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Nuclei and Radioactivity

NIOS · Class 12 · Physics

Quick revision notes for Nuclei and Radioactivity — NIOS Class 12 Physics. Key concepts, formulas, and definitions for last-minute revision.

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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 Chapter Overview – Nuclei and Radioactivity, Correct vs Incorrect Thinking in Radioactive Decay, Nuclei and Radioactivity — Complete Chapter Mind Map. These are the concepts NIOS Class 12 examiners draw on most — study them first, then practise related questions.
How to score full marks in Nuclei and Radioactivity — NIOS Class 12 Physics?
Understand the core concepts first, then work through the 45 practice questions available for this chapter. Revise formulas and 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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