Development of the Model of the Atom

• What does atomic theory propose about matter?

  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 elements?

  They are made of different types of spheres
• What model did J.J. Thompson propose in 1897?

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

  Atoms contain negatively charged particles
• How did Thompson describe the atom in his model?

  A ball of 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 and gold foil?

  They should pass through if positive charge is spread out
• What unexpected result did Rutherford observe in his experiment?

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

  Atoms have a compact nucleus with positive charge
• What flaw did Rutherford's model have?

  No explanation for electron stability around nucleus
• Who proposed a solution to Rutherford's model in 1913?

  Niels Bohr
• How did Bohr suggest electrons behave around the nucleus?

  They orbit the nucleus in shells
• Why is Bohr's model important?

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

  It contains small discrete particles called 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 orbit nucleus in shells (1913)
  • Chadwick: Discovered neutrons
• What is the current understanding of the atom's structure?

  Similar to Bohr's model with minor changes
• What does the term "atomic theory" refer to?

  The idea that matter is made of atoms
• How did the understanding of the atom evolve over time?

  Through experiments and new theories by scientists