History of the atom

• Who described atoms as solid spheres at the start of the 19th century?

  John Dalton
• What did John Dalton say about different elements?


  Different spheres made up the different elements
• What did J.J. Thomson conclude about atoms in 1897?


  Atoms weren’t solid spheres
• What did Thomson's measurements show about atoms?


  Atoms must contain smaller, negatively charged particles called electrons
• What was the new theory of atomic structure proposed by J.J. Thomson?


  The 'plum pudding model'
• How did the plum pudding model depict the atom?


  As a ball of positive charge with electrons stuck in it
• Who conducted the alpha particle scattering experiments in 1909?

  Ernest Rutherford and Ernest Marsden
• What were Rutherford and Marsden expecting to happen in their experiments?


  They expected the particles to pass straight through or be slightly deflected
• What unexpected result did Rutherford and Marsden observe during their experiments?


  Some particles were deflected more than expected, and a small number were deflected backwards
• What did Rutherford propose to explain the new evidence from his experiments?


  The nuclear model of the atom
• What does the nuclear model of the atom state about the nucleus?


  It has a tiny, positively charged nucleus at the center where most of the mass is concentrated
• What surrounds the nucleus in the nuclear model?


  A 'cloud' of negative electrons
• Why were alpha particles deflected in Rutherford's experiments?


  They were deflected by the concentrated, positive charge of the nucleus
• What did scientists realize about electrons in a 'cloud' around the nucleus?


  They would be attracted to the nucleus, causing the atom to collapse
• What did Niels Bohr propose about the arrangement of electrons?


  Electrons orbit the nucleus in fixed shells
• What is a characteristic of the shells proposed by Bohr?


  Each shell is a fixed distance from the nucleus
• How was Bohr's theory of atomic structure received by the scientific community?


  It was supported by many experiments and helped explain other observations
• What did further experiments by Rutherford and others reveal about the nucleus?


  The nucleus can be divided into smaller particles called protons
• What charge do protons have?


  They have the same charge as a hydrogen nucleus
• Who provided evidence for neutral particles in the nucleus?


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


  Neutrons
• What is the modern accepted version of the atomic model?

  • The nuclear model
  • Contains a nucleus with protons and neutrons
  • Electrons orbit in fixed shells around the nucleus
• How has the understanding of atomic structure evolved over time?


  1. John Dalton: Atoms as solid spheres
  2. J.J. Thomson: Plum pudding model with electrons
  3. Ernest Rutherford: Nuclear model with a nucleus
  4. Niels Bohr: Electrons in fixed shells
  5. Discovery of protons and neutrons
• What did John Dalton propose about the atomic model in the early 1800s?


  He proposed that atoms are spheres that cannot be divided.
• Who discovered the electron and in what year?


  1. J. Thomson discovered the electron in 1897.
• What is the plum pudding model of the atom?


  It describes the atom as a ball of positive charge containing small negative electrons.
• What model did Ernest Rutherford propose in 1909?


  He proposed the nuclear model of the atom.
• What does the nuclear model state about the atom's structure?


  The mass is concentrated in a small, dense nucleus with electrons orbiting around it.
• What did Niels Bohr contribute to the atomic model in 1913?


  He proposed that electrons orbit the nucleus in fixed paths.
• What is the significance of the nucleus in the atomic model?


  The nucleus contains most of the atom's mass and is made up of protons and neutrons.
• Who discovered the neutron and in what year?


  James Chadwick discovered the neutron in 1932.
• What are the main components of the atomic nucleus?


  • Protons (positively charged)
  • Neutrons (neutral charge)
• How does the atomic model evolve from Dalton to Rutherford?


  It evolves from a solid sphere model to a model with a dense nucleus and orbiting electrons.
• What is the role of electrons in the atomic model?


  Electrons orbit the nucleus and are responsible for chemical bonding.