Atomic Structure and the Development of the Model
The model of the atom has changed as new experimental evidence appeared: atoms were once thought to be indivisible spheres until the discovery of the electron led to the plum pudding model, a ball of positive charge with negative electrons embedded in it. The alpha particle scattering experiment then showed that an atom is mostly empty space with its mass and positive charge concentrated in a tiny nucleus, Bohr showed electrons orbit at specific energy levels, and Chadwick later provided evidence for the neutron.
Method
- Learn the sequence in order with the evidence attached to each step, because a question about the model is really a question about evidence: indivisible sphere, then plum pudding after the electron was discovered, then the nuclear model after alpha scattering, then Bohr's energy levels, then protons, then the neutron.
- Describe the alpha scattering observations before the conclusions: most alpha particles passed straight through the gold foil, some were deflected through small angles, and a very small number were deflected through more than 90 degrees.
- Draw each conclusion from its own observation: passing straight through means the atom is mostly empty space; large deflections mean there is a concentrated positive charge that repels the positive alpha particles; the rarity of those deflections means that region is tiny; and the fact that some come straight back means it is far more massive than an alpha particle.
- Explain why the plum pudding model failed rather than simply saying it was wrong: it predicted that alpha particles would pass through with only slight deflections everywhere, because charge and mass were spread thinly and evenly throughout the atom, so it could not account for the large-angle deflections.
- State Bohr's contribution accurately: electrons orbit the nucleus at fixed distances in energy levels, and his theoretical calculations agreed with experimental observations, which is why the model was accepted.
- Use the scale figures when a question asks about size: an atom has a radius of about 1 x 10^-10 m and the nucleus is less than 1/10 000 of that, yet the nucleus contains nearly all the mass, which is why the density of nuclear matter is so extraordinarily high.
Worked example
An atom has a radius of 1 x 10^-10 m and its nucleus has a radius of 1 x 10^-14 m. Calculate how many times larger the atomic radius is than the nuclear radius, and comment on what this means for the structure of the atom.
- Write the ratio: atomic radius / nuclear radius = (1 x 10^-10) / (1 x 10^-14).
- Subtract the powers of ten: -10 - (-14) = 4.
- So the ratio is 1 x 10^4, that is 10 000 times.
- Interpret: the atom's radius is about ten thousand times the nucleus's radius, so the nucleus occupies a minute fraction of the atom's volume.
- Connect to the evidence: this is exactly why almost all alpha particles passed straight through the gold foil in the scattering experiment.
- Add the mass point: despite that tiny volume, the nucleus contains nearly all the atom's mass, so an alpha particle that does strike it can be deflected straight back.
Practice questions
Try each question, then tap to reveal the answer.
Q1Describe the plum pudding model.Show answer
Answer: A ball of positive charge with negatively charged electrons embedded within it.
Q2What observation showed that the atom is mostly empty space?Show answer
Answer: Most alpha particles passed straight through the thin gold foil without deflection.
Q3What observation showed the nucleus is positively charged and very small?Show answer
Answer: A small number of alpha particles, which are positively charged, were deflected through large angles or came straight back, which requires a concentrated positive charge in a very small volume.
Q4What did Bohr add to the nuclear model?Show answer
Answer: That electrons orbit the nucleus at specific fixed distances, in energy levels, supported by calculations that agreed with experimental observations.
Q5Who provided the evidence for the existence of the neutron, and roughly when relative to the nuclear model?Show answer
Answer: Chadwick, about twenty years after the nuclear model was accepted.
Q6State the approximate radius of a nucleus compared with the radius of an atom.Show answer
Answer: Less than 1/10 000 of the atomic radius, roughly 1 x 10^-14 m against 1 x 10^-10 m.
Q7Why is the atomic model a good example of how scientific models change?Show answer
Answer: Each model was accepted while it explained the available evidence, and was replaced when new experimental evidence appeared that it could not explain.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Describe the alpha particle scattering experiment and explain how its results led to the nuclear model of the atom.
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Explain why the plum pudding model was replaced, and what this shows about how scientific models develop.
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This topic is chapter 26 of GCSE Physics Workbook, the whole course as one free printable PDF.
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