The History of The Atom

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The History of The Atom

The theory of the atomic structure:

  • 1) At the beginning of the 19th century, John Dalton described atoms as solid spheres, and different spheres made up different elements. 
  • 2) In 1897, J J Thompson found out that atoms weren't solid spheres and showed that an atom must contain even smaller, negatively charged particles- electrons.
    • This became known as the 'plum pudding model' which showed the atom as a ball of positive charge with electrons stuck in it.

The alpha particle scattering experiment:

  • In 1909, Ernest Rutherford conducted an experiment by which he fired positively charged alpha particles at an extremely thin sheet of gold.
  • Most of the particles passed straight through the gold sheet, however some particles were deflected more than expected and a small number deflected backwards.
  • Rutherford came up with the 'nuclear model' of the atom. In this, there's a tiny positively charged nucleus at the centre, surrounded by a 'cloud' of negative electrons -------------->
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The History of The Atom 2

The alpha particle scattering experiment (continued):

  • Most of the atom was empty space. 
  • When alpha particles came near the positive charge of the nucleus, they were deflected. If they were fired directly at the nucleus, they would deflect backwards, otherwise they would just pass through empty space.

Bohr's nuclear model:

  • 1) Scientists realised that electrons surrounding the nucleus would be attracted to the nuceus, therefore causing the atom to collapse.
  • 2) Niels Bohr's nuclear model of the atom suggested that electrons were contained in shells.
  • 3) Bohr said that electrons orbit around the nucleus in fixed shells and each shell is a fixed distance from the nucleus.
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The existence of protons:

  • 1) Rutherford and others came to a conclusion that the nuceus could be divided into smaller particles, each of which has the same charge as a hydrogen nucleus.
  • 2) James Chadwick, 20 years later, carried out an experiment which proved there were neutral particles in the nucleus- neutrons

Electronic structure:

1st shell: 2   2nd shell: 8   3rd shell: 8

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