Dalton's atom: a solid sphere with nothing inside it

1. An atomic model is a scientific description of what an atom contains and how its parts are arranged.
2. In the early 1800s John Dalton described each atom as a tiny solid sphere that could not be split.
3. Dalton's model treated atoms as indivisible, so it allowed nothing smaller to exist inside them.
4. It fitted the evidence of the time, because elements combined in fixed ratios as though built from identical unsplittable units.
5. Every later change to the model came from finding a subatomic particle that Dalton's solid sphere had no room for.

Thomson: the electron turns the sphere into a plum pudding

1. In 1897 J. J. Thomson used cathode-ray experiments to show that atoms contain electrons, which carry a negative charge.
2. An electron is far smaller than an atom, so atoms could no longer be described as indivisible.
3. Thomson proposed the plum pudding model, with electrons embedded in a ball of positive charge.
4. The positive charge balanced the negative electrons, so the atom as a whole stayed electrically neutral.

Rutherford: alpha scattering puts the positive charge in a nucleus

1. In the alpha-scattering experiment, positively charged alpha particles were fired at a very thin sheet of gold foil.
2. Most alpha particles passed straight through, which showed that most of the atom is empty space.
3. A small number were deflected through large angles, which showed a concentrated region of positive charge.
4. A very small number bounced almost straight back, which showed that this region is both tiny and dense.
5. Rutherford concluded that nearly all the positive charge and nearly all the mass sit in a tiny central nucleus.
6. That replaced the spread-out positive charge of the plum pudding model and left the electrons somewhere outside the nucleus.

Bohr and Chadwick: shells for the electrons, neutrons in the nucleus

1. Niels Bohr showed that electrons orbit at fixed distances from the nucleus rather than anywhere outside it.
2. Calculations based on fixed energy levels matched the experimental observations, which Rutherford's model could not explain.
3. Later work identified the positively charged particles inside the nucleus as protons.
4. James Chadwick provided evidence for the neutron in 1932, explaining why nuclei are heavier than their protons alone can account for.
5. The current model therefore has a tiny nucleus of protons and neutrons, with electrons in shells around it.

Why the model changed: each discovery exposed a limit in the one before

1. Dalton's solid sphere became Thomson's plum pudding once the electron was found inside the atom.
2. The plum pudding became Rutherford's nuclear model once alpha scattering showed the positive charge was concentrated.
3. The nuclear model became Bohr's shell model once fixed energy levels were needed to match what was observed.
4. Bohr's model gained neutrons once Chadwick accounted for the missing nuclear mass.
5. A model survives only while it explains every observation, so new evidence is what forces each change.