The Development of the Atomic Model

A scientific model is our best current explanation of something we cannot see directly. When new experimental evidence appears that a model cannot explain, scientists change the model or replace it entirely. The story of the atom is the clearest example on the GCSE course.

The model of the atom was not worked out by one person in one moment. It was built up in stages, each stage triggered by a new experiment, over roughly a hundred years.

The atom is far too small to see, so every model is inferred from what atoms do in experiments. A model survives only as long as it fits the evidence.

Understanding why each model changed matters more for the exam than memorising dates. Examiners reward answers that link a specific piece of evidence to the specific change it forced in the model.

Before the electron was discovered, atoms were thought to be tiny spheres that could not be divided. This is often called the "solid sphere" idea: the atom was the smallest possible piece of matter, with no internal parts.

That picture had to change once the electron was discovered. The electron is a tiny, negatively charged particle, and its discovery proved the atom is not indivisible after all: it contains smaller parts inside it.

If atoms contain negative electrons but are neutral overall, there must also be positive charge somewhere to cancel it out. Explaining where that positive charge sits was the next problem, and it led directly to the first detailed model of atomic structure. The key logical step is simple: a neutral atom that contains negative electrons must also contain an equal amount of positive charge.

The discovery of the electron led to the plum pudding model. In this model the atom is a ball of positive charge with negative electrons embedded in it, like pieces of fruit dotted through a pudding.

The positive charge was spread evenly throughout the whole atom, and the electrons were scattered inside it. There was no nucleus in this model, and there was no empty space: the positive charge filled the entire atom.

Feature	Plum pudding model
Positive charge	Spread evenly through the whole atom
Electrons	Embedded throughout the positive ball
Nucleus	None
Empty space	None

The plum pudding model fitted what was known at the time: atoms are neutral and contain negative electrons balanced by positive charge. It stood until an experiment produced results it simply could not explain.

The alpha particle scattering experiment fired positively charged alpha particles at a very thin sheet of gold foil. If the plum pudding model were correct, the positive charge would be spread out thinly, so every alpha particle should pass almost straight through with only tiny deflections.

The actual results were different:

Observation	What it showed
Most alpha particles passed straight through	The atom is mostly empty space
Some were deflected through large angles	There is a concentrated positive charge to repel them
A very few bounced almost straight back	The mass and charge are packed into a tiny, dense centre

These results could not fit the plum pudding model. They led to the nuclear model: the mass of an atom is concentrated at the centre in a nucleus, and the nucleus is charged. The electrons occupy the space around it. Because the nuclear model explained the scattering results and the plum pudding model could not, the nuclear model replaced it.

The early nuclear model still had a problem: if electrons simply sat around a positive nucleus, they should spiral inwards. Niels Bohr adapted the nuclear model to fix this.

Bohr proposed that electrons orbit the nucleus at specific distances from it, called energy levels (or shells). Electrons in a given energy level stay at that fixed distance rather than falling in. Crucially, the results calculated from Bohr's model agreed with experimental observations, which is why it was accepted.

The diagram shows the key features of the model after Bohr: a small central nucleus containing the positive charge and the mass, with electrons arranged in fixed shells around it.

(The detail of the experimental work behind Bohr's model is not required by AQA. You need the idea that electrons orbit at fixed energy levels and that Bohr's calculations matched observation.)

This shell arrangement is the foundation of electronic structure, which explains chemical bonding and the whole shape of the periodic table.

Later experiments refined the nucleus itself. Scientists found that the positive charge of the nucleus could be subdivided into a whole number of smaller particles, each carrying the same positive charge. These particles are the protons.

About 20 years later, James Chadwick carried out work that provided the evidence for neutrons in the nucleus. Neutrons have no charge but do have mass, which explained why the mass of an atom is greater than the protons alone could account for.

Discovery	What it added to the model
Protons	The nucleus contains whole-number positive particles
Neutrons (Chadwick)	The nucleus also contains uncharged particles with mass

(The detail of Chadwick's experimental work is not required by AQA. You need only that his work gave evidence for the neutron, roughly 20 years after protons.)

With protons and neutrons in the nucleus and electrons in energy levels outside it, the model reached the form you use throughout GCSE chemistry.

1. Saying the plum pudding model has a nucleus

The plum pudding model has no nucleus. Its positive charge is spread evenly through the whole atom. The nucleus only appears in the nuclear model, after alpha scattering.

2. Not linking the evidence to the change

An answer that just says "alpha particles were fired at gold foil" scores little. You must link each observation to a conclusion: particles passing through means mostly empty space; particles bouncing back means a tiny dense nucleus.

3. Crediting the wrong scientist for the neutron

Protons came first; Chadwick provided the evidence for the neutron about 20 years later. Do not attribute the neutron to the scientist who found the nucleus.

4. Describing experimental detail that is not required

AQA does not require the experimental detail behind the Bohr model or Chadwick's work. Focus on what each stage concluded about the atom, not on apparatus you are not asked for.

5. Thinking the model was correct from the start

Each model was the best fit for the evidence available at the time, then was changed when new evidence appeared. Describe the sequence as a response to new experiments, not as a single correct answer discovered at once.