AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Atomic structure › Atoms and isotopes
Practise The development of the model of the atom. 8 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.
Key facts
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
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
diagram
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.
Paths of alpha particles near a gold nucleus (the nucleus is drawn far bigger than it really is).
So the nuclear model replaced the plum pudding model: a tiny, positively charged nucleus containing most of the mass, with electrons around it.
Before and after the alpha scattering experiment (red + = positive charge, blue = electrons).
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.
How to answer each type of question
Compare the plum pudding and nuclear models
2 marksGrade 5
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 answerHide 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.
Don’t lose marks
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.
More tips
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.
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'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Put the models of the atom in orderTiny spheres, plum pudding, nuclear model, Bohr's energy levels, then protons and neutrons.
Grade 4
Describe the plum pudding modelThe atom is a ball of positive charge with negative electrons embedded in it.
Grade 5
Describe the nuclear modelA tiny, positively charged nucleus containing most of the mass, with electrons around it.
Grade 5
Compare plum pudding and nuclear modelsCharge and mass spread out, compared with concentrated in a nucleus with empty space around it.
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