Menu

Nuclei

NCERT Class 12 Physics Chapter 13

Nuclei for Class 12 Physics, JEE and NEET: nuclear size and composition, mass defect, binding energy, nuclear force, fission and fusion.

6 topics Updated 8 Nov 2026

For: Class 12, JEE and NEET, unless a topic says otherwise

In the syllabus: JEE Class 12 syllabus, NEET Class 12 syllabus

With your textbook

NCERT Physics Part II (Class 12), Chapter 13 “Nuclei”, reprint 2026-27.

Open the official NCERT chapter (PDF, 1.8 MB, ncert.nic.in)

Checked on 19 Oct 2026.

Learn in this order

  1. Atomic masses and composition of the nucleus

    Protons and neutrons, atomic mass unit, discovery of the neutron, isotopes, isobars and isotones.

    NCERT Section 13.2 12 min read 4 solved examples 8 practice questions

  2. Size of the nucleus

    Nuclear radius formula R = RA1/3, R = 1.2 fm, why nuclear density is constant, and solved examples.

    NCERT Section 13.3 9 min read 4 solved examples 8 practice questions

  3. Mass-energy and nuclear binding energy

    E = mc, mass defect, how to calculate binding energy per nucleon, its curve and solved examples.

    NCERT Section 13.4 14 min read 4 solved examples 10 practice questions

  4. Nuclear force

    Properties of nuclear force (strong, short range, charge independent), saturation, the potential energy graph.

    NCERT Section 13.5 11 min read 4 solved examples 7 practice questions

  5. Radioactivity

    Becquerel’s discovery, alpha, beta and gamma decay, how each changes A and Z, what half-life means, and solved examples.

    Class 12 only NCERT Section 13.6 11 min read 4 solved examples 8 practice questions

  6. Nuclear energy: fission and fusion

    Why both release energy, Q values, 200 MeV per fission, fusion in the sun and controlled fusion.

    NCERT Section 13.7 17 min read 5 solved examples 10 practice questions

Key formulas in this chapter

Composition of the nucleus

  • 1 u = 1.660539 × 10−27 kg
  • A = Z + N
  • mp = 1.00727 u
  • mn = 1.00866 u

Size of the nucleus

  • R = R0 A1/3, R0 = 1.2 fm
  • ρ = 3m/(4π R0^3) ≈ 2.3 × 1017 kg m−3

Binding energy

  • E = mc2
  • 1 u = 931.5 MeV/c2
  • ΔM = Z mp + (A − Z) mn − M
  • Eb = ΔM c2
  • ΔM = Z mH + N mn − Matom
  • Sn = (MA−1 + mn − MA) c2
  • Ebn = Eb/A

Nuclear force

  • F = −dU/dr
  • U(r) is least at r0 ≈ 0.8 fm

Radioactivity

  • α decay: A → A − 4, Z → Z − 2
  • β decay: Z → Z + 1; β decay: Z → Z − 1
  • N = N0/2n

Nuclear energy

  • Q = (minitial − mfinal) c2
  • (3/2) k T ≈ K
  • 4 H-1 atoms → He-4 atom + 2 neutrinos, releasing about 26.7 MeV

More in Physics