History 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 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 reveal about atoms?

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

  It has a positive charge with electrons stuck in it
• What 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 the 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?

  Electrons should collapse into the nucleus
• Who proposed a solution to Rutherford's model in 1913?

  Niels Bohr
• What did Bohr suggest about electron movement?

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

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

  It 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 (1897)
  • Rutherford: Nuclear model (1909)
  • Bohr: Electrons orbiting in shells (1913)
  • Chadwick: Evidence for neutrons
• What are the key differences between the models of the atom proposed by Thompson and Rutherford?

  • Thompson: Atoms are solid spheres with electrons
  • Rutherford: Atoms have a compact nucleus with electrons in a cloud
• What are the main components of the modern atomic model?

  • Nucleus containing protons and neutrons
  • Electrons orbiting in defined shells