Class 11 Chemistry: Atomic Structure

Introduction

• Chapter: Atomic Structure (Structure of Atom)
• Previous Topics Covered:
  • Discovery of subatomic particles: Electrons, Protons, Neutrons
  • Charge to mass ratio of electron

Current Topic: Atomic Models

• Importance of understanding the arrangement of subatomic particles within an atom
• Various atomic models proposed by scientists:
  • Thomson's Model
  • Rutherford's Model
  • Bohr's Model
  • Quantum Model (widely accepted today)

Thomson's Atomic Model

• Key Features:
  • Atom is spherical with a radius of 10^-10 meters
  • Protons (positive charges) are uniformly distributed
  • Electrons are embedded throughout, like seeds in a watermelon
  • Known as the "Watermelon Model" or "Plum Pudding Model"
  • Did not account for neutrons
  • Suggested electrostatic attraction between protons and electrons
  • Mass is uniformly distributed
  • Limitations:
    • Could not explain stability of atoms
    • Electrons are not stationary; they move
    • Lacked explanation for neutron presence

Rutherford's Atomic Model

• Experiment: Alpha Ray Scattering Experiment
  • Alpha rays (helium nuclei, positive charge) were used
  • Observations:
    • Most alpha rays passed straight through gold foil
    • Few deflected at small angles
    • Very few (1 in 20,000) bounced back
  • Conclusions:
    • Most of atom's space is empty
    • Nucleus is small, centrally located, and positively charged
    • Electrons revolve around the nucleus
    • Electrons held by electrostatic attraction to nucleus
• Postulates:
  • Atom is spherical with most space empty
  • Nucleus contains most of atom's mass
  • Electrons orbit nucleus in circular paths
  • Compared to solar system (nucleus as sun, electrons as planets)
• Limitations:
  • Did not explain electron energy loss and atom stability
  • Failed to explain electronic structure
  • Did not describe how atoms form molecules or compounds

Next Steps

• Study "Developments towards Bohr's Model"
• Understand concepts necessary for Bohr's Model
• Transition to Bohr's atomic model in the next class

Conclusion

• Today's class covered Thomson and Rutherford models and their limitations
• Next class will focus on Bohr's Model and necessary concepts

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• Note: Keep revisiting these models to understand the evolution of atomic theory and its current understanding with the Quantum Model.