loj feb 2004 models of the atom gcse level loj feb 2004
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LOJ Feb 2004
Models of the atom
GCSE Level
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LOJ Feb 2004
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LOJ Feb 2004
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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LOJ Feb 2004
Current model of the atom
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LOJ Feb 2004
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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LOJ Feb 2004
Plum Pudding Model
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LOJ Feb 2004
Plum Pudding Model
• JJ Thomson’s 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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LOJ Feb 2004
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LOJ Feb 2004
Rutherford’s 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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LOJ Feb 2004
Rutherford’s 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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LOJ Feb 2004
Rutherford’s 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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LOJ Feb 2004
Why did his new model become so widely accepted?
• Because it was backed up by experimental evidence!
• It wasn’t 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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LOJ Feb 2004
• 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.