Radioactivity Short notes and Selected PYQs | SmartScience ICSE Class 10

Radioactivity: The spontaneous emission of radiation from unstable atomic nuclei.
Discovery: Discovered by Henri Becquerel in 1896; studied extensively by Marie and Pierre Curie.
Radioactive Substances: Uranium, radium, thorium, polonium, etc.
Cause: Occurs in unstable nuclei with excess energy; nuclei try to become stable.
Nature: Spontaneous process, unaffected by external physical or chemical conditions.

Alpha (α) Particles: Helium nuclei (2 protons + 2 neutrons). Symbol: ⁴₂He. Charge: +2, Mass: 4 u.
Properties of α: Least penetrating, stopped by paper; highly ionizing; travel few cm in air.
Beta (β) Particles: High-energy electrons. Symbol: ⁰₋₁e. Charge: -1, Mass: negligible.
Properties of β: Moderate penetration, stopped by thin aluminum; moderate ionization; travel few meters in air.
Gamma (γ) Rays: Electromagnetic radiation (photons). No charge or mass.
Properties of γ: Most penetrating, stopped by thick lead or concrete; least ionizing; travel long distances.

Penetrating Power: γ > β > α. Gamma rays are most penetrating.
Ionizing Power: α > β > γ. Alpha particles cause most ionization.
Effect in Magnetic Field: α and β are deflected (opposite directions); γ is undeflected.
Effect in Electric Field: α deflected towards negative plate, β towards positive; γ undeflected.
Speed: α particles: ~10⁷ m/s, β particles: ~10⁸ m/s, γ rays: 3 × 10⁸ m/s (speed of light).

Alpha Decay: ᴬ₂X → ᴬ⁻⁴₂₋₂Y + ⁴₂He (mass number decreases by 4, atomic number by 2).
Beta Decay: ᴬ₂X → ᴬ₂₊₁Y + ⁰₋₁e (mass number unchanged, atomic number increases by 1).
Gamma Emission: No change in mass or atomic number; only energy is released.
Conservation Laws: Mass number and atomic number are conserved in nuclear reactions.
Example: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He (alpha decay of uranium).

Half-Life (t½): Time taken for half the radioactive nuclei in a sample to decay.
Characteristic: Each radioactive element has a fixed half-life, independent of conditions.
Examples: Uranium-238: 4.5 billion years; Carbon-14: 5730 years; Radium-226: 1600 years.
Activity: Number of disintegrations per second. Decreases exponentially with time.
Calculation: After n half-lives, remaining nuclei = N₀/(2ⁿ), where N₀ is initial amount.

Medical Uses: Cancer treatment (radiotherapy), sterilization of equipment, diagnostic imaging (PET scans).
Industrial Uses: Radiography (detecting flaws), thickness gauges, smoke detectors.
Carbon Dating: Uses C-14 to determine age of fossils and ancient artifacts.
Nuclear Energy: Controlled fission in reactors; releases huge energy from small mass.
Safety Precautions: Use lead shields, maintain distance, limit exposure time, proper disposal of waste.
Harmful Effects: Cell damage, mutations, cancer, burns. High doses can be fatal.
Background Radiation: Natural radiation from cosmic rays, rocks, soil; always present at low levels.

Radioactivity