Development of Atomic Models

• Describe daltons model (discovery)

  Atoms are indivisible and indestructible particles that combine to form compounds in fixed ratios.

  a solid sphere with no internal structure
• What is a limitation of daltons model?

  Didn't account for the internal structure of the atom (e.g., protons, neutrons, electrons).
• Describe Thomson’s Plum Pudding Model (1897)

  J.J. Thomson discovered the electron while experimenting with cathode rays.

  • The atom is a sphere of positive charge with electrons embedded in it (like "plums" in a "pudding").
  • Model: The atom is mostly positive, with negatively charged electrons scattered inside, balancing the atom's overall neutrality.
• Limitation of the plum pudding model

  Couldn't explain the results of the later gold foil experiment (Rutherford).
• What model did rutherford create?

  Nuclear model
• Describe the nuclear model

  Ernest Rutherford discovered the nucleus after his gold foil experiment.

  • Most of the atom is empty space.
  • A small, dense, positively charged nucleus in the center.
  • Electrons orbit the nucleus at a distance, like planets orbiting the Sun.
  • Model: A tiny, dense nucleus at the center, with electrons moving around the outside in orbits.
• What types of particles did rutherford shoot at the gold foil?

  Alpha
• Limitations of the nuclear model

  • This model couldn’t explain why electrons don't spiral into the nucleus (as classical physics would suggest).
  • It also didn’t explain atomic spectra (the colors of light emitted by atoms).
• Describe bohrs model

  Niels Bohr proposed that electrons exist in fixed energy levels (or shells) around the nucleus and that they can jump between these levels without radiating energy.

  Electrons orbit the nucleus in specific energy levels (like the orbits of planets around the Sun).

  Model: Electrons are found in specific orbits or shells, and these orbits correspond to different energy levels.
• Limitation of Bohrs model

  • This model worked well for hydrogen but couldn't explain the spectra of atoms with more than one electron.
  • Couldn’t fully explain more complex atoms and their behavior.
• describe the Quantum Mechanical Model (modern)

  The quantum mechanical model emerged with contributions from Schrödinger, Heisenberg, and others.

  Electrons do not have fixed orbits. Instead, they are found in regions of probability called orbitals, which define the most likely locations of electrons.

  • Electrons have both particle-like and wave-like properties.
  • Heisenberg's Uncertainty Principle: We cannot know both the position and the momentum of an electron exactly at the same time.
• Describe the key differences between the models of the atom proposed by Bohr and Rutherford.

  Bohr’s Model:

  • Electrons are arranged in fixed, circular orbits around the nucleus.

  Rutherford’s Model:

  • Electrons are located outside the dense nucleus and orbit it like planets around the sun.
  • Electrons don’t have fixed energy levels in Rutherford’s model
• Explain the gold foil experiment

  fired alpha particles (helium nuclei) at a very thin sheet of gold foil.

  Alpha particles were emitted from a radioactive source

  • The gold foil was very thin (only a few atoms thick).
  • A fluorescent screen was used to detect where the alpha particles hit after passing through the gold foil.
• What was the expected and observed outcome in the foil experiment

  • According to the Plum Pudding Model, the positive charge in the atom was spread out evenly. Therefore, the alpha particles should pass through the foil with little deflection.
  • If they encountered the atom, they would mostly experience a small force, causing only a slight deflection.
  • Most alpha particles passed straight through the gold foil with no deflection or only slight deflections.
  • However, some alpha particles were deflected at large angles, and a few were even deflected backward (around 180 degrees)!