1.1 - How atomic models changed
Discoveries of smaller particles changed the atom from Dalton’s indivisible sphere into a model with electrons surrounding a nucleus. You will connect each change to what scientists learned about atomic structure.
Dalton's starting model
Dalton pictured matter as made of tiny atoms. Atoms of each element could combine with atoms of other elements to form compounds. This helped chemists explain substances and reactions.
Dalton model
Atoms are tiny, solid, indivisible spheres: the model has no smaller parts inside an atom.
A scientific model is a representation used to explain observations. A useful model can still be incomplete; new evidence may reveal features that it cannot explain.
Electrons reveal internal structure
The discovery of the electron, a negatively charged subatomic particle, showed that atoms had smaller parts. That contradicted the idea that an atom was indivisible.
Thomson’s plum pudding model included negative electrons embedded in a spread of positive charge. It kept the sphere shape but introduced internal structure. It did not have a central nucleus.
Developing the nuclear model
Later evidence led to the nuclear model: positive charge and most of the mass are concentrated in a tiny central nucleus, with electrons outside it. The identification of protons connected the nuclear positive charge to particles. Discovery of neutrons added uncharged particles to the model of the nucleus.
| Model | What it includes |
|---|---|
| Dalton | Indivisible solid spheres |
| Thomson | Electrons embedded in positive material |
| Nuclear model, later refined | Electrons outside a nucleus containing protons and neutrons |
This is a sequence of changes in the explanation of atoms. Finding electrons established internal structure; it did not, by itself, establish where the positive charge was located.