The development of the model of the atom (common content with chemistry) — AQA GCSE Physics

The earliest scientific model of the atom, set out by John Dalton at the beginning of the nineteenth century, treated each atom as a tiny solid sphere with no internal structure and no parts. It was a useful model because it explained how elements combine in fixed ratios to form compounds, and why mass is conserved in a reaction. What it said nothing about was electric charge, because at that time no smaller particle was known. Once Thomson identified the electron, a particle far lighter than any atom and carrying a negative charge, the idea that an atom could not be divided had to be abandoned, and the modelling of what is inside the atom began.

Thomson's experiments on the rays inside a discharge tube showed the existence of a particle much lighter than any atom and carrying a negative charge, the electron. Two consequences followed at once. First, the atom must contain smaller parts, so it is not indivisible. Second, since ordinary atoms carry no overall charge, something positive must also be present to cancel the electrons. The simplest picture that fitted both facts was a ball of positive charge with the negative electrons dotted through it, which became known as the plum pudding model. It explained neutrality and the existence of electrons, but it placed no concentration of mass or charge anywhere in particular.

In the plum pudding model the whole atom is a sphere of positive charge, spread out evenly through its volume, with negatively charged electrons embedded in it like fruit in a pudding. There is no nucleus and no empty space: matter and charge fill the atom uniformly. The amount of positive charge exactly matches the total negative charge of the electrons, so the atom as a whole is neutral. This picture makes a clear prediction about the alpha scattering experiment. Because the positive charge is thinly spread, no part of the atom could push hard on a fast, heavy alpha particle, so every alpha particle should pass through with at most a small deflection.

Geiger and Marsden fired alpha particles, which are positively charged and relatively heavy, at a very thin gold foil and recorded where they went. Three results mattered. Most passed straight through undeflected, so the atom must be mostly empty space. A small number were deflected through large angles, so somewhere inside there is a concentration of positive charge repelling them. A very few bounced almost straight back, which can only happen if they met something both positively charged and far more massive than themselves. Put together, the atom has a tiny central nucleus that is positively charged and holds almost all the mass, with the electrons far outside it.

The plum pudding model predicted that alpha particles would sail through a foil with only slight deflections, because its positive charge is spread thinly everywhere. The observed large deflections and rebounds could not be produced by such a diffuse charge, so the model failed on its own prediction and was replaced. Rutherford's nuclear model puts a tiny, positively charged, massive nucleus at the centre with the electrons well outside it and empty space in between, which accounts for all three observations at once. Comparison questions are level marked: a top answer links each observation to the feature of the model that explains it, rather than simply listing the two models side by side.

Rutherford's nuclear model left a problem: electrons circling a positive nucleus should lose energy continuously and spiral into it, so atoms should collapse. Bohr adapted the model rather than replacing it. He kept the nucleus and the orbiting electrons, but proposed that the electrons can only occupy certain fixed distances from the nucleus, not any distance at all. Each allowed distance corresponds to a definite energy, which is where the energy levels used in electron arrangements come from. An electron can move between levels by absorbing or emitting electromagnetic radiation, but it cannot sit anywhere in between, so the atom is stable.

A suggestion becomes an accepted model only when its predictions are tested against measurement. Bohr's idea of fixed orbits allowed him to calculate values that could be compared directly with what experimenters had measured, and the two agreed. That agreement, not Bohr's reputation, is why the scientific community adopted his version of the atom. The process is general: a model makes a prediction, experiments test it, and other scientists check the work by peer review and by repeating the measurements themselves, which is done to make sure the observations are correct. Agreement never makes a model permanent, since later evidence can still force a further change.

Once the nucleus was accepted, the next question was whether its positive charge was a single lump or made of parts. Experiments showed that the charge of every nucleus is a whole-number multiple of one fixed basic amount: never one and a half times it, never a fraction. That points to the nucleus being built from identical positively charged particles, with one element differing from the next by the number of them it contains. Hydrogen has one, helium two, lithium three, and so on up the periodic table. The idea came from experimental evidence, not from theory alone, which is why it took further work after the nuclear model was established.

The identical positively charged particles that make up the charge of a nucleus were named protons. A proton has a relative charge of +1 and a relative mass of 1, the same mass as a neutron and about two thousand times the mass of an electron. The number of protons in a nucleus is the atomic number, and it is what fixes the identity of the element: six protons is always carbon, whatever the number of neutrons or electrons. Because the proton number determines the element, the periodic table is ordered by it. Losing or gaining electrons makes an ion but leaves the proton number, and so the element, unchanged.

By the 1920s the nucleus was known to contain protons, but the measured mass of a nucleus was roughly twice what the protons alone could account for. In 1932 James Chadwick produced the experimental evidence for a third particle, the neutron, sitting in the nucleus alongside the protons. A neutron has a relative mass of 1, the same as a proton, but carries no charge, so adding neutrons changes the mass of a nucleus without changing its charge or the element. That immediately explained the missing mass and also explained isotopes, atoms of one element with the same proton number but different numbers of neutrons and so different masses.

The nuclear model was established around 1911, when the alpha scattering results were explained, and Bohr adapted it soon afterwards. The evidence for the neutron did not arrive until about twenty years later. That gap is worth noticing, because it shows how a model develops: the nuclear model was not thrown away when the neutron turned up, it was completed. Each stage waited on new experimental techniques and on evidence that the existing picture could not explain. The order to remember is solid sphere, plum pudding, nuclear model, Bohr's fixed orbits, the subdivision of nuclear charge into protons, and finally the neutron, about two decades after the nucleus itself was accepted.