Structure of Atom Lecture Notes

Jul 16, 2024

Structure of Atom

Introduction

  • Importance of the chapter in competitive exams (NEET, JEE, etc.)
  • Deep dive compared to basic school-level concepts

Dalton's Atomic Theory

  • Matter is composed of small particles called atoms
  • Atoms combine to form chemical compounds
  • Chemical reactions involve rearrangement of atoms
  • Atoms are indivisible

Discovery of Subatomic Particles

  • Late 19th century: Discovery of subatomic particles within atoms
  • Main subatomic particles: Electrons, Protons, Neutrons

Atomic Models

  • Various atomic models explaining the arrangement of subatomic particles
    • Thomson's Atomic Model
    • Rutherford's Model (studied in school)
    • Bohr's Atomic Model
    • Quantum Mechanical Model: The accepted model

Development toward Bohr's Model

  • Electromagnetic radiation and atomic spectra
  • Hydrogen spectra
  • Quantum theories leading to Bohr's Model
  • Introduction to Quantum Mechanical Model

Discovery of Electrons

  • Scientist: J.J. Thomson
  • Experiment: Cathode Discharge Tube Experiment
    • Glass tube with two metal electrodes connected to high voltage
    • Negative terminal: Cathode, Positive terminal: Anode
    • Used 10,000 volts, low pressure, often filled with hydrogen gas
    • Zinc Sulfide coated ends to act as a fluorescent material
    • Observation of rays traveling from cathode to anode
    • Rays deflect towards the positive side of electric and magnetic fields, indicating they have a negative charge (electrons)

Characteristics of Cathode Rays (Electrons)

  1. Travel in straight lines
  2. Deflected towards the positive side of electric and magnetic fields
  3. Properties do not depend on the nature of the gas within the discharge tube

Charge to Mass Ratio of an Electron

  • Scientist: J.J. Thomson
  • Experiment to determine charge-to-mass ratio (e/m) using electric and magnetic field deflections
  • Calculations leading to e/m ratio: 1.75 x 10^11 C/kg

Factors Affecting Deflection

  1. Magnitude of charge: Greater charge, greater deflection
  2. Strength of electric and magnetic fields: Greater field strength, greater deflection
  3. Mass of particles: Lighter particles deflect more

Charge and Mass of an Electron

  • Scientist: Millikan
  • Experiment: Oil Drop Method
  • Charge of Electron: -1.6 x 10^-19 Coulombs
  • Mass of Electron: 9.1 x 10^-31 kg

Conclusion

  • Three crucial values to remember:
    1. Charge of an electron: -1.6 x 10^-19 Coulombs
    2. Charge to mass ratio: 1.75 x 10^11 C/kg
    3. Mass of an electron: 9.1 x 10^-31 kg