Models of the atom

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Models of the atom

• GCSE Level

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The atom

• The atoms are listed in the periodic table.
• What do you know about the atom?
• What have you been taught about it?

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Current model of the atom
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You need to know

• In a neutral atom, the number of electrons is
  equal to the number of protons in the nucleus.
  The atom as a whole has no electrical charge.
• The number of protons determines which element
  you are dealing with
• The number of neutrons determines which isotope
  of the element you are dealing with.

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Plum Pudding Model
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Plum Pudding Model

• JJ Thomsons model was sensible!
• They knew that electrons were tiny specks that
  could be removed from atoms so they were seen
  as tiny plums in a large plum pudding.
• The whole atom was thought of as a positive
  sphere embedded with negative electrons just as a
  plum pudding was embedded with plums.
• This was the pre - 1911 understanding of what the
  atom was like!

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Rutherfords Experiment

• The plum pudding atom indicated that the atom
  was made of smeared out positive charge with
  concentrated specks of negative charge embedded
  in it
• So if you fired alpha particles at it you
  expected them to go straight through
• Any slight change in trajectory would indicate
  how the charge gradient altered within the sphere
  
• Was it getting gradually more dense towards the
  centre? Or was it denser around the edge like a
  shell? How was the charge distributed?
• In 1911 Rutherford hoped to find out!

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Rutherfords Experiment

• His findings astounded him!
• The fact that the vast majority of the alpha
  particles got straight through led Rutherford to
  propose that the atom was composed primarily of
  empty space.

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Rutherfords Experiment

• The fact that backscattering occurred in 1 in
  8000 alpha particles indicated that the nucleus
  in the centre was
• small (that was why so few were affected)
• massive (meaning containing lots of mass - he
  knew the electrons had very little mass and the
  fact that all of the positive charges were
  concentrated into a small area meant that the
  mass was concentrated there too)
• positively charged (because it repelled the alpha
  particles)

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Why did his new model become so widely accepted?

• Because it was backed up by experimental
  evidence!
• It wasnt common sense but the evidence showed
  it was true!
• Others were able to repeat the experiment and
  find this out for themselves
• So the old model was replaced by the new one

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• The proton number Z (or atomic number) indicates
  the element to which the atom belongs
• All atoms of a particular element X have the
  same number of protons. Atoms of different
  elements have different numbers of protons.
• The total number of protons and neutrons
  (nucleons) in an atom is called its nucleon
  number A.