Nuclei

part 1

Nuclear Structure

• Atomic masses and nuclear size
• Composition of nucleus — protons and neutrons
• Nuclear binding energy
• Binding energy per nucleon vs mass number graph

part 2

Nuclear Forces and Radioactivity

• Nuclear force and its properties
• Radioactivity — discovery
• Alpha, beta and gamma decay
• Laws of radioactive decay

part 3

Nuclear Reactions and Energy

• Half-life and mean life
• Radioactive decay law — N = N₀e^(-λt)
• Nuclear fission
• Nuclear fusion
• Nuclear reactor and its components

Mass defect

$\Delta m = [Zm_p + (A-Z)m_n] - M_{nucleus}$

★ board favourite

Binding energy

$BE = \Delta m \times c^2 = \Delta m \times 931.5$ MeV/u

★ board favourite

Radioactive decay law

$N = N_0 e^{-\lambda t}$

★ board favourite

Half-life

$T_{1/2} = \frac{0.693}{\lambda}$

★ board favourite

Activity

$A = \lambda N = A_0 e^{-\lambda t}$ [unit: Becquerel]

★ board favourite

Mass-energy equivalence

$E = mc^2$

★ board favourite

Nuclear radius

$R = R_0 A^{1/3}$ (R₀ = 1.2×10⁻¹⁵ m = 1.2 fm)

★ frequently asked

Binding energy per nucleon

$BE/A$

★ frequently asked

Mean life (average life)

$\tau = \frac{1}{\lambda} = \frac{T_{1/2}}{0.693}$

★ frequently asked

Q-value of nuclear reaction

$Q = (m_{reactants} - m_{products})c^2$

★ frequently asked

Calculate the binding energy per nucleon of a ⁵⁶₂₆Fe nucleus. Given: mass of proton = 1.00783 u, mass of neutron = 1.00867 u, mass of ⁵⁶Fe nucleus = 55.93494 u, 1 u = 931.5 MeV.

Two nuclei have mass numbers in the ratio 1:8. What is the ratio of their nuclear radii?

State the law of radioactive decay. Derive the expression N = N₀e^(−λt). The half-life of a radioactive element is 30 days. How much of 64g of the element remains after 120 days?

Draw the graph showing the variation of binding energy per nucleon with mass number A. Using it, explain (i) the release of energy in nuclear fission and (ii) the release of energy in nuclear fusion.

Define binding energy per nucleon. Why is it that heavy nuclei undergo fission while light nuclei undergo fusion?

Define activity of a radioactive sample and write its SI unit. The half-life of a radioactive nuclide is 20 hours. What fraction of the original activity will remain after 40 hours?

State two characteristic properties of nuclear forces that distinguish them from electrostatic forces.

A radioactive nucleus ²³⁸₉₂U undergoes a series of decays emitting alpha and beta particles to finally become ²⁰⁶₈₂Pb. Calculate the number of alpha and beta particles emitted in the complete decay chain.

How many marks is Nuclei worth in the Class 12 board exam?

Around 4 marks of the 70-mark CBSE Physics theory paper, based on the official unit-wise weightage. Half-life and binding energy numerics are standard.

Is Nuclei easy or hard?

It's rated medium — a moderate-difficulty chapter in Class 12 Physics. Most students need about 3 hours to cover it well.

What are the important topics in Nuclei?

The chapter covers 11 NCERT topics in 3 areas: Nuclear Structure; Nuclear Forces and Radioactivity; Nuclear Reactions and Energy.

What questions come from Nuclei in board exams?

Between 2019–2024, CBSE board papers asked questions from this chapter on Mass-Energy & Binding Energy, Nuclear Density, Radioactivity, Binding Energy Curve — 1- to 5-mark questions.