Early Models of the Atom

John Dalton's Model

• Early 1800s: First formalized atomic theory.
• Inspired by Greek Philosopher Democritus
  • Democritus coined "atomos": smallest indivisible particle.
  • Greeks were philosophers, not experimenters.

Key Statements

1. Matter is composed of tiny indivisible particles (atoms)
2. All atoms of a given element are identical
   • E.g., all hydrogen atoms are the same.
3. Atoms of one element are different from atoms of any other element
   • E.g., hydrogen atom ≠ carbon atom.
4. Additional statements on atoms combining to form compounds
   • Discussed in later chapters.

Dalton's Model

• Visual Representation: Solid sphere with no internal structure.
• No protons, neutrons, electrons: Technology to observe these particles nonexistent.

J.J. Thompson's Model (1897)

• Key Contribution: Introduced internal structure to the atom.
• Discovery of the Electron
  • Negative particles (electrons) stream out from metal.
  • Positively charged sphere embedded with electrons (negatively charged particles).

Thompson's Model: Plum Pudding/Chocolate Chip Cookie

• Plum Pudding Analogy: British dessert with nuts and berries inside.
• Chocolate Chip Cookie Analogy: Cookie (positively charged sphere), Chocolate chips (electrons).

Ernest Rutherford's Model

• Rutherford's Background: Student of J.J. Thompson.
• Gold Foil Experiment
  • Setup: Radioactive substance emitting alpha particles aimed at a thin sheet of gold foil.
  • Outcome: Detected large deflections of alpha particles, contrary to expectations.
• Key Discovery: Nucleus
  • Nucleus: Dense central region, positively charged, contains most of the atom's mass.
  • Electrons: Still present but orbit around the nucleus.
  • Result: Large deflections when alpha particles hit the positively charged nucleus.

Rutherford's Model: Nuclear Model

• Key Change: Introduction of the nucleus with concentrated positive charge.

Future Topics

• Bohr Model: Next atomic model to be discussed.
• Quantum Model: Follow-up topic after Bohr model.