4.2 Developing the model of the atom
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Which model of the atom was replaced as a result of the alpha scattering experiment?
Choose one answer.
Correct Not quite: the answer is Plum pudding model
Mark scheme
B: Plum pudding model
Model answer
Plum pudding model (B). The experiment gave results that the plum pudding model could not explain.
Which scientist provided the evidence for the neutron?
Choose one answer.
Correct Not quite: the answer is Chadwick
Mark scheme
C: Chadwick
Model answer
Chadwick (C).
Describe the plum pudding model of the atom.
Mark scheme
a ball of positive charge (1); with negative electrons embedded / spread through it (1)
Model answer
The atom is a ball of positive charge with the negative electrons embedded in it, like plums in a pudding.
In the alpha scattering experiment, most of the alpha particles went straight through the gold foil. State what this showed about the atom.
Mark scheme
most of the atom is empty space
Model answer
Most of an atom is empty space.
In the alpha scattering experiment, a very small number of alpha particles bounced straight back. Explain what this shows about the nucleus.
Mark scheme
the nucleus is positively charged (1); because positive alpha particles are repelled (1); the nucleus is very small and dense / contains most of the mass (1)
Model answer
The alpha particles are positive and were repelled by the nucleus, so the nucleus is positive. Only a very few came straight back, so the nucleus is very small and dense, with most of the atom's mass.
Some alpha particles were deflected through small angles in the alpha scattering experiment. Explain what this shows.
Mark scheme
the alpha particles were positive and were repelled (1); by a positive charge in the centre / nucleus of the atom (1)
Model answer
The alpha particles passed close to the positively charged nucleus and were repelled, so the centre of the atom must be positive.
Explain why the plum pudding model could not explain the results of the alpha scattering experiment.
Mark scheme
in the plum pudding model the positive charge is spread out through the atom (1); so alpha particles would only be slightly deflected / pass straight through (1); but some were deflected through large angles / bounced back (1)
Model answer
In the plum pudding model the positive charge is spread out, so the alpha particles should be deflected only slightly. But a few were deflected through very large angles, or bounced straight back, so the model had to be replaced.
Describe how Bohr changed the nuclear model of the atom, and explain why his model was accepted.
Mark scheme
Bohr suggested electrons orbit the nucleus at specific distances / in energy levels (1); his calculations agreed with experimental observations (1); of the radiation emitted / absorbed by atoms (1)
Model answer
Bohr suggested that electrons orbit the nucleus at fixed distances, in energy levels. His model was accepted because its predictions agreed with experiments on the radiation that atoms emit and absorb.
Explain why scientists accepted the nuclear model of the atom and rejected the plum pudding model, and why the results of experiments are shared with other scientists.
Mark scheme
the nuclear model explained the alpha scattering results / all the evidence (1); the plum pudding model could not explain them (1); other scientists repeat the experiment to check the results are reliable / valid (1); peer review checks the work before it is accepted (1)
Model answer
The nuclear model explained the alpha scattering observations, which the plum pudding model could not. Results are shared so that other scientists can repeat the experiment and check the results are reliable, and peer review checks the work before it is accepted.
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