Practise The development of the model of the atom. 14 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How the model of the atom changed as new evidence was found: from tiny indivisible spheres, to the plum pudding model, to the nuclear model, then Bohr's energy levels, protons and neutrons. Expect 'describe', 'compare' and 'explain' questions, especially on why the alpha particle scattering experiment led to the nuclear model, and sometimes a 6-mark answer.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Put the models of the atom in orderTiny spheres, plum pudding, nuclear model, Bohr's energy levels, then protons and neutrons.
4
Describe the plum pudding modelThe atom is a ball of positive charge with negative electrons embedded in it.
5
Describe the nuclear modelA tiny, positively charged nucleus containing most of the mass, with electrons around it.
5
Compare plum pudding and nuclear modelsCharge and mass spread out, compared with concentrated in a nucleus with empty space around it.
6
Explain each alpha scattering observationLink 'straight through', 'deflected' and 'bounced back' to what each shows about the atom.
7
Explain why the plum pudding model was replacedIt predicted only tiny deflections, so large deflections needed a new model with a concentrated, charged nucleus.
7
Explain how new evidence changes modelsWhen experiments give results a model cannot explain, scientists change or replace the model.
Notes
Early models
Scientific models change when new experimental evidence cannot be explained by the old model.
Before the electron was discovered, atoms were thought to be tiny spheres that could not be divided.
The discovery of the electron showed that atoms contain smaller particles. This led to the plum pudding model: the atom is a ball of positive charge with negative electrons embedded in it.
The alpha particle scattering experiment
Positively charged alpha particles were fired at a very thin sheet of gold foil, and detectors recorded where they went.
The plum pudding model predicted that they would all pass through with very small deflections, because the positive charge and mass were spread out.
Most alpha particles passed straight through → most of the atom is empty space.
A few were deflected through large angles → the positive charge is concentrated in a tiny region at the centre, which repels the alpha particles.
A very small number bounced back → most of the mass is concentrated in that tiny region, the nucleus.
So the nuclear model replaced the plum pudding model: a tiny, positively charged nucleus containing most of the mass, with electrons around it.
Later developments
Niels Bohr adapted the nuclear model: electrons orbit the nucleus at specific distances (energy levels). His theoretical calculations agreed with experimental observations.
Later experiments showed that the positive charge of any nucleus could be divided into smaller particles, each with the same amount of positive charge. These are protons.
James Chadwick's experiments provided evidence for neutrons in the nucleus, about 20 years after the nucleus became an accepted idea.
You do not need the details of Bohr's or Chadwick's experimental work.
Cheatsheet
Order: tiny spheres → plum pudding → nuclear model → Bohr's energy levels → protons → neutrons (Chadwick)
Plum pudding: ball of positive charge with electrons embedded in it
Nuclear model: tiny positive nucleus with most of the mass, electrons around it
Most alpha particles went straight through → atom is mostly empty space
A few deflected through large angles → positive charge concentrated at the centre
Very few bounced back → mass concentrated in a tiny nucleus
Bohr: electrons orbit at specific distances
Chadwick: evidence for neutrons
How to answer each type of question
Compare the plum pudding and nuclear models
2 marks5
Pick a feature: where the positive charge is, where the mass is, or empty space.
Say what each model says about it in the same sentence.
Use 'whereas' to make the comparison clear.
Example. Describe two differences between the plum pudding model and the nuclear model of the atom.
Show the model answer
In the plum pudding model the positive charge is spread through the whole atom, whereas in the nuclear model it is concentrated in a tiny nucleus (1). In the plum pudding model the mass is spread evenly, whereas in the nuclear model most of the mass is in the nucleus (1). Also accept: the nuclear model is mostly empty space; the plum pudding model has no nucleus.
Explain the alpha scattering observations
3 marks6
Match each observation to one conclusion.
Straight through → empty space.
Deflected → positive charge concentrated in the nucleus. Bounced back → mass concentrated in the nucleus.
Example. In the alpha particle scattering experiment, alpha particles were fired at thin gold foil. Explain what each observation shows about the structure of the atom. (a) Most alpha particles passed straight through the foil. (b) A few alpha particles were deflected through large angles. (c) A very small number of alpha particles bounced back.
Show the model answer
(a) Most of the atom is empty space (1) (b) The centre of the atom (the nucleus) is positively charged, so it repels the positive alpha particles (1) (c) Most of the mass of the atom is concentrated in a very small nucleus (1)
Explain why the model was replaced
2 marks7
Say what the old model predicted would happen.
Say what the experiment actually showed, and that the old model could not explain it.
Example. Explain why the alpha particle scattering experiment led to the plum pudding model being replaced.
Show the model answer
The plum pudding model predicted that alpha particles would pass through the foil with little or no deflection (1). Some alpha particles were deflected through large angles or bounced back, which the plum pudding model could not explain (1).
6-mark: describe how the model of the atom developed
6 marks (level of response)7
Go in time order, starting from the plum pudding model.
For each change, give the evidence and the new model.
Include the alpha scattering results and what they showed, then Bohr, protons and Chadwick.
Example. Describe how the model of the atom has changed from the plum pudding model to the model used today. Include the evidence for each change.
Show the model answer
Model answer: In the plum pudding model, the atom was a ball of positive charge with negative electrons embedded in it. In the alpha particle scattering experiment, positive alpha particles were fired at thin gold foil. The plum pudding model predicted they would pass through with very little deflection. Most did pass straight through, showing that the atom is mostly empty space. But a few were deflected through large angles and a very small number bounced back, showing that the positive charge and most of the mass are concentrated in a tiny nucleus. So the nuclear model replaced the plum pudding model. Niels Bohr then suggested that electrons orbit the nucleus at specific distances, and his calculations agreed with experimental observations. Later experiments showed that the positive charge of the nucleus is made of smaller particles with the same charge, called protons. About 20 years after the nucleus was accepted, James Chadwick provided evidence that the nucleus also contains neutrons. Level 3 (5 to 6 marks): the models described in the correct order, each change clearly linked to its evidence, including what the alpha scattering results showed. Level 2 (3 to 4 marks): most models described in order, with some evidence given. Level 1 (1 to 2 marks): some models named or described, with little order or evidence.
