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Nuclei – Class 12 Physics notes, important questions and MCQs

NCERT Class 12 Physics, chapter 13. Notes, key terms, important questions with answers and MCQs.

Chapter overview

The nucleus contains protons and neutrons held by the strong nuclear force. Mass defect gives binding energy, and fission and fusion release huge energy.

Nuclear structure

Radius R = R₀A^(1/3) (R₀ ≈ 1.2 fm); nuclear density is nearly constant.
Mass defect Δm = [Zm_p + (A − Z)m_n] − M; binding energy E_b = Δm c² (1 u = 931.5 MeV).
Binding energy per nucleon peaks near iron (≈ 8.75 MeV).

Nuclear energy

Fission: heavy nucleus splits (U-235 + n), used in reactors with controlled chain reaction.
Fusion: light nuclei join (hydrogen → helium) — the Sun's energy source.
Nuclear force: strongest, short-range, charge-independent.

Key terms

Mass defect: difference between mass of separate nucleons and the nucleus
Binding energy: energy needed to separate a nucleus into its nucleons
Half-life: time in which half of the radioactive nuclei in a sample decay, T½ = 0.693/λ

Formulas

E = Δm c²
1 u = 931.5 MeV
R = R₀A^(1/3)

Important questions with answers

What is mass defect? How is it related to binding energy?2 marks

Mass defect Δm = [Zmₚ + (A − Z)mₙ] − M_nucleus. The missing mass appears as binding energy: E_b = Δm c² (1 u ≈ 931.5 MeV).

Draw (describe) the binding energy per nucleon curve and explain how it accounts for fission and fusion.3 marks

BE/A rises sharply for light nuclei, peaks at about 8.8 MeV near A ≈ 56 (iron), and slowly falls to about 7.6 MeV for heavy nuclei like uranium. Splitting a heavy nucleus (fission) or joining light nuclei (fusion) forms nuclei with higher BE per nucleon, so energy is released in both.

Compare nuclear fission and fusion. Calculate the energy released when 1 g of matter is completely converted into energy, and explain why fusion needs very high temperatures.5 marks

Fission: a heavy nucleus (²³⁵U) absorbs a neutron and splits into two medium nuclei plus 2–3 neutrons, releasing about 200 MeV; it can form a chain reaction, used in reactors. Fusion: light nuclei (hydrogen isotopes) combine to form heavier ones, releasing energy; it powers the Sun and gives more energy per unit mass. E = mc² = 10⁻³ × (3 × 10⁸)² = 9 × 10¹³ J. Fusion needs about 10⁷ K so that nuclei have enough kinetic energy to overcome their electrostatic repulsion and come within the range of the nuclear force.

Quick revision

Nuclear force is short-range.
Sun works by fusion.
Reactors use fission.

Exam tips

Both fission of heavy nuclei and fusion of light nuclei move towards higher binding energy per nucleon.
Nuclear radius R = R₀A^(1/3), so nuclear density is nearly the same for all nuclei (about 2.3 × 10¹⁷ kg/m³).

Practice MCQs

Energy of the Sun comes from
1. fission
2. radioactive decay only
3. combustion
4. fusion
Show answer
Answer: (d) fusion. Fusion.
1 u of mass is equivalent to
1. 931.5 MeV
2. 9.31 MeV
3. 1 MeV
4. 13.6 eV
Show answer
Answer: (a) 931.5 MeV. Standard value.
Nuclear radius of a nucleus with A = 64 compared to A = 1 is
1. 64 times
2. 2 times
3. 8 times
4. 4 times
Show answer
Answer: (d) 4 times. (64)^(1/3) = 4.
Nuclear forces are
1. long-range
2. gravitational
3. electrostatic
4. short-range and very strong
Show answer
Answer: (d) short-range and very strong. Act within ~fm.
After 3 half-lives, the fraction of a radioactive sample left is
1. 1/3
2. 1/8
3. 1/6
4. 1/9
Show answer
Answer: (b) 1/8. (1/2)³.
Binding energy per nucleon is maximum near
1. iron
2. helium
3. hydrogen
4. uranium
Show answer
Answer: (a) iron. Most stable nuclei.
In a nuclear reactor, moderators are used to
1. slow down neutrons
2. absorb neutrons
3. speed up neutrons
4. cool the core only
Show answer
Answer: (a) slow down neutrons. Slow neutrons cause U-235 fission.
Ratio of nuclear radii of ²⁷Al and ⁶⁴Cu is about
1. 3 : 4
2. 27 : 64
3. 9 : 16
4. 1 : 2
Show answer
Answer: (a) 3 : 4. (27/64)^(1/3).

All chapters of Class 12 Physics

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.