AQA GCSE Chemistry Foundation (8462), Foundation tier · Atomic structure and the periodic table › Atoms, isotopes and electronic structure
Practise 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 electron shells, protons and neutrons. Expect 'describe the differences' and 'explain how the evidence changed the model' questions, often worth 3 to 6 marks.
Plum pudding: ball of positive charge with electrons embedded in it
Most alpha particles pass straight through: atom is mostly empty space
Some alpha particles deflected: the nucleus is (positively) charged
Very few bounce back: mass concentrated in a tiny nucleus
Bohr: electrons orbit the nucleus at specific distances
Chadwick: evidence for neutrons, about 20 years after the nucleus was accepted
Notes
Why models change
A scientific model is changed or replaced when new experimental evidence cannot be explained by it.
Before the electron was discovered, atoms were thought to be tiny spheres that could not be divided.
The plum pudding model
The discovery of the electron showed that atoms contain even smaller, negatively charged particles, so atoms can be divided.
This led to the plum pudding model: the atom is a ball of positive charge with negative electrons embedded in it. The charges balance, so the atom is neutral.
Alpha particle scattering and the nuclear model
diagram
In the alpha particle scattering experiment, positively charged alpha particles were fired at a very thin sheet of gold.
Most alpha particles went straight through → most of the atom is empty space.
A few were deflected (changed direction) → the centre of the atom is charged: it is positive, because it repels the positive alpha particles.
Very few bounced back → the mass is concentrated in a tiny nucleus at the centre.
Alpha scattering: the paths show that the positive charge and mass are in a tiny nucleus.
The plum pudding model could not explain this, because its positive charge and mass are spread out. So the nuclear model (a tiny, positive nucleus with electrons around it) replaced it.
Bohr, protons and neutrons
Niels Bohr adapted the nuclear model: electrons orbit the nucleus at specific distances (in shells, or energy levels). His theoretical calculations agreed with experimental observations.
Later experiments showed that the positive charge of a nucleus can be divided into a whole number of smaller particles, each with the same positive charge. These were named protons.
The experimental work of James Chadwick provided evidence for neutrons in the nucleus, about 20 years after the nucleus became an accepted idea.
Plum pudding compared with nuclear
diagram
Plum pudding: positive charge spread through the whole atom; mass spread evenly; electrons embedded in the positive charge; no nucleus.
Nuclear: positive charge and almost all the mass concentrated in a tiny nucleus; electrons outside the nucleus; most of the atom is empty space.
Plum pudding (left) and nuclear model (right).
How to answer each type of question
Recall scientists and what they found
1 mark eachGrade 4
Bohr: electrons orbit at specific distances.
Chadwick: evidence for neutrons.
You don't need details of their experiments, only what they found.
Example. (a) Name the scientist whose experimental work provided evidence for neutrons. (b) Describe how Niels Bohr changed the nuclear model of the atom.
Show the model answerHide the model answer
(a) (James) Chadwick (1) (b) He suggested that electrons orbit the nucleus at specific distances / in fixed energy levels (1).
Don’t lose marks
Saying the alpha particles bounced off electrons. They were repelled by the positive nucleus.
Writing that most alpha particles were deflected. Most went straight through.
Describing the plum pudding model as having a nucleus.
Getting the order wrong: Bohr's shells came after the nuclear model, and protons were identified before neutrons.
Describing only one model when the question asks for differences.
More tips
Memory tricks
Order of models: 'Some People Never Bake Proper Nougat' = Spheres, Plum pudding, Nuclear, Bohr, Protons, Neutrons.
Alpha results in six words: 'most through, some bent, few back'. Then: empty space, charged nucleus, tiny massive nucleus.
The neutral neutron was the last particle found, about 20 years after the nucleus.
Exam technique
For 'describe the differences', write matched pairs: 'In the plum pudding model…, but in the nuclear model…'.
For scattering questions, give each observation and then its conclusion, linked with 'so'.
Only Bohr and Chadwick are named in the specification; you don't need details of their experiments.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State that models change with new evidenceA scientific model may be changed or replaced when new experimental evidence doesn't fit it.
Grade 4
Describe the plum pudding modelThe atom is a ball of positive charge with negative electrons embedded in it.
Grade 4
Put the atomic models in orderIndivisible spheres → plum pudding → nuclear model → Bohr's shells → protons → neutrons (Chadwick).
Grade 5
Describe the nuclear modelA tiny, positively charged nucleus that contains almost all the mass, with electrons outside it.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Scientists’ ideas about atoms have changed over time. Describe the plum pudding model of the atom.
A ball of positive charge with negative electrons embedded in it.
Experiments showed that the positive charge of a nucleus can be divided into a whole number of smaller particles. Each particle has the same amount of positive 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 1Easy5 marks
Scientists’ ideas about atoms have changed over time.
(a) Which statement describes the model of the atom that scientists used before the plum pudding model? Tick (✓) one box.[1]
Tiny spheres that could not be divided
Particles made of protons, neutrons and electrons
Nuclei with electrons orbiting at fixed distances
Mostly empty space with a tiny nucleus
(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]
(d) Name the scientist whose experimental work provided evidence for neutrons.[1]
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 / scattered through it
(d)Answer: (James) Chadwick
(James) Chadwick
Question 2Medium6 marks
In the alpha particle scattering experiment, positively charged alpha particles were fired at a very thin sheet of gold.
(a) Most of the alpha particles passed straight through the gold. What does this show about atoms?[1]
(b) A very small number of alpha particles bounced back towards the source. Explain what this shows about the nucleus of an atom.[3]
(c) Explain why these results could not be explained by the plum pudding model.[2]
Show the answer and mark scheme
(a)Answer: Most of an atom is empty space.
most of the atom is empty space
(b)Answer: The nucleus is positively charged (it repels the alpha particles), it contains most of the mass of the atom, and it is very small.
the nucleus is positively charged so it repels the (positive) alpha particles
most of the mass of the atom is concentrated in the nucleus
the nucleus is very small compared with the atom (because very few alpha particles bounced back)
the nucleus is at the centre of the atom
(c)Answer: In the plum pudding model the positive charge and mass are spread out, so the alpha particles would all pass through with little or no deflection.
(in the plum pudding model) the positive charge / mass is spread out through the atom
so the alpha particles would (all) pass through with little or no deflection / none would bounce back