In-Depth Notes on Atomic Models and Structure

Models of the Atom

  • Democritus/Dalton Model
    • Earliest model of the atom, proposed by Democritus who coined the term atomos (meaning uncuttable).
  • Law of Conservation of Mass (1789)
    • Mass is neither created nor destroyed, only transformed.
    • Key contributors: Antoine Lavoisier & Marie-Anne Paulze Lavoisier.
  • Other Conservation Laws
    • Matter, energy, charge, and momentum.

Dalton’s Atomic Theory (1808)

  • Formulated by John Dalton, consisting of several postulates:
    1. All matter consists of extremely small particles called atoms.
    2. Atoms of the same element are identical; atoms of different elements vary in size, mass, and properties.
    3. Atoms cannot be created, subdivided, or destroyed by chemical or physical means.
    4. Atoms of different elements combine in whole number ratios to form compounds.
    5. In chemical reactions, atoms are rearranged.

Discoveries of Subatomic Particles

  • J.J Thomson (1897)
    • Discovered the electron using a Crookes tube (cathode ray tube).
    • Thomson Model or Plum Pudding Model:
      • Electrons (plums) embedded in a positively charged sphere (pudding).
  • Robert Millikan (1909)
    • Determined the mass of the electron via the oil drop experiment.
  • Eugen Goldstein (1886)
    • Discovered protons using cathode ray tubes; initially referred to as Canal Rays.
  • Hantaro Nagaoka (1904)
    • Proposed a planetary model of the atom with electrons orbiting a massive nucleus (similar to rings of Saturn).
  • Rutherford’s Gold Foil Experiment (1908)
    • Discovered the atomic nucleus:
    • Found it contains positively charged protons and has a small volume, mostly empty space.

Atomic Structure and Models

  • James Chadwick (1932)
    • Discovered the neutron, distinguishing it from protons and electrons.
  • Bohr Model (1913)
    • Builds on observations from flame experiments and spectroscopy.
    • Electrons occupy quantized energy levels and transitions emit/absorb electromagnetic radiation.
  • Energy and Electron Levels
    • Valence electrons are outer shell electrons influencing chemical properties.
    • Core electrons are stable, inner electrons representing noble gases.

Quantum Mechanical Model

  • Quantum Theory
    • Describes where electrons reside via probabilities, with electrons found in electron clouds.
    • Each electron identified by four quantum numbers:
    1. Principal quantum number (n): energy level.
    2. Angular momentum quantum number (l): orbital shape.
    3. Magnetic quantum number (mℓ): orientation in space.
    4. Spin quantum number (ms): electron spin direction.

Electron Configuration and Orbital Filling

  • Filling Rules:
    • Aufbau Principle: Fill orbitals starting from lowest energy.
    • Pauli Exclusion Principle: Maximum of 2 electrons per orbital (opposite spins).
    • Hund’s Rule: Electrons must occupy all orbitals singly before pairing.
  • Types of Electrons:
    • Core Electrons: No role in chemical bonding.
    • Valence Electrons: Participates in bonding; same number across groups in the periodic table.
    • Unpaired Electrons: Single in orbitals, energies dictate reactivity.

Noble Gas Configuration

  • Noble gases (He, Ne, Ar, Kr, Xe, Rn):
    • Known for being unreactive.
  • Abbreviated electron configurations (noble gas notations) summarize electron arrangements in elements.

Ions and Electron Loss/Gain

  • Cations: Positive ions formed from loss of electrons (more protons than electrons).
  • Anions: Negative ions formed from gain of electrons (more electrons than protons).

Advanced Topics in Electron Configuration

  • Discusses the filling order for d and f orbitals in higher periods (4th and 5th), including specifics on energy levels and orientations.

Overall Summary of Electron Configurations

  • Review example ions and noble gas configurations to solidify understanding of atomic configurations in various elements.