1.1.5

Model of the Atom

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The Model of the Atom

Our understanding of atoms has improved over time. The models that we have used to understand atoms have also changed.

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Thomson's plum pudding model

  • In 1897, an English physicist called J. J. Thomson discovered electrons.
  • Thomson modelled the atom as a 'plum pudding' - a ball of positive charge (dough), with negatively charged electrons (currants in pudding) mixed in with the 'dough'.
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Rutherford's nuclear model

  • In 1909, Ernest Rutherford discovered that alpha particles could bounce back off atoms.
  • Rutherford concluded that an atom's mass is concentrated in the atom's centre. This was called the "nucleus" and it contained positively charged particles called protons.
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The modern model

  • Niels Bohr discovered that electrons orbit (fly around) the nucleus at fixed distances.
  • In 1932, James Chadwick discovered that some particles in the nucleus have no charge at all. He called them neutrons.

The Charges of Sub-Atomic Particles

The 3 different sub-atomic particles (protons, neutrons and electrons) have different relative charges. In an atom, these charges all cancel each other out., meaning that atoms have zero overall charge (are neutral).

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Protons (+1)

  • Protons have a relative charge of +1.
  • Protons are found in the nucleus.
  • An element’s atomic number is the number of protons it has.
    • All atoms of the same element will have an identical number of protons.
Illustrative background for Electrons (-1)Illustrative background for Electrons (-1) ?? "content

Electrons (-1)

  • Electrons have a relative charge of -1.
  • Electrons are found in fixed orbits around the nucleus.
  • In any atom, the total number of negative electrons equals the number of positive protons, meaning atoms have no overall electric charge.
Illustrative background for Neutrons (0)Illustrative background for Neutrons (0) ?? "content

Neutrons (0)

  • Neutrons have a relative charge of 0 - they are neutral.
  • Like protons, they are found in the nucleus.

Jump to other topics

1Atomic Structure

2Chemical Bonding

3Quantitative Chemistry

4Chemical Changes

5Energy Changes

6The Rate & Extent of Chemical Change

7Organic Chemistry

8Chemical Analysis

9Chemistry of the Atmosphere

10Using Resources

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