Structure of Atom Lecture

Introduction

• Context: Clarifying misunderstandings about covering the structure of the atom.
• Purpose: Start from the basics, cover the structure, history, fundamental particles, and properties.

Basics of Atom

• Definition: Atom from Greek word 'Atmos' meaning indivisible.
• Greek View: Physical property studied in physics; smallest particles of elements.
• Modern Understanding: Atoms are complex and invisible to the naked eye.

Dalton's Atomic Theory

• Basic Chemical Laws: Law of Conservation of Mass, Law of Definite Proportion, Law of Multiple Proportion.
• **Postulates:
  • All elements are composed of atoms.
  • Atoms of the same element are identical; different elements have different atoms.
  • Atoms combine to form compounds in fixed ratios.
• Fundamental particles considered are protons, neutrons, and electrons.

Discovery of Electrons - Cathode Ray Experiment

• Experiments by Crookes
  • Perforated quartz glass tube with low pressure gas (H2).
  • Cathode and anode electrodes connected to a high voltage source.
  • Observation: Greenish light and dark space in the tube; cathode rays formed.
• JJ Thompson's Work (1897)
  • Repeated Crookes' experiment to confirm electrons.
  • Found cathode rays travel straight, have charge density, and create fluorescence.
• **Properties of Cathode Rays:
  • Negatively charged (electron).
  • Travel in straight lines.
  • Produce shadows and fluorescence.
  • Can pass through thin metals and have high kinetic energy.
  • Deflected by magnetic and electric fields.

Charge to Mass Ratio (

• Charge by mass ratio (e/m) of electron: Initially unknown.
• J.J. Thompson calculated e/m ratio for electron as 1.75882 × 10^8 C/g.
• Millikan's Oil Drop Experiment: Determined charge of electron: -1.6 × 10^-19 C.
• Electron Mass (Using e/m & Charge values): 9.1 × 10^-28 g or 9.1 × 10^-31 kg.

Discovery of Protons - Canal Rays Experiment

• *Goldstein's Canal Rays Experiment:
  • Used perforated cathode, observed positive rays (canal rays).
  • Properties:
    • Positively charged (proton).
    • Same properties as cathode rays but travel in opposite direction.
• Specific Charge by Mass Ratio: E/m ratio varies based on gas used; highest for hydrogen.***

Rudinger's Formula for Proton Size

Thomson’s Watermelon/Pudding Model of Atom

• Concept: Atom is a positively charged sphere with embedded electrons (like watermelon seeds).
• Issues: Failed to explain arrangement/stability of particles and scattering observed by Rutherford.

Rutherford's Alpha Particle Scattering Experiment

• Setup: Alpha particles directed at thin gold foil.
• **Observations:
  • Most particles passed through, few deflected, some rebounded.
• *Conclusions:
  • Most of atom is empty space.
  • Small, dense, positively charged nucleus.
  • Established nuclear model of atom.*

Discovery of Neutrons

• *James Chadwick:
  • Alpha particle bombardment on elements (e.g., Beryllium).
  • Unaffected by electric/magnetic fields -> Neutral particles (neutrons).
  • Neutron mass: 1.675 × 10^-24 kg (similar to proton).*

General Properties

of Nuclear Particles

• Electrons: Negative charge, very small mass (9.1 × 10^-31 kg). Protons: Positive charge, mass ~1.6 × 10^-27 kg.
• Neutrons: No charge, mass ~1.6 × 10^-24 kg.

Summary & Next Steps

• Importance of understanding the history and fundamental concepts.
• Future focus on modern atomic models and quantum mechanics.