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Nuclei and Radioactivity — Revision Notes

NIOS · Class 12 · Physics

Nuclei and Radioactivity revision notes for NIOS Class 12 Physics: 4 topics in quick points. Part of the NIOS Class 12 Physics syllabus.

45 questions36 flashcards18 formulas & key relations5 concepts

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A diagram showing a nucleus composed of positively charged protons and neutral neutrons, collectively called nucleons.
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Key Topics to Revise

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1. The Atomic Nucleus – Structure and Properties

  • The nucleus contains two types of particles: positively charged protons and neutral neutrons. Together they are called NUCLEONS.
  • Atomic Number (Z) = Number of protons in the nucleus = Number of electrons in the atom.
  • Mass Number (A) = Total number of nucleons (protons + neutrons). So, Number of neutrons N = A – Z.
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2. Mass Defect and Binding Energy

  • The measured mass of a nucleus is ALWAYS LESS than the sum of masses of its individual nucleons. This difference is called MASS DEFECT (Δm).
  • Mass defect: Δm = [Z·mₚ + (A–Z)·mₙ] – M, where M is the actual mass of the nucleus.
  • By Einstein's mass-energy equivalence (E = mc²), this lost mass appears as the energy that binds the nucleons together — called BINDING ENERGY (BE).
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3. Nuclear Forces

  • The force that holds nucleons together inside the nucleus is called NUCLEAR FORCE (also called Strong Nuclear Force).
  • Nuclear forces CANNOT be gravitational — gravitational forces between nucleons are about 10³⁹ times weaker and cannot account for the large binding energies.
  • Nuclear forces CANNOT be electrostatic — protons repel each other electrostatically; if only electrostatic forces existed, the nucleus would fly apart.
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4. Radioactivity – Discovery and Nature of Radiation

  • Radioactivity was DISCOVERED BY ACCIDENT by Henri Becquerel in 1896, when he found that uranium salt fogged his photographic plates even in the dark.
  • Marie Curie and Pierre Curie isolated radium and polonium — two new radioactive elements — from uranium ore.
  • RADIOACTIVITY is the spontaneous emission of radiation (α, β, or γ) from unstable atomic nuclei to achieve stability.

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Full Notes

Key Concepts

The nucleus is the extremely tinyThese three terms describe relationships betweenThe radius of a nucleusSince nuclear masses are extremely smallA nucleus weighs LESS than

Frequently Asked Questions

What are the important topics in Nuclei and Radioactivity for NIOS Class 12 Physics?
Key topics in Nuclei and Radioactivity include The Atomic Nucleus – Structure and Properties, Mass Defect and Binding Energy, Nuclear Forces, Radioactivity – Discovery and Nature of Radiation. Study these first, then practise questions on each for the NIOS Class 12 board exam.
How should I revise Nuclei and Radioactivity for the NIOS Class 12 board exam?
Learn the core ideas first, then work through the 45 practice questions on Nuclei and Radioactivity. Revise definitions regularly and use flashcards for quick recall before the exam.

Sources & Official References

Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.

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