Scatter Shot

Scatter Shot: the alpha particle scattering experiment and the model of the atom

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

Why the model of the atom keeps changing

Nobody can see inside an atom, so scientists use a model: a simple picture that explains what experiments show. For AQA GCSE Chemistry (spec point 4.1.1.3) you need to know that new experimental evidence may lead to a scientific model being changed or replