Important Questions Class 12 Physics Chapter 13 Nuclei

A nucleus is the small, dense, positively charged central part of an atom that contains protons and neutrons. It holds more than 99.9% of atomic mass and has a radius much smaller than the atomic radius.

Nuclear physics explains why most atomic mass stays packed inside a tiny centre. Important Questions Class 12 Physics Chapter 13 help students practise nuclear size, isotopes, mass defect, binding energy, nuclear force, radioactivity, fission, and fusion. The NCERT 2026 chapter uses direct formulae such as R = R0A^(1/3), E = mc^2, and Eb = ΔMc^2. These topics appear in short answers, derivations, numerical questions, and board-style PYQ patterns.

Key Takeaways

• Size of Nucleus: Nuclear radius depends on mass number as R = R₀A¹ᐟ³.
• Mass Defect: The nucleus has less mass than its separate protons and neutrons.
• Binding Energy: Binding energy explains nuclear stability, fission, and fusion.
• Nuclear Energy: Fission splits heavy nuclei, while fusion combines light nuclei.

Important Questions Class 12 Physics Chapter 13 Structure 2026

Important Questions Class 12 Physics Chapter 13 Structure 2026

Important Questions Class 12 Physics Chapter 13 Overview

Important Questions Class 12 Physics Chapter 13 focus on nuclear structure and nuclear energy. The chapter explains how protons and neutrons form nuclei, why nuclei have huge density, and how mass converts into energy.

Q1. What is the nucleus of an atom?

The nucleus is the dense, positively charged central part of an atom. It contains protons and neutrons.

More than 99.9% of the atom’s mass lies inside the nucleus.

Q2. Why is an atom mostly empty?

An atom is mostly empty because its nucleus is much smaller than the atom. The nuclear radius is smaller than atomic radius by about 10^4.

Example: If an atom becomes as large as a classroom, the nucleus becomes about the size of a pinhead.

Q3. What does the chapter Nuclei class 12 explain?

Nuclei class 12 explains nuclear composition, nuclear size, binding energy, radioactivity, fission, and fusion. It connects nuclear mass with energy release.

Example: Fission of 1 kg uranium releases about 10^14 J energy.

Nuclei Class 12 Important Questions on Atomic Mass and Composition

Nuclei class 12 important questions often start with atomic mass and nuclear notation. These concepts build the base for mass defect and binding energy.

Q4. What is atomic mass unit?

Atomic mass unit is one-twelfth of the mass of one carbon-12 atom. It measures very small atomic and nuclear masses.

Formula:
1 u = 1/12 mass of one carbon-12 atom

Value:
1 u = 1.660539 × 10^-27 kg

Q5. What is the mass of a proton?

The mass of a proton is 1.00727 u. In kilogram, it equals 1.67262 × 10^-27 kg.

Final value: mp = 1.00727 u

Q6. What is the mass of a neutron?

The mass of a neutron is 1.00866 u. In kilogram, it equals 1.6749 × 10^-27 kg.

A free neutron is unstable, but it remains stable inside a nucleus.

Q7. What are protons and neutrons together called?

Protons and neutrons are together called nucleons. The total number of nucleons gives the mass number.

Formula:
A = Z + N

Here, A = mass number, Z = atomic number, and N = neutron number.

Q8. What is atomic number?

Atomic number is the number of protons in the nucleus. It is denoted by Z.

For a neutral atom, atomic number also equals the number of electrons.

Q9. What is mass number?

Mass number is the total number of protons and neutrons in a nucleus. It is denoted by A.

Formula:
A = Z + N

Example: Gold-197 has 197 nucleons.

Q10. What is nuclide notation?

Nuclide notation represents a nucleus using its chemical symbol, mass number, and atomic number. It is written as AXZ in standard form.

Example:
Gold nucleus is written as 197/79 Au.

It has 79 protons and 118 neutrons.

Q11. What are isotopes?

Isotopes are atoms of the same element with the same atomic number but different mass numbers. They contain the same protons but different neutrons.

Example:
Hydrogen has protium, deuterium, and tritium.

Q12. What are isobars?

Isobars are nuclides with the same mass number but different atomic numbers. They belong to different elements.

Example:
3/1 H and 3/2 He are isobars.

Q13. What are isotones?

Isotones are nuclides with the same neutron number but different atomic numbers. They differ in proton number.

Example:
198/80 Hg and 197/79 Au are isotones.

Q14. Why do isotopes show similar chemical properties?

Isotopes show similar chemical properties because they have identical electronic structures. Chemical behaviour depends mainly on electrons.

Example:
Hydrogen isotopes differ in mass but occupy the same periodic table position.

Size of Nucleus Class 12 Physics Chapter Nuclei Important Questions

Size of nucleus questions are important because they connect nuclear radius with nuclear density. Students should remember R = R0A^(1/3).

Q15. What is the size of nucleus?

The size of nucleus is of the order of femtometres. One femtometre equals 10^-15 m.

Formula:
R = R0A^(1/3)

Here, R0 = 1.2 × 10^-15 m.

Q16. What does R = R0A^(1/3) mean?

The formula shows that nuclear radius increases with the cube root of mass number. It does not increase directly with A.

Formula:
R = R0A^(1/3)

This means nuclear volume is proportional to A.

Q17. Why is nuclear density nearly constant?

Nuclear density is nearly constant because nuclear volume is proportional to mass number A. Nuclear mass is also proportional to A.

Steps:

1. R = R0A^(1/3)
2. Volume V is proportional to R^3.
3. Therefore, V is proportional to A.
4. Nuclear mass is also proportional to A.
5. Density = mass/volume becomes independent of A.

Final result: Nuclear density is nearly constant for all nuclei

Q18. What is the approximate density of nuclear matter?

The approximate density of nuclear matter is 2.3 × 10^17 kg/m^3. It is much larger than ordinary matter.

Example:
Water density is about 10^3 kg/m^3.

Q19. Why is nuclear density so high?

Nuclear density is high because most atomic mass lies inside a very small volume. The rest of the atom contains mostly empty space.

Example:
A nucleus contains more than 99.9% of atomic mass.

Q20. Find the approximate ratio of nuclear radii of gold-197 and silver-107.

The radius ratio is approximately 1.23. Use R proportional to A^(1/3).

Given:

1. A_gold = 197
2. A_silver = 107

Formula used:
R1/R2 = (A1/A2)^(1/3)

Calculation:

1. R_gold/R_silver = (197/107)^(1/3)
2. R_gold/R_silver = (1.841)^(1/3)
3. R_gold/R_silver ≈ 1.23

Final result: R_gold : R_silver = 1.23 : 1

Nuclei Important Questions Class 12 on Mass Defect and Binding Energy

Nuclei important questions class 12 often test energy conversion in nuclei. Use mass defect carefully before calculating binding energy.

Q21. What is Einstein’s mass-energy relation?

Einstein’s mass-energy relation states that mass is another form of energy. A mass m has energy equivalent mc^2.

Formula:
E = mc^2

Here, c = 3 × 10^8 m/s.

Q22. Find the energy equivalent of 1 g of matter.

The energy equivalent of 1 g matter is 9 × 10^13 J. Use E = mc^2.

Given:

1. m = 1 g = 10^-3 kg
2. c = 3 × 10^8 m/s

Formula used:
E = mc^2

Calculation:

1. E = 10^-3 × (3 × 10^8)^2
2. E = 10^-3 × 9 × 10^16
3. E = 9 × 10^13 J

Final result: 9 × 10^13 J

Q23. What is mass defect?

Mass defect is the difference between total mass of free nucleons and actual nuclear mass. It is denoted by ΔM.

Formula:
ΔM = [Zmp + (A - Z)mn] - M

Here, M is actual nuclear mass.

Q24. What is nuclear binding energy?

Nuclear binding energy is the energy required to separate a nucleus into its nucleons. It equals the energy equivalent of mass defect.

Formula:
Eb = ΔMc^2

A higher binding energy means stronger nuclear stability.

Q25. What is binding energy per nucleon?

Binding energy per nucleon is binding energy divided by mass number. It measures average stability per nucleon.

Formula:
Ebn = Eb/A

For many medium-mass nuclei, it is nearly 8 MeV per nucleon.

Q26. Find the energy equivalent of 1 atomic mass unit.

The energy equivalent of 1 u is 931.5 MeV. This value is used in nuclear numericals.

Given:

1. 1 u = 1.6605 × 10^-27 kg
2. c = 2.9979 × 10^8 m/s

Result:
1 u = 1.4924 × 10^-10 J

Conversion:
1 u = 931.5 MeV/c^2

Final result: 1 u = 931.5 MeV/c^2

Q27. What does the binding energy curve show?

The binding energy curve shows variation of binding energy per nucleon with mass number. It helps explain fission and fusion.

Key facts:

1. Maximum value occurs near A = 56.
2. Middle nuclei have high stability.
3. Heavy nuclei can release energy by fission.
4. Light nuclei can release energy by fusion.

Q28. Why is iron-56 highly stable?

Iron-56 is highly stable because its binding energy per nucleon is near maximum. It lies close to the peak of the curve.

Fact:
Binding energy per nucleon near A = 56 is about 8.75 MeV.

Q29. Why do heavy nuclei release energy during fission?

Heavy nuclei release energy because their fragments have higher binding energy per nucleon. The final nuclei become more tightly bound.

Example:
A nucleus with A = 240 can split into two nuclei near A = 120.

Q30. Why do light nuclei release energy during fusion?

Light nuclei release energy because the fused nucleus has higher binding energy per nucleon. The final nucleus becomes more stable.

Example:
Hydrogen nuclei fuse to form helium inside stars.

Class 12 Physics Nuclei Important Questions on Nuclear Force and Radioactivity

Class 12 physics nuclei important questions test why the nucleus remains stable. Nuclear force explains why protons do not fly apart.

Q31. What is nuclear force?

Nuclear force is the strong attractive force that binds protons and neutrons inside the nucleus. It overcomes electrostatic repulsion between protons.

It acts between proton-proton, neutron-neutron, and proton-neutron pairs.

Q32. State the main properties of nuclear force.

Nuclear force is strong, short-ranged, and charge-independent. It acts only over distances of a few femtometres.

Properties:

1. It is stronger than Coulomb force.
2. It becomes negligible beyond a few fm.
3. It is attractive beyond about 0.8 fm.
4. It becomes repulsive below about 0.8 fm.
5. It does not depend on electric charge.

Q33. Why is nuclear force short-ranged?

Nuclear force is short-ranged because it falls rapidly to zero beyond a few femtometres. Each nucleon interacts mainly with nearby nucleons.

This explains saturation of nuclear force.

Q34. What is radioactivity?

Radioactivity is the spontaneous decay of unstable nuclei with emission of radiation. It is a nuclear phenomenon.

It was discovered by A. H. Becquerel in 1896.

Q35. What are the three types of radioactive decay?

The three natural radioactive decays are alpha decay, beta decay, and gamma decay. Each releases a different type of radiation.

Types:

1. Alpha decay emits helium nucleus.
2. Beta decay emits electron or positron.
3. Gamma decay emits high-energy photons.

Q36. Why is radioactivity a nuclear phenomenon?

Radioactivity is a nuclear phenomenon because it changes the nucleus, not outer electrons. Chemical conditions do not control nuclear decay.

Example:
Alpha decay changes both mass number and atomic number.

Nuclei Class 12 PYQ Boards on Fission and Fusion

Nuclei class 12 pyq boards often ask why fission and fusion release energy. Both depend on the binding energy per nucleon curve.

Q37. What is nuclear fission?

Nuclear fission is the splitting of a heavy nucleus into two medium-mass fragments. It releases energy and neutrons.

Example:
n + U-235 gives Ba, Kr, and 3 neutrons in one possible fission reaction.

Q38. What is the energy released in uranium fission?

Uranium fission releases about 200 MeV energy per fission. The exact fragment pair may vary.

This energy appears first as kinetic energy of fragments and neutrons.

Q39. Why does fission release energy?

Fission releases energy because the product nuclei have greater binding energy per nucleon. The final system has lower mass.

Example:
The gain in binding energy for A = 240 splitting can be about 216 MeV.

Q40. What is nuclear fusion?

Nuclear fusion is the combination of two light nuclei to form a heavier nucleus. It releases energy when the product is more tightly bound.

Example:
Two deuterium nuclei can fuse to form helium-3 and a neutron.

Q41. Why does fusion need very high temperature?

Fusion needs high temperature because nuclei must overcome Coulomb repulsion. High temperature gives nuclei enough kinetic energy.

For two protons, the Coulomb barrier is about 400 keV.

Q42. What is thermonuclear fusion?

Thermonuclear fusion is fusion achieved by raising temperature extremely high. The fuel becomes plasma at such temperatures.

Controlled fusion reactors aim to heat fuel to about 10^8 K.

Q43. What is the energy source of the Sun?

The Sun gets energy from fusion of hydrogen into helium. The proton-proton cycle releases energy in several steps.

Net result:
Four hydrogen atoms form one helium atom with release of 26.7 MeV energy.

Q44. What is the difference between fission and fusion?

Fission splits heavy nuclei, while fusion combines light nuclei. Both release energy by forming more tightly bound nuclei.

Example:
Uranium undergoes fission. Hydrogen undergoes fusion in stars.

Q45. Why can nuclear reactions release more energy than chemical reactions?

Nuclear reactions release more energy because nuclear binding energies are in MeV. Chemical energies are usually in eV.

Example:
Fission of 1 kg uranium gives about 10^14 J, while 1 kg coal gives about 10^7 J.

Atom and Nuclei Class 12 Important Questions for Board Exam Pattern

Atom and nuclei class 12 important questions should be revised through formulas, definitions, and PYQ-style numerical patterns. These areas match nuclei pyq cbse class 12 search intent.

Most repeated question types:

• Define atomic mass unit.
• Find number of protons and neutrons.
• Explain isotopes, isobars, and isotones.
• Derive nuclear density independence from A.
• Calculate nuclear radius using R = R0A^(1/3).
• Explain mass defect and binding energy.
• Calculate energy using E = mc^2.
• Interpret binding energy per nucleon curve.
• State properties of nuclear force.
• Explain alpha, beta, and gamma decay.
• Compare fission and fusion.
• Explain energy generation in stars.

Important formulas:

• A = Z + N
• R = R0A^(1/3)
• E = mc^2
• ΔM = [Zmp + (A - Z)mn] - M
• Eb = ΔMc^2
• Ebn = Eb/A
• 1 u = 931.5 MeV/c^2
• Q = (sum of initial masses - sum of final masses)c^2

High-scoring nuclei class 12 important topics:

• Nuclear composition
• Size of nucleus
• Nuclear density
• Mass-energy equivalence
• Mass defect
• Binding energy
• Nuclear force
• Radioactivity
• Nuclear fission
• Nuclear fusion

Class 12 Physics Chapter List