Shortcuts and memory tricks
Order of models: 'Some Pupils Need Better Physics Notes' = Spheres, Plum pudding, Nuclear, Bohr, Protons, Neutrons.
Alpha results in three words: through (empty space), deflected (positive nucleus), back (mass in the nucleus).
Chadwick found the chargeless particle: the neutron.
Where marks are lost
Describing the plum pudding model without saying the 'pudding' is positive charge.
Saying alpha particles bounced back because they hit electrons. Electrons are far too light to turn an alpha particle back.
Saying most alpha particles were deflected. Most went straight through; only a few were deflected through large angles.
Mixing up the scientists: Bohr = electrons at specific distances; Chadwick = neutrons.
Saying the old model was 'wrong' without saying what new evidence it could not explain.
Exam technique
For each alpha scattering observation, give the observation AND what it shows about the atom.
In 6-mark answers, keep a clear time order and link every change to the evidence that caused it.
'Compare' means each point must mention both models, e.g. using 'whereas'.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Describe the plum pudding model of the atom.
A ball of positive charge with negative electrons embedded in it.
Niels Bohr adapted the nuclear model of the atom. What did Bohr suggest about the electrons?
The electrons orbit the nucleus at specific distances.
Later experiments showed that the positive charge of a nucleus could be subdivided into a whole number of smaller particles, each with the same charge. What name was given to these particles?
Protons
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Before the electron was discovered, what did scientists think atoms were like? Tick (✓) one box.[1]
Tiny spheres that could not be divided
Balls of positive charge with electrons embedded in them
Tiny positive nuclei with electrons orbiting them
Nuclei containing protons and neutrons
(b) The discovery of which particle led to the plum pudding model of the atom? Tick (✓) one box.[1]
Alpha particle
Electron
Neutron
Proton
(c) Describe the plum pudding model of the atom.[2]
Show the answer and mark scheme
(a)Answer: Tiny spheres that could not be divided
(b)Answer: Electron
(c)Answer: A ball of positive charge with negative electrons embedded in it.
a ball / sphere of positive charge
with (negative) electrons embedded in it
Question 2Medium5 marks
The results of the alpha particle scattering experiment led to the plum pudding model of the atom being replaced by the nuclear model.
(a) Give two differences between the plum pudding model and the nuclear model of the atom.[2]
(b) Explain why the plum pudding model had to be replaced.[2]
(c) Later experiments showed that the positive charge of a nucleus could be subdivided into a whole number of smaller particles, each with the same charge. What name was given to these particles?[1]
Show the answer and mark scheme
(a)Answer: In the nuclear model the mass and the positive charge are concentrated in a tiny central nucleus and most of the atom is empty space; in the plum pudding model they are spread through the whole atom.
nuclear model: mass concentrated in the nucleus; plum pudding: mass spread throughout the atom
nuclear model: positive charge concentrated in the nucleus; plum pudding: positive charge spread throughout the atom
nuclear model: the atom is mostly empty space; plum pudding: no empty space (a solid ball)
nuclear model: electrons outside the nucleus / in orbits; plum pudding: electrons embedded in the ball of positive charge
(b)Answer: The new evidence (large deflections and alpha particles bouncing back) could not be explained by the plum pudding model.
the scattering experiment gave new (experimental) evidence / results that the plum pudding model could not explain
(for example) some alpha particles were deflected through large angles / bounced back, which needs a small, concentrated nucleus
(c)Answer: Protons
protons
Question 3Hard8 marks
In 1909, scientists fired alpha particles at a very thin sheet of gold foil in a vacuum. At the time, most scientists accepted the plum pudding model of the atom.
(a) Explain how the results of the alpha particle scattering experiment led to the plum pudding model being replaced by the nuclear model of the atom.[6]
(b) Suggest why the experiment was done in a vacuum.[1]
(c) Suggest why the gold foil was very thin.[1]
Show the answer and mark scheme
(a)Answer: The plum pudding model predicted almost no deflection. Most alpha particles went straight through (mostly empty space), some were deflected (concentrated positive charge) and a few bounced back (mass concentrated in a tiny nucleus). The old model could not explain this, so it was replaced by the nuclear model.
alpha particles are positively charged (and are fired at thin gold foil)
the plum pudding model has its positive charge and mass spread out, so all the alpha particles were expected to pass through with little or no deflection
most alpha particles passed straight through, so the atom must be mostly empty space
some alpha particles were deflected, so there must be a concentrated region of positive charge that repels them
a very small number bounced back, so the mass (and charge) must be concentrated in a very small central nucleus
the plum pudding model could not explain the large deflections / the alpha particles that bounced back
so the new evidence led to the nuclear model: a tiny, positively charged nucleus containing most of the mass, surrounded by electrons
new experimental evidence can lead to a scientific model being changed or replaced
Marked with levels of response: the full level descriptors are in the app.
(b)Answer: Alpha particles would be stopped by just a few centimetres of air.
alpha particles have a short range in air / would be absorbed (stopped) by the air
(c)Answer: So that the alpha particles could pass through, with each one passing only a few layers of atoms.
so that the alpha particles could pass through it / were not absorbed by the foil / each alpha particle passed only a few atoms