Models of the atom change as new evidence appears
Definition
Scientific model
A simplified representation used to describe and explain scientific observations.
- A scientific model is a way of picturing something we cannot see directly, such as the atom.
- A model is only kept while it fits the evidence.
- When new experimental evidence does not fit, the model is changed or replaced by a better one.
- The accepted model of the atom has changed several times over about 100 years as experiments revealed more about its structure.
Key Idea
- Models in science are provisional: they are the best current explanation, not a final truth.
- New evidence is what forces a model to change, not opinion.
From solid spheres to the plum pudding model
Definition
Electron
A negatively charged subatomic particle that can be transferred between species during a redox reaction.
- Before the electron was known, atoms were pictured as tiny solid spheres that could not be divided.
- In 1897, J. J. Thomson discovered the electron, showing that atoms contain even smaller charged particles.
- Because the electron is negatively charged but atoms are neutral overall, there had to be positive charge too.
- Thomson suggested the plum pudding model: a ball of positive charge with negative electrons dotted through it, like fruit in a pudding.
Note
The plum pudding model had no nucleus and no empty space; charge was spread evenly.
Alpha scattering: the evidence for a tiny nucleus
Definition
Alpha particle
A positively charged particle used to investigate atomic structure in the scattering experiment.
- Rutherford and his co-workers Geiger and Marsden fired alpha particles (which are positively charged) at very thin gold foil.
- The plum pudding model predicted the alpha particles would all pass almost straight through, barely bending.
- Most of the alpha particles did pass straight through, showing the atom is mostly empty space.
- A few were deflected through large angles, and a very few bounced almost straight back.
- A back-scattered particle can only happen if it meets something small, dense and positively charged.
- Rutherford concluded that the positive charge and nearly all the mass sit in a tiny central nucleus.
- This replaced the plum pudding model with the nuclear model of the atom.
Example
- Deflected particles were pushed away by the positive nucleus as they passed close to it.
- The rare bounce-back showed the nucleus is extremely small but very dense.
Bohr's shells and the discovery of the neutron
Definition
Theoretical calculation
A result worked out from scientific ideas and mathematical relationships rather than measured directly.
- Niels Bohr suggested that electrons orbit the nucleus in fixed energy levels, also called shells, at set distances.
- Bohr's theoretical calculations agreed with experimental evidence, so this idea was accepted.
- Further experiments showed the nucleus contains smaller positive particles called protons.
- About 20 years after the nucleus was found, James Chadwick found evidence for the neutron, a particle in the nucleus with no charge.
- The modern picture is a nucleus of protons and neutrons surrounded by electrons in shells.
Self review
- What did J. J. Thomson discover, and which model followed?
- Answer: the electron; this led to the plum pudding model.
- In alpha scattering, what did the few particles that bounced back show?
- Answer: the atom has a tiny, dense, positively charged nucleus.
- What did Bohr add to the model?
- Answer: electrons orbit in fixed energy levels (shells).
- Who provided evidence for the neutron?
- Answer: James Chadwick.
- Why do scientific models change over time?
- Answer: new experimental evidence appears that the old model cannot explain.
