Interactive Chemistry Worksheets for Students |
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Scientists |
Model |
Description |
Evidence |
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The atom is the smallest particle of an element. The atom is a solid, indestructible unit. Atoms of different elements have different masses. |
Law of Constant Proportions. (% mass composition) Law of Multiple Proportions. E.g. Two compounds of nitrogen oxide. The ratio of oxygen between both compounds is 2:3. |
J. Dalton |
The Billiard Ball model (1808) |
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The ‘pudding’ is the positive material of an atom. The embedded ‘raisins’ are negative electrons. |
Cathode rays (negative particles or electrons) Canal rays (positive particles with large masses) |
J. J. Thomson |
The Plum Pudding model (1903) |
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The mass and positive charge of an atom is concentrated in a small core called the nucleus. Negative electrons orbit the nucleus. The atom is mostly made up of empty space. |
The scattering of alpha particles through thin gold foil. A small percentage of particles were deflected at large angles, some even returning. ‘it is like a cannonball rebounding off a piece of paper’ |
Ernest Rutherford |
The Nuclear model (1911) |
Protons & later neutrons are identified. |
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Electrons are arranged around the nucleus in discrete energy levels or shells. |
Explains emission spectra (flame test) Explains patterns in the successive ionisation energies of an element. |
Neils Bohr |
The Shell model (1923) |
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s-orbital |
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Electrons exist in orbitals. i.e. an area surrounding the nucleus that has a 90% probability of containing an electron. |
Better explains anomalies in successive ionisation energies and emission spectra. |
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Orbitals differ in shape(s, p, d, & f orbitals) and size(dependent upon energy level). No orbital may contain more than two electrons. |
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Various Scientists |
The Quantum model (1935) |
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