The development of the model of the atom (common content with physics)

What you'll learn

• How ideas about atoms changed as new evidence was discovered.
• The difference between the plum pudding model and the nuclear model.
• Why the alpha particle scattering experiment changed scientists’ view of the atom.
• How Bohr, protons and neutrons fitted into the later atomic model.

Big idea: scientific models can change

Scientists use models to explain things we cannot directly see. Atoms are far too small to see with an ordinary microscope, so scientists built models of atomic structure using evidence from experiments.

This is one of the most important ideas in this topic: the atomic model did not appear all at once. It developed step by step as scientists found evidence that older models could not explain.

Here is the overall sequence you need to know.

Before the electron: tiny indivisible spheres

An atom is a tiny particle of an element. Before the electron was discovered, atoms were thought to be tiny solid spheres that could not be divided into anything smaller.

The word indivisible means “cannot be split into smaller parts”. So, in this early model, the atom had no internal structure: no nucleus, no electrons, no protons and no neutrons.

This model was simple, but it became inadequate once scientists discovered that atoms contain smaller particles.

Discovery of the electron and the plum pudding model

An electron is a negatively charged subatomic particle. A subatomic particle is a particle smaller than an atom.

The discovery of the electron showed that atoms were not indivisible after all. They contained smaller negative particles. Scientists therefore needed a new model of the atom.

The new model was called the plum pudding model.

In this model:

• the “pudding” is the spread-out positive charge
• the “plums” are the negative electrons
• there is no central nucleus
• the mass and charge are not concentrated in one tiny region

The alpha particle scattering experiment

The next major change came from the alpha particle scattering experiment, often linked with Rutherford and his team.

An alpha particle is a positively charged particle. In the experiment, alpha particles were fired at a very thin sheet of gold foil. A detector screen showed where the alpha particles went after hitting, or passing through, the foil.

Scattering means changing direction after an interaction.

The results were surprising:

• most alpha particles passed straight through the gold foil
• some alpha particles were deflected slightly
• a very small number were deflected through large angles or bounced back

The plum pudding model could not explain these results well. If positive charge were spread out evenly through the atom, alpha particles should mostly pass through with only tiny deflections.

Instead, some alpha particles were strongly repelled. This suggested that the positive charge was concentrated in a very small central region.

Why the scattering evidence changed the model

The alpha scattering experiment led to the nuclear model of the atom.

The reasoning is very important for exams:

• Most alpha particles passed straight through → atoms are mostly empty space.
• Some alpha particles were deflected → there is positive charge in the atom, repelling the positive alpha particles.
• A few alpha particles bounced back → the positive charge and mass must be concentrated in a tiny, dense nucleus.

Plum pudding model vs nuclear model

You must be able to describe the difference between these two models clearly.

Plum pudding model

The plum pudding model has:

• positive charge spread throughout the atom
• negative electrons embedded in the positive charge
• no nucleus
• no mostly empty space

Nuclear model

The nuclear model has:

• a tiny central nucleus
• positive charge concentrated in the nucleus
• most of the mass concentrated in the nucleus
• electrons outside the nucleus
• the atom mostly empty space

Bohr’s improvement: electrons in shells

The nuclear model still needed improving. Niels Bohr adapted it by suggesting that electrons orbit the nucleus at specific distances.

At GCSE, these specific distances are usually described as electron shells or energy levels.

Bohr’s theoretical calculations agreed with experimental observations, so his model was accepted as an improvement.

You do not need to know the details of the experiments or calculations that supported Bohr’s model for this specification. You just need to know that Bohr suggested electrons are not anywhere at random: they are arranged at specific distances from the nucleus.

Protons: positive charge in smaller units

Later experiments showed that the positive charge of a nucleus could be divided into a whole number of smaller particles. Each of these particles had the same amount of positive charge.

These particles were named protons.

This improved the nuclear model because the nucleus was no longer just described as a vague “positive centre”. It was made of identifiable positive particles.

Neutrons: Chadwick’s evidence

The final part of this story, for GCSE, is the neutron.

A neutron is a neutral subatomic particle found in the nucleus. Neutral means it has no charge.

James Chadwick provided experimental evidence for the existence of neutrons. This was about 20 years after the nucleus had become accepted.

You do not need to know the details of Chadwick’s experimental work. You do need to know that his work provided evidence for neutrons in the nucleus.

Why this topic matters

This topic is not just about memorising names. It shows how science works:

• scientists propose a model
• experiments produce evidence
• the evidence is compared with the model
• if the model cannot explain the evidence, it is changed or replaced

That is why the alpha scattering experiment is so important: it gave results that the plum pudding model could not explain, but the nuclear model could.