Summary of "Structure of Atom Complete Chapter🔥| CLASS 9th Science | NCERT covered | Prashant Kirad"

Summary of "Structure of Atom Complete Chapter🔥| CLASS 9th Science | NCERT covered | Prashant Kirad"

Main Ideas and Concepts Covered:

  1. Introduction and Motivation
    • Prashant Bhaiya encourages students not to fear exams.
    • The chapter on the structure of the atom will be covered fully and in an easy-to-understand, story-like manner.
    • Emphasis on making the subject interesting and memorable.
  2. Historical Development of Atomic Theory
    • Dalton’s Atomic Theory (1808):
      • Atom is indivisible and the smallest particle.
      • Atoms cannot be broken down further.
    • Thomson’s Discovery of Electron (Cathode Ray Experiment):
      • Atom is divisible and contains negatively charged particles called electrons.
      • Cathode ray tube experiment details: vacuum tube, high voltage, zinc sulphide screen glowing due to electrons.
      • Electron charge = -1.6 × 10⁻¹⁹ Coulombs, mass negligible (~9.1 × 10⁻³¹ kg).
      • Thomson’s "Plum Pudding" or "Watermelon" model: positive charge spread uniformly with electrons embedded like seeds.
    • Goldstein’s Discovery of Proton (Canal Ray / Anode Ray Experiment):
      • Positively charged particles (protons) inside atom.
      • Proton charge = +1.6 × 10⁻¹⁹ Coulombs, mass ~1.67 × 10⁻²⁴ g (about 2000 times electron mass).
    • Chadwick’s Discovery of Neutron (1932):
      • Neutral particle inside nucleus, mass almost equal to proton.
      • No charge, explains additional mass in atom.
  3. Rutherford’s Gold Foil Experiment and Model
    • Alpha particles (He²⁺) fired at thin gold foil.
    • Most passed through (empty space), some deflected, very few bounced back.
    • Conclusions:
      • Atom mostly empty space.
      • Positive charge and most mass concentrated in a tiny nucleus.
      • Electrons revolve around nucleus.
    • Nucleus is very small compared to atom size.
  4. Bohr’s Model of Atom
    • Electrons revolve in fixed, discrete orbits (energy shells) around nucleus without radiating energy.
    • Orbits named K, L, M, N shells.
    • Electrons can jump between shells by absorbing or emitting energy.
    • Ground state = lowest energy level (closest shell to nucleus).
    • Electrons only emit or absorb energy when jumping between orbits.
  5. Electron Distribution and Shell Capacity
    • Electron filling rule: Maximum electrons in a shell = 2n² (n = shell number).
      • K shell (n=1): 2 electrons max
      • L shell (n=2): 8 electrons max
      • M shell (n=3): 18 electrons max
      • N shell (n=4): 32 electrons max
    • Outer shell can hold max 8 electrons for stability (Octet rule).
    • Electrons fill lower shells first before filling higher shells.
  6. Atomic Number, Mass Number, and Notation
    • Atomic number (Z) = number of protons = number of electrons (in neutral atom).
    • Mass number (A) = number of protons + neutrons.
    • Notation of element:

      ^AZX where X = element symbol, A = mass number, Z = atomic number.

    • Number of neutrons = Mass number - Atomic number.
  7. Isotopes and Isobars
    • Isotopes: Atoms of the same element (same atomic number) but different mass numbers (different neutrons).
      • Chemical properties same, physical properties differ.
      • Examples: Hydrogen isotopes – Protium, Deuterium, Tritium.
      • Applications: Uranium isotopes in nuclear reactors, Cobalt isotopes in cancer treatment, Iodine isotopes for goiter treatment.
    • Isobars: Different elements with different atomic numbers but same mass number.
      • Chemical properties differ, physical properties similar.
      • Examples: Argon (Z=18), Potassium (Z=19), Calcium (Z=20) with mass number 40.
  8. Valency and Valence Electrons
    • Valence electrons = electrons in the outermost shell.
    • Valency = combining capacity of an atom (number of electrons lost, gained, or shared).
    • Rule for valency:

Category ?

Educational

Share this summary

Summarize another video

Video