing an electron or positron.
• Gamma decay: After alpha or beta decay, the nucleus releases energy as gamma radiation. Each type changes the nuclear composition differently.

Common Mistakes to Avoid

🚫 Thinking protons and neutrons have the same mass, they are close but not identical
🚫 Confusing mass number (A) with atomic number (Z)

🚫 Mixing up fission and fusion processes
🚫 Assuming all radioactive decay emits harmful radiation, some are low energy
🚫 Forgetting that binding energy increases stability, not decreases it

Creative Ways to Make Notes for Nuclei Class 12

1. Concept Mapping: Make a concept map linking mass defect → binding energy → nuclear stability.
2. Decay Timeline Charts: Create a timeline visualising decay, half-life, and exponential drop.
3. Formula Box Cards: Keep flashcards of key formulas (decay law, BE, mass-energy).
4. Table of Particles: Compile α, β, γ properties like mass, charge, penetration power.
5. Use Colour Codes: Mark theory (blue), formulae (red), and derivations (black) for easy scanning.

How Can Notes Help?

• Faster revision before exams with summarised formulas and derivations.
• Prevent confusion with unit conversions and signs in energy equations.
• Act as a checklist to ensure full syllabus coverage for the chapter.
• Great for solving numericals from NCERT, Exemplar, and sample papers.
• Helps in recognising patterns and relationships among nuclear concepts.

The Nuclei chapter in Class 12 Physics is both theoretical and numerical. A good grasp of formulas, unit conversions, and nuclear processes is essential for success in board exams and entrance tests like NEET or JEE. These notes are carefully structured to help you cover all aspects - definitions, derivations, examples, and practice questions. Regular revision using well-organised notes will boost both your speed and accuracy.

Frequently Asked Questions

1. Why does the nucleus have such a high density compared to ordinary matter?

Answer:
The nucleus is made up of nucleons (protons and neutrons) packed very closely within a tiny volume (~10⁻¹⁵ m radius). This extremely small size combined with a significant mass causes nuclear density to be about 10¹⁷ kg/m³, much higher than ordinary matter.

2. What is the physical significance of the curve of binding energy per nucleon?

Answer:
The curve shows that binding energy per nucleon increases sharply for light nuclei, peaks near iron (Fe-56), and then slowly decreases for heavier nuclei. This explains why fusion of light nuclei and fission of heavy nuclei both release energy - they move toward nuclei with higher binding energy per nucleon (more stability).

3. How does beta decay change the composition of a nucleus?

Answer:
In beta-minus decay, a neutron converts into a proton, emitting an electron and an antineutrino, increasing the atomic number by 1 but keeping the mass number the same. In beta-plus decay (positron emission), a proton changes into a neutron, decreasing the atomic number by 1 without changing the mass number.

4. What role do neutrons play in the stability of the nucleus?

Answer:
Neutrons provide an attractive nuclear force without adding electrostatic repulsion (because they are neutral), helping to bind protons together. An optimal neutron-to-proton ratio stabilizes the nucleus; too few or too many neutrons make the nucleus unstable and radioactive.

5. Why is nuclear fusion difficult to achieve on Earth, though it powers stars easily?

Answer:
Fusion requires extremely high temperature and pressure to overcome the electrostatic repulsion between positively charged nuclei. Stars naturally achieve this through their immense gravity and temperature, but recreating such conditions on Earth needs advanced technology (like tokamaks) and is very challenging.