Atomic Structure and History Study Notes

Student Learning Objectives

  • Find the atomic mass, atomic number, and symbols of elements on the Periodic Table.
  • Calculate the number of neutrons in a particular atom.
  • Differentiate the properties for isotopes of a particular element.
  • Explain the contributions of various scientists to atomic theory.
  • Explain the observations and interpret results from the Gold Foil and Cathode Ray Tube Experiments.

Evolution of Atomic Models

Evolution of Atomic Models from Dalton to Schrodinger

  • Solid Sphere Model:

    • Scientist: John Dalton
    • Year: 1803
    • Key Contributions: Formulated the first theory of the atom, known as Dalton's Atomic Theory, which consisted of 4 Postulates.
    • Limitations and Failures: Did not account for subatomic particles (protons, neutrons, and electrons) or isotopes.

  • Plum Pudding Model:

    • Scientist: J.J. Thomson
    • Year: 1904
    • Key Contributions: Identified electrons as fundamental components of the atom using the Cathode Ray Experiment.
    • Limitations and Failures: Did not account for the existence of a central nucleus containing protons and neutrons.

  • Nuclear Model:

    • Scientist: Ernest Rutherford
    • Year: 1911
    • Key Contributions: Accounted for the positive and dense mass in the center of an atom, defined as the nucleus, using the Gold Foil Experiment.
    • Limitations and Failures: Did not differentiate between protons and neutrons; failed to explain how electrons remain in orbit without emitting energy and collapsing into the nucleus.

  • Planetary Model:

    • Scientist: Niels Bohr
    • Year: 1913
    • Key Contributions: Included nucleons within the nucleus; proposed stable, quantized orbits for electrons; successfully explained atomic emission spectra.
    • Limitations and Failures: Failed to work well for heavier atoms, where classical electrodynamics predicted electrons would emit energy and collapse into the nucleus.

  • Quantum Model:

    • Scientist: Erwin Schrodinger
    • Year: 1926
    • Key Contributions: Mathematically demonstrated the existence of electron clouds where electrons behave like waves rather than particles in fixed tracks.
    • Current Status: Remains the most widely accepted scientific model of the atom.

John Dalton and Atomic Theory

  • Lifespan: 1766–1844
  • Historical Significance: John Dalton was the first person with experimental evidence to support his theory of the atom.
  • Dalton's Four Postulates:
    • Postulate 1: All elements are composed of invisible particles called atoms.
    • Postulate 2: Atoms of the same element are identical. The atoms from any one element are different from the atoms of other elements.
    • Postulate 3: Atoms of different elements mix or combine in simple whole numbers.
    • Example: Oxygen combines with hydrogen to form water in a 2:12:1 ratio of hydrogen to oxygen.
    • Postulate 4: Chemical reactions occur when atoms separate, combine, or rearrange. In a chemical reaction, atoms of one element never change into atoms of another element.

J.J. Thomson and the Cathode Ray Tube Experiment

  • Lifespan: 1859–1940
  • Experimental Method: Conducted the Cathode Ray Tube Experiment.
  • Observations: Observed that a ray of particles inside the tube bent towards a positive charge.
  • Discovery: Identified a negatively charged subatomic particle within the ray, which was identified as the electron.

Ernest Rutherford and the Gold Foil Experiment

  • Experimental Setup: Shot a high-energy beam of alpha particles into thin gold foil.
  • Observations:
    • Most of the alpha particles passed straight through the gold foil.
    • Some alpha particles bounced back or were deflected at large angles.
  • Discovery:
    • Discovered the nucleus of the atom.
    • Determined that the positive charge and mass of an atom are concentrated in a small, highly dense region at the center called the nucleus.

Discovery of Subatomic Particles

  • Proton:
    • Discovered By: Eugene Goldstein (1850–1930)
    • Identity: Positively charged subatomic particle.
  • Neutron:
    • Discovered By: James Chadwick (1891–1974)
    • Identity: Neutral subatomic particle located within the nucleus.