History of atom

• • In 1803, John Dalton presented his atomic theory based on three key ideas:
  • Matter is made of atoms which are tiny particles that cannot be created, destroyed, or divided
  • Atoms of the same element are identical, and atoms of different elements are different
  • Different atoms combine together to form new substances
  • At the time, the theory was correct but as science developed some parts of Dalton's theory were disproved
• new experimental evidence may lead to a scientific model being changed or replaced
• The Discovery of the electron led to the plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.
• • In 1909, Ernest Rutherford presented his model of the atom based on the famous gold foil experiment
• • Rutherford shot a beam of positively charged particles at a thin sheet of gold foil and based on the plum pudding model, expected the particles to pass through the foil because the positive charge of the nucleus was thought to be evenly spread out
  • Some particles were scattered, however, and a few were deflected directly back, which led him to postulate that most of an atom's mass is concentrated in a region of space at the centre of the atom called the nucleus
• • The results of Rutherford’s scattering experiments did not support the idea that atoms were as described in the plum pudding model, so the model had to be changed
  • In Rutherford's model, the atom consists mainly of empty spacewith the nucleus at the centre and the electrons orbiting in paths around the nucleus
  • This model was known as the nuclear model of the atom
• Plum pudding model 


  Image of the plum pudding model

  
• Nuclear model atom 


  Image of the nuclear model atom

  
• • In Rutherford's model, the atom consists mainly of empty spacewith the nucleus at the centre and the electrons orbiting in paths around the nucleus
  • This model was known as the nuclear model of the atom
• • In 1913, Niels Bohr further developed the nuclear model by proposing that electrons orbit the nucleus in fixed shells or orbitals located at set distances from the nucleus
  • Each orbital has a different energy associated with it, with the higher energy orbitals being located further away from the nucleus
• • Bohr’s theory and calculations agreed with experimental results
• • Further investigation and experimentation revealed that the nucleus could be divided into smaller particles, each one having the same mass and charge
  • This work led to the discovery of the proton
• The Bohr model

  Image of the Bohr model

  
• • In 1932, James Chadwick published a paper based on an experiment which provided evidence for the existence of these neutral particles which were called neutrons