Lecture on Atomic Theory and Atomic Structure

Introduction to Matter and Atoms

• Definition of Matter: Any substance that has mass and occupies space.
• Atoms: Small particles that make up matter; cannot be further divided.
• Historical Perspective:
  • Democritus (460 BC) proposed the idea of atoms.
  • John Dalton (1808) proposed Dalton’s Atomic Theory.

Dalton’s Atomic Theory

• Key Postulates:
  • Matter is composed of indivisible atoms.
  • Atoms cannot be created or destroyed.
  • Atoms of the same element are identical.
  • Different elements have different types of atoms.
  • Chemical reactions involve the rearrangement of atoms.
  • Compounds are combinations of two or more different types of atoms.

Discoveries and Experiments

Michael Faraday’s Experiment (1830)

• Electric Nature of Matter:
  • Discovered that electricity consists of charged particles.
  • Chemical reactions in electrolyte can deposit matter at electrodes.

William Crookes’ Cathode Ray Experiment (1879)

• Cathode Ray Tubes:
  • Used to study electrical discharge through gases.
  • Discovered cathode rays emitted from the cathode moving to the anode.

J.J. Thomson’s Experiment (1897)

• Discovery of Electrons:
  • Cathode rays deflected by electric and magnetic fields.
  • Determined cathode rays are negatively charged particles (electrons).
  • Calculated charge-to-mass ratio of electrons.

Robert Millikan’s Oil-Drop Experiment (1913)

• Charge and Mass of Electrons:
  • Calculated electron charge: 1.6 x 10^-19 C.
  • Combined with Thomson’s findings, determined electron mass.

Discovery of Protons and Neutrons

• Protons:
  • Discovered by Goldstein through channel rays experiments.
  • Positive ions in discharge tubes; hydrogen ions named protons.
• Neutrons:
  • Discovered by James Chadwick (1932).
  • Neutral particles with mass similar to protons.

Atomic Models

Thomson’s Atomic Model

• Plum Pudding Model:
  • Atom as a sphere with positive charge; electrons embedded within.

Rutherford’s Nuclear Model

• Gold Foil Experiment:
  • Disproved uniform charge distribution.
  • Proposed nucleus-centered model.
  • Nucleus contains protons and neutrons; electrons orbit in space.

Isotopes and Isobars

• Isotopes: Atoms of the same element with different mass numbers.
• Isobars: Different elements with the same mass numbers.

Electromagnetic Radiation and Spectra

Characteristics of Waves

• Wavelength, Frequency, and Velocity:
  • Wavelength: Distance between two wave crests.
  • Frequency: Waves passing a point per second (Hertz).
  • Velocity: Speed of wave in a medium.

Electromagnetic Spectrum

• Order of Waves by Wavelength:
  • Visible light, UV, Infrared, Radio waves, Microwaves, X-rays, Gamma rays.

Planck’s Quantum Theory

• Blackbody Radiation:
  • Energy emitted in discrete quanta.
• Photoelectric Effect:
  • Observed by Hertz; explained by Einstein using quantized light energy (photons).

Bohr’s Model of the Atom

• Explained Hydrogen's Line Spectrum:
  • Electrons occupy quantized orbits.
  • Energy levels explain atomic spectra.

Spectroscopy

• Emission and Absorption Spectra:
  • Emission: Discrete lines due to specific wavelengths.
  • Absorption: Dark lines in a continuous spectrum.

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These notes summarize the key developments in atomic theory, experiments that led to the discovery of subatomic particles, and the models that have shaped our understanding of atomic structure. The lecture also covers the fundamentals of electromagnetic radiation and its role in forming atomic spectra. This content serves as a comprehensive guide to the historical and theoretical aspects of atomic structure.