What is an atom made of, and how did our model of it develop?
The structure of the atom: the sizes and charges of protons, neutrons and electrons, the nuclear model, and how the model developed from the plum pudding model.
A focused answer to AQA GCSE Physics 4.4.1, covering the relative sizes and charges of protons, neutrons and electrons, the nuclear model of the atom, and how the alpha scattering experiment replaced the plum pudding model.
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What this dot point is asking
AQA wants you to describe the structure of the atom, give the relative charges and locations of protons, neutrons and electrons, state the approximate sizes involved, and explain how the alpha scattering experiment led from the plum pudding model to the nuclear model. This is topic 4.4.1 of the AQA GCSE Physics (8463) specification.
Inside the atom
Almost all of an atom's mass is concentrated in the nucleus, because protons and neutrons are far more massive than electrons. A proton and a neutron each have a relative mass of about , while an electron has a relative mass of only about , so the electrons contribute almost nothing to the total mass even though they take up nearly all the volume.
Electrons occupy fixed energy levels (also called shells) at set distances from the nucleus, not just anywhere. This matters for an AQA-specific point: if an atom absorbs electromagnetic radiation, an electron can move to a higher energy level (further from the nucleus); when an electron falls back to a lower energy level it emits electromagnetic radiation. This idea links the structure of the atom to the way atoms give out and take in light.
The sizes involved
To picture the scale, if the nucleus were the size of a pea placed in the centre of a large sports stadium, the electrons would be moving around near the outer stands, with empty space in between. This emptiness is exactly why most alpha particles in the scattering experiment passed straight through the foil: they rarely came close to a nucleus.
How the model developed
Before the nucleus was discovered, the plum pudding model pictured the atom as a ball of positive charge with electrons scattered through it like fruit in a pudding.
Later, Bohr suggested electrons orbit at set distances in energy levels, and experiments showed the nucleus contains positive protons and, later still, neutral neutrons.
Try this
Q1. State the relative charges of a proton, a neutron and an electron. [2 marks]
- Cue. Proton , neutron , electron .
Q2. Explain what the alpha scattering experiment showed about the atom. [3 marks]
- Cue. Most alpha particles passed straight through (mostly empty space); a few were deflected or bounced back (a small, dense, positively charged nucleus).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20196 marksDescribe how the model of the atom changed from the plum pudding model to the nuclear model, and explain how the results of the alpha particle scattering experiment provided evidence for this change.Show worked answer →
A top-band answer first describes the plum pudding model: a ball of positive charge with negative electrons embedded throughout it, like fruit in a pudding, with no concentrated nucleus. It then describes the alpha scattering experiment: alpha particles were fired at a thin sheet of gold foil and their paths observed. The key observations are that most alpha particles passed straight through with little or no deflection, a small number were deflected through large angles, and a very few bounced almost straight back. The answer then links each observation to a conclusion: most passing straight through shows the atom is mostly empty space; the large deflections and back-scattering show that the positive charge and almost all the mass are concentrated in a tiny, dense, positively charged nucleus, because only a concentrated positive charge could repel a fast positive alpha particle so strongly. This evidence could not be explained by the plum pudding model, so it was replaced by the nuclear model. Markers reward describing both models, the observations, and the explicit observation-to-conclusion reasoning.
AQA 20213 marksState the relative charges of a proton, a neutron and an electron, and explain why an atom has no overall charge.Show worked answer →
The relative charges are: proton , neutron , electron (1 mark for all three correct). An atom has no overall charge because it contains equal numbers of protons and electrons (1 mark), so the total positive charge of the protons exactly cancels the total negative charge of the electrons (1 mark). Markers reward all three charges and a clear statement that equal proton and electron numbers give a net charge of zero. A common error is to give the neutron a small charge; it is exactly zero.
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Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)