History of the Model of the Atom

• What does atomic theory propose?

  Everything is made of tiny, indivisible particles
• Who originally proposed atomic theory?

  Democritus
• When did Democritus live?

  Around 500 BC
• What did John Dalton describe atoms as?

  Solid spheres
• What did Dalton suggest about different types of spheres?

  They make up different elements
• What model did J.J. Thompson propose in 1897?

  The plum pudding model
• What did Thompson's experiments show about atoms?

  Atoms contain negatively charged particles
• What did Thompson propose about the structure of the atom?

  It has a positive charge with electrons
• What significant experiment did Ernest Rutherford conduct in 1909?

  Fired alpha particles at gold foil
• What was Rutherford's expectation about alpha particles passing through gold?

  They should pass through without deflection
• What unexpected result did Rutherford observe in his experiment?

  Some alpha particles were deflected
• What did Rutherford conclude from his experiment with gold foil?

  Atoms have a compact nucleus
• What did Rutherford suggest about the negative charge in atoms?

  It exists in a cloud around the nucleus
• What flaw did Rutherford's model have regarding electron behavior?

  Electrons should collapse into the nucleus
• What solution did Niels Bohr propose in 1913?

  Electrons orbit the nucleus in shells
• Why is Bohr's idea about electron orbits important?

  It prevents the atom from collapsing
• What did further experiments by Rutherford discover about the nucleus?

  It contains protons
• Who provided evidence for neutral particles in the nucleus?

  James Chadwick
• What are neutral particles in the nucleus called?

  Neutrons
• What are the key developments in atomic theory from Democritus to Chadwick?

  • Democritus: Proposed atomic theory (500 BC)
  • Dalton: Described atoms as solid spheres
  • Thompson: Plum pudding model (1897)
  • Rutherford: Nuclear model (1909)
  • Bohr: Electrons orbiting in shells (1913)
  • Chadwick: Discovered neutrons