CBSE Important Questions Ch 13 Class 12 Physics 2025 PDF

Nuclei is one of the important chapters of the Class 12 Physics syllabus, which discusses the atomic nuclei structure and its behaviour, nuclear reactions, and related phenomena. This chapter demands an in-depth understanding of concepts such as radioactivity, nuclear fission and fusion, binding energy, and decay laws. For this topic, it is advisable to focus on a set of Chapter 13 Nuclei Class 12 Physics Important Questions that cover theoretical concepts, derivations, and numerical problems. In this article, we'll be discussing the most important and frequently asked questions relating to the nuclei chapter in Class 12 Physics.

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Important Questions on Nuclei

This section will answer the most important questions of the Class 12 Nuclei. 

What are alpha, beta, and gamma radiations?  

Radioactivity may be described as a process where unstable atomic nuclei emit energy because of the emission of radiation. The main source of radiation is: 

Alpha radiation (α): This is a type that comprises helium nuclei, which are two protons and two neutrons out of the nucleus. It has low penetrable properties and is blocked by even a sheet of paper and human skin.

Beta radiation (β): It consists of the emission of electrons in beta-minus decay, and in beta-plus decay, it involves positrons that emerge from the nucleus. Beta particles penetrate more than alpha particles and are stopped by a sheet of aluminium or plastic.

Gamma radiation (γ): Gamma rays are high-energy electromagnetic waves (photons) produced after the nucleus undergoes alpha or beta decay. These particles possess very high penetrating power and usually are shielded using thick lead or concrete.

Explain the difference between fission and fusion reactions.

Nuclear reactions can be broadly categorised into two: fission and fusion, which both liberate a tremendous amount of energy.

Nuclear Fission: It involves the breaking off of a massive nucleus like that of uranium-235 or plutonium-239 into two minor nuclei. While doing so, it releases huge amounts of energy and also leads to the expulsion of neutrons, which itself can cause the fissioning of more nuclear material and help in the release of a further chain reaction; hence, providing the basis behind nuclear reactors as well as an atomic bomb.

Nuclear Fusion: In contrast, fusion is a process in which two light nuclei, such as isotopes of hydrogen, combine to form a heavier nucleus while releasing energy. This is the process that powers the Sun and other stars. However, fusion produces hundreds of times more energy than fission; it is not currently practical to be harnessed as power on Earth because of the very extreme conditions required for the reaction to take place.

The energy released in both fusion and fission can be estimated using the mass-energy equivalence principle.

Explain the concept of binding energy per nucleon.

Binding energy per nucleon is the average energy needed to remove a nucleon from the nucleus. A higher binding energy per nucleon suggests a more stable nucleus. This concept is useful for explaining the stability differences among various nuclei.

What is the mass defect of a nucleus?

The mass defect is the difference between the sum of the masses of individual protons and neutrons and the actual mass of the nucleus. This "missing" mass has been converted into binding energy, which is what holds the nucleus together.

Formula:

Mass defect(Δm)=Sum of masses of nucleons−Mass of the nucleus

How is the Q-value related to the mass defect in a nuclear reaction?

The Q-value is directly related to the mass defect, which is the difference between the total mass of individual nucleons and the actual mass of the nucleus or system. This mass defect is the origin of the energy that is released or absorbed in a nuclear reaction because mass is converted into energy, according to Einstein's equation E=Δm⋅c².

For a reaction, the formula is:

Q = (Mass of reactants - Mass of products) ⋅ c²

The mass defect in both the reactants and products is what accounts for the energy change during the reaction.

What is the Q-value for the fission of uranium-235 by thermal neutrons?

In the process of fission of uranium-235 (U-235), when it absorbs a neutron, it breaks into two smaller nuclei, namely barium and krypton, and releases several neutrons and a great amount of energy.

For this reaction, the Q-value is positive; thus, energy is emitted. In general, the Q-value for fission of uranium-235 with thermal neutrons is about 200 MeV (million electron volts). It is mainly the kinetic energy of the fission fragments and the energy of the neutrons emitted during fission.

How do you determine whether a nuclear reaction is exothermic or endothermic based on its Q-value?

To determine if a nuclear reaction is exothermic, releasing energy, or endothermic, requiring energy, you should examine the Q-value:

• When Q > 0, that means the reaction is exothermic; it is releasing energy.
• When Q < 0, this indicates that the reaction is endothermic because it is absorbing energy.

This may be understood by comparing the mass of the reactants and products:

• If the mass of the products is less than that of the reactants, the difference (mass defect) is converted into energy, and the Q-value is positive.  
• If the mass of the products is greater than that of the reactants, the Q-value will be negative, meaning that the reaction requires energy.

How is energy released in a nuclear reaction?

In a nuclear reaction, energy is released if the mass of the products is smaller than that of the reactants. This "missing mass" is then used to create energy, as shown by the principle of mass-energy equivalence, which states E=Δm⋅c². Fission, where a heavy nucleus splits, and fusion, the combining of light nuclei, follow this principle. The difference between the masses of the reactants and the product is known as the mass defect, which in turn accounts for the energy in nuclear reactions.

Numerical Problems in Nuclei

Apart from theoretical questions, students should also be able to solve numerical problems, which are often encountered in exams. Some examples of these problems include:

Calculation of Binding Energy: Using the mass defect, calculate the binding energy of a nucleus with the formula E=Δm⋅c².

Radioactive Decay: Problems that have to do with finding the number of undecayed nuclei at a given time, knowing the decay constant, and knowing the initial quantity of nuclei.

Energy in Fission and Fusion: Problems that require the calculation of the energy released in nuclear fission or fusion, using the concept of mass-energy equivalence.

The chapter on nuclei in Class 12 Physics is essential for understanding the key processes that control atomic behaviour and nuclear reactions. By focusing on important topics such as nuclear structure, radioactivity, nuclear reactions, and nuclear models, students can develop a solid base in nuclear physics. Class 12 Nuclei Important questions and numerical problems will be worked out to help students reinforce theoretical concepts and adequately prepare for exams.

The mastery of this chapter comes only if a student understands that memorization is not enough. The concepts on which these formulas are based have to be remembered. Revision regularly, solving problems, and application of these principles to situations will ensure an increased understanding of the subject matter, which in turn will make students perform better at exams.

We hope that you practice the above Chapter 13 Nuclei Class 12 Physics Important Questions and achieve your dream marks.

All the best!