models of atom

• What did most people in ancient Greece believe matter was made of?

  Combinations of four elements: earth, air, fire, and water
• What was the alternative idea some Greeks had about matter?

  That matter could be cut into smaller pieces until it became uncuttable
• Who was the ancient Greek philosopher that proposed the idea of uncuttable pieces of matter?

  Demokritos
• What does the word 'atom' mean?

  'Uncuttable'
• What is the plum pudding model of the atom?

  • Proposed by J J Thomson in 1897
  • Atom consists of a big positive 'dough' with negative electrons embedded
  • Represents the atom like a cookie or plum pudding
• What significant particle did J J Thomson discover in 1897?

  The electron
• What evidence supported Thomson's plum pudding model?

  Solids cannot be squashed, indicating atoms must be solid throughout
• What happens when two solids are rubbed together, according to Thomson's model?

  It results in static charge due to the transfer of electrons
• Who conducted the experiment that tested the plum pudding model in 1905?

  Ernest Rutherford
• What did Rutherford's experiment involve?

  Directing a beam of alpha particles at thin gold leaf
• What are alpha particles?

  A form of nuclear radiation with a large positive charge
• Why was a vacuum used in Rutherford's experiment?

  To ensure deflections were only due to collisions with the gold foil
• Why was gold chosen for Rutherford's experiment?

  It could be rolled out to be very thin without cracking
• What were the expected outcomes of Rutherford's experiment if the plum pudding model was correct?

  All alpha particles would pass straight through undeflected
• What did Rutherford conclude from his observations during the experiment?

  Most alpha particles passing through indicated the atom is mostly empty space
• What did the deflection of a small number of alpha particles suggest?

  There is a concentration of positive charge in the atom
• What did the very small number of alpha particles coming straight back indicate?

  The positive charge and mass are concentrated in a tiny volume (the nucleus)
• What did Rutherford discover about the structure of the atom?

  The nuclear atom has a small, positively-charged nucleus surrounded by empty space
• What was the size of the nucleus compared to the atom?

  About 1/10,000th the size of the atom
• What were the further developments to the atomic model after Rutherford?

  • Niels Bohr revised the model in 1913
  • Suggested electrons orbit the nucleus in different energy levels
  • Explained that energy released by electrons corresponds to specific colors in chemical reactions
• How did Bohr's model differ from Rutherford's model?

  Bohr suggested electrons orbit the nucleus at specific distances
• What happens to electrons when atoms absorb energy?

  Electrons are pushed to higher energy levels
• What occurs when electrons jump back down to lower energy levels?

  They release light of definite frequencies
• Who proposed the existence of the neutron?

  James Chadwick
• What was the purpose of Chadwick's experiment?

  To prove the existence of a neutral particle the same size as a proton
• What material did Chadwick use in his experiment?

  A sheet of beryllium and a paraffin block
• What was the outcome of Chadwick's experiment?

  He proved that a proton-sized neutral particle, now known as the neutron, existed
• How did the discovery of the neutron contribute to the understanding of atomic structure?

  It explained the difference between atomic number and atomic mass