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Edexcel GCSE Physics · 1PH0
Edexcel 1PH0 · 6.17 Check the specification (PDF) (opens in a new tab)
Atoms are far too small to see directly, so scientists use models to explain their structure. A useful model must fit experimental evidence. When an experiment produces a result the model cannot explain, the model needs to change.
In the early 1800s, John Dalton pictured atoms as tiny solid spheres. The discovery of negatively charged electrons by J. J. Thomson in 1897 showed that atoms had smaller parts inside them. Thomson proposed the plum pudding model: electrons were embedded throughout a sphere of positive charge. The positive and negative charges balanced, making the atom neutral. This model had no nucleus.
Hans Geiger and Ernest Marsden, working with Ernest Rutherford, directed positively charged alpha particles at a very thin sheet of gold foil. A surrounding screen showed where the particles went. If positive charge was spread throughout each atom, as Thomson’s model suggested, the particles should pass through with at most small changes in direction.
The different paths of alpha particles through thin gold foil provided evidence for a small, dense, positively charged nucleus.
Most alpha particles passed straight through. Rutherford inferred that most of an atom is empty space. Some were deflected, showing that they had encountered a concentration of positive charge that repelled them. Very few were deflected through a large angle or came back towards the source. Such a strong change in direction suggested that the positive charge, and most of the atom’s mass, was concentrated in a tiny, dense centre: the nucleus. The results did not fit a diffuse sphere of positive charge.
Rutherford’s nuclear model therefore placed a small positive nucleus at the centre, with electrons around it and mostly empty space between. The scattering experiment changed the model because it gave evidence about where the atom’s charge and mass were located, not merely a new drawing of an atom.
Rutherford’s model left a problem: it did not adequately explain how electrons could remain around a positive nucleus rather than being drawn in. In 1913, Niels Bohr proposed that electrons occupy particular shells, or energy levels, around the nucleus rather than any position or energy. This retained the small nucleus supported by scattering evidence while providing a more stable model of the atom.
New evidence changed the model from a solid sphere to an atom with a nucleus and electrons in shells.
The sequence is solid sphere → plum pudding → nuclear model → Bohr model. Each step kept ideas that still fitted the evidence and changed those that did not.
Dalton: solid spheres. Thomson: electrons embedded in a positive sphere. Rutherford: a tiny, dense, positive nucleus with electrons around it; the atom is mostly empty space. Bohr: electrons occupy particular shells or energy levels around the nucleus.
In alpha-particle scattering, most particles passed through gold foil, some were deflected and very few turned through large angles. These results supported Rutherford’s nuclear model rather than the plum pudding model. Bohr then addressed the stability problem left by Rutherford’s model.
Link each gold-foil observation to its own conclusion: most particles passed through → mostly empty space; some were deflected → a positive centre; very few turned through a large angle → a small, dense centre.
Do not put a nucleus in the plum pudding model or describe Bohr’s model as the first to propose one.
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Atomic model
An explanation of an atom’s structure that can be tested and changed when new evidence is found.
Plum pudding model
Thomson’s model of an atom as a sphere of positive charge with negative electrons embedded throughout it.
Nucleus
The small, dense, positively charged centre of an atom.
Alpha particle
A positively charged particle made of two protons and two neutrons.
Put your knowledge into practice — try past paper questions for Physics
Atomic model
An explanation of an atom’s structure that can be tested and changed when new evidence is found.
Plum pudding model
Thomson’s model of an atom as a sphere of positive charge with negative electrons embedded throughout it.
Nucleus
The small, dense, positively charged centre of an atom.
Alpha particle
A positively charged particle made of two protons and two neutrons.