# Nuclei Notes, Class 12 Physics | SwaVid Nuclei: NCERT Class 12 Physics notes with key ideas, formulas, common mistakes and 3 quick checks with worked answers. Written by SwaVid from the textbook. Canonical: https://www.swavid.com/cbse/class-12/physics/chapter/nuclei Source: https://www.swavid.com/cbse/class-12/physics/chapter/nuclei # Nuclei ## Formulas and definitions ## Key ideas ## Common mistakes ## Quick checks ## Keep going ## Questions parents ask ## Class 12 Physics on SwaVid ### Nuclear composition and size ### Nuclear density is constant ### Mass defect and binding energy ### Binding energy per nucleon curve ### The nuclear force ### Nuclear fission and fusion ### Next chapter ### Related chapter ### Related chapter ### All chapters SwaVid › CBSE › Class 12 › Physics › Nuclei Notes written by SwaVid from the NCERT textbook. Class 12 Physics, Chapter 13. Key ideas, formulas and common mistakes. CBSE prints 12 marks for Units VII-VIII together, not for this chapter alone. Short answer. This chapter describes the atomic nucleus, including its constituents, size, mass defect, binding energy, and the nuclear force. It explains how nuclear mass converts into energy during fission and fusion reactions. Source: NCERT Physics Part II (Reprint 2026-27) , Chapter 13, Nuclei A nucleus consists of Z protons and N neutrons, giving a total mass number A = Z + N. Its volume is proportional to A, so the nuclear radius follows R = R₀ A^(1/3), where R₀ is about 1.2 × 10⁻¹⁵ m. Because both the mass and volume of a nucleus are directly proportional to the mass number A, the nuclear density is independent of A. For all nuclei, this density is extremely high, approximately 2.3 × 10¹⁷ kg m⁻³. The mass of a stable nucleus is always less than the sum of the masses of its constituent nucleons. This mass difference is called the mass defect, and the equivalent energy is the binding energy that holds the nucleus together. The binding energy per nucleon is roughly constant at around 8 MeV for nuclei with mass numbers between 30 and 170, peaking near iron-56. Very light nuclei can release energy by fusion, while very heavy nuclei can release energy by fission. The nuclear force binds protons and neutrons together inside the tiny nuclear volume. It is a very strong, short-range, attractive force that does not depend on electric charge and saturates as the number of nucleons increases. Nuclear fission splits a heavy nucleus into lighter fragments, releasing energy and neutrons. Nuclear fusion joins light nuclei to form a heavier nucleus at extremely high temperatures and pressures, powering stars like the Sun. Try each question first. Then tap it to see the answer. R₁ / R₂ = (A₁ / A₂)^(1/3) R₁ / R₂ = (27 / 125)^(1/3) = 3 / 5 = 0.6 0.6 E = m c² E = 1.6605 × 10⁻²⁷ × (3.0 × 10⁸)² E = 1.6605 × 10⁻²⁷ × 9.0 × 10¹⁶ = 1.49445 × 10⁻¹⁰ J 1.49445 × 10⁻¹⁰ J E bn = E b / A E bn = 120 MeV / 15 = 8.0 MeV / nucleon 8.0 MeV / nucleon Semiconductor Electronics: Materials, Devices and Simple Circuits . Atoms . Dual Nature of Radiation and Matter . The 14 chapters of Class 12 Physics . CBSE does not give marks to one chapter. It prints 12 marks for Units VII-VIII together (Dual Nature of Radiation and Matter; Atoms and Nuclei). Nuclei is part of that group. No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed. Semiconductor Electronics: Materials, Devices and Simple Circuits. SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works. - Ch 13 NCERT book - 12 marks, Units VII-VIII together - 3 quick checks - Assuming nuclear density increases as mass number A increases . Remember that volume is proportional to A and mass is also proportional to A, so the mass number cancels out, leaving nuclear density roughly constant. - Confusing atomic mass with nuclear mass when finding mass defect . When using atomic masses, include the masses of Z electrons on both sides of the mass defect equation so that the electron masses cancel correctly. - Thinking the nuclear force acts over atomic distances . The strong nuclear force drops sharply to zero beyond a distance of a few femtometres (10⁻¹⁵ m), unlike the long-range Coulomb and gravitational forces. ## Key Links - [CBSE](https://www.swavid.com/cbse) - [Class 12](https://www.swavid.com/cbse/class-12) - [Physics](https://www.swavid.com/cbse/class-12/physics/chapters) - [Start free →](https://www.swavid.com/signup) - [All 14 chapters](https://www.swavid.com/cbse/class-12/physics/chapters) - [NCERT Physics Part II (Reprint 2026-27)](https://ncert.nic.in/textbook.php?leph2=0-6) - [Open notes](https://www.swavid.com/cbse/class-12/physics/chapter/semiconductor-electronics-materials-devices-and-simple-circuits) - [Open notes](https://www.swavid.com/cbse/class-12/physics/chapter/atoms) - [Open notes](https://www.swavid.com/cbse/class-12/physics/chapter/dual-nature-of-radiation-and-matter) - [Class 12 Physics](https://www.swavid.com/cbse/class-12/physics/chapters) - [Start free →](https://www.swavid.com/signup)