Honors Chemistry Section 4.2: Understanding the Atom

Introduction

• Focus: Parts of the atom and discovery of atomic structure.
• Importance of experiments in discovering atomic structure, as atoms can't be seen directly.

Subatomic Particles

• Types:
  • Electrons
  • Protons
  • Neutrons
• Charge Interaction:
  • Like charges repel; opposite charges attract.

Characteristics of Subatomic Particles

• Electron (e⁻):
  • Charge: -1
  • Relative Mass: 1/1840
• Proton (p⁺):
  • Charge: +1
  • Relative Mass: 1
• Neutron (n⁰):
  • Charge: 0
  • Relative Mass: 1
• Mass number calculation excludes electrons due to their negligible mass.

Electron Location

• Found in the electron cloud, also referred to as energy levels or orbitals.
• Protons and neutrons are located within the nucleus.

Discoveries in Atomic Theory

Discovery of Electrons

• Scientist: J.J. Thompson (1897)
• Method: Cathode Ray Tube Experiment
  • Cathode rays attracted to positive plates, indicating presence of negative particles (electrons).

Discovery of Protons

• Pioneers:
  • Eugene Goldstein (1886): Found evidence of positive particles.
  • Ernest Rutherford (1911): Rediscovered positive particles, named them protons.
• Charge of Proton: Same as electron, but positive (1.6 x 10⁻¹⁹ C).

Discovery of Neutrons

• Scientist: James Chadwick (1932)
• Nature: Neutrons are neutral (no charge).

Atomic Models

Plum Pudding Model

• Proposed by J.J. Thompson.
• Electrons in a positively charged "pudding".
• Analogy: Like chocolate chips in a cookie.

Rutherford's Gold Foil Experiment

• Purpose: Test the Plum Pudding Model.
• Method:
  • Alpha particles shot at thin gold foil.
  • Expected to pass through with slight deflection.
• Results:
  • Most particles passed through, some deflected, some bounced back.
  • Conclusion: Most of the atom is empty space; concentrated positive mass (nucleus) causing deflections.

Rutherford's Model

• Structure:
  • Nucleus at center containing protons and neutrons.
  • Electrons distributed around nucleus in an electron cloud.
• Key Points:
  • Nucleus: Small, dense, and contains most of atom's mass.
  • Electron cloud: Large volume but little mass.

Section Assessment

1. Subatomic Particles:
   • Proton, Neutron, Electron
2. Rutherford Model Structure:
   • Positive nucleus at center; electrons in cloud.
3. Charges and Relative Masses:
   • Proton: +1 charge, mass 1.
   • Neutron: 0 charge, mass 1.
   • Electron: -1 charge, mass 1/1840.
4. Gold Foil Experiment Outcomes:
   • Expected: Deflection of alpha particles.
   • Actual: Most pass through, some deflect, some bounce back.
5. Evidence for Empty Space in Atoms:
   • Most alpha particles passed through unaffected.
6. Comparison of Models:
   • Plum Pudding: Positive "pudding" with embedded electrons.
   • Rutherford: Central positive nucleus, electrons in cloud.

Conclusion

• Next Lesson Preview (4.3): Focus on atomic details like isotopes and atomic mass calculation.