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?

  Different types of spheres make up different elements
• 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 did Rutherford expect to happen in his gold foil experiment?

  Alpha particles should pass through gold
• What was the actual result of Rutherford's experiment?

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

  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?

  No explanation for electron stability
• Who proposed a solution to the flaw in Rutherford's model?

  Niels Bohr
• What did Bohr suggest about electron orbits?

  Electrons orbit the nucleus like planets
• Why is Bohr's idea important?

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

  The nucleus contains protons
• Who provided evidence for neutrons in the nucleus?

  James Chadwick
• What are neutrons?

  Neutral particles in the nucleus
• What is the progression of atomic theory from Democritus to Chadwick?

  • Democritus: Proposed atomic theory (500 BC)
  • Dalton: Described atoms as solid spheres
  • Thompson: Plum pudding model (negatively charged electrons)
  • Rutherford: Nuclear model (compact nucleus)
  • Bohr: Electrons orbit nucleus in shells
  • Chadwick: Discovered neutrons in the nucleus