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Discovery of Main Particles of Atoms

• Introduction to Atomic Structure
    - Focus on discovery of main particles in atoms.
    - Description of Coulomb's Law:
      - Opposite charges attract (positive and negative) and like charges repel.
    - Importance in understanding atomic structure:
      - Established that atoms consist of multiple charged particles.

Historical Context

• Chemical History and Atomic Theory
    - Initial theory by John Dalton (early 1800s):
      - Atoms envisioned as solid spheres.
      - Most of Dalton's theories were later disproved or revised.

Key Experiments and Discoveries

Cathode Ray Experiment

  - Conducted by J.J. Thomson in 1897.
    - Used cathode ray tubes to investigate the nature of electric current.
    - Discovered electrons as negatively charged particles.
  - Experiment Details:
    - Two electrodes (cathode and anode) connected to a battery.
    - Electric current produces a beam of electrons.
    -
    - If no external influence, the beam travels straight.
    - Presence of magnetic fields causes the beam to curve, demonstrating the negative charge of electrons.
  - Importance of the Cathode Ray Experiment:
    - Showed that there are particles within atoms (electrons) and confirmed their negative charge.

Oil Drop Experiment

  - Conducted by Robert Millikan in 1909.
    - Aimed to determine the charge of the electron.
  - Experiment Details:
    - Atomized oil droplets into a chamber.
    - Charged droplets fall through a slit into a parallel plate capacitor.
    - Droplets are held suspended by balancing gravitational and electric forces.
  - Results:
    - Determined that the charge of oil droplets were multiples of a fundamental charge: $1.602 imes 10^{-19} ext{ coulombs}$.
    - Found the charge of one electron from consistent ratios observed.

Gold Foil Experiment

  - Conducted by Ernest Rutherford in 1909.
    - Aimed to probe the structure of the atom.
  - Experiment Details:
    - Alpha particle beam directed at a thin gold foil.
    - Expected model (Plum Pudding Model) suggested particles should pass straight through.
  - Observations:
    - Most particles passed through, but some deflected at large angles.
    - Discovery of the nucleus, a dense center containing most of the atom's mass.
  - Conclusions:
    - Most of the atom is empty space, and electrons orbit around a dense nucleus.

Proton Discovery

  - Movement towards understanding protons post-Rutherford experiments.
  - Experiment by bombarding air with alpha particles.
  - Findings:
    - Emission of hydrogen nuclei, later recognized as protons.
    - All atoms contain protons in their nuclei.

Discovery of Neutrons

  - Neutrons discovered through experiments involving alpha particles and various metals.
  - Characteristics of Neutrons:
    - No electric charge (neutral).
    - Approximately the same mass as protons.
  - Importance:
    - Neutrons contribute to the mass of an atom but do not affect electrical charge.

Summary of Subatomic Particles

• Protons
    - Charge: +1
    - Mass: approximately $1.007 ext{ AMU}$ or $1.67 imes 10^{-24} ext{ g}$
• Neutrons
    - Charge: 0 (neutral)
    - Mass: approximately $1.0087 ext{ AMU}$ or $1.675 imes 10^{-24} ext{ g}$
• Electrons
    - Charge: -1
    - Mass: negligible, approximately $9.109 imes 10^{-31} ext{ g}$

Conclusion: Atomic Structure

• Atoms are composed mainly of protons and neutrons in a dense nucleus, with electrons in orbit around.
• Understanding the atomic structure is foundational for studying chemistry.
• Important for future lessons: distinguishing between protons, neutrons, and electrons, and how to quantify and identify atoms based on these particles.

Additional Notes

• Next lecture will cover reading the periodic table and calculations related to atomic structure.
• Importance of understanding and retaining these foundational concepts for success in chemistry.
• A study guide on the first experiments (Cathode Ray and Gold Foil) may be beneficial for assessments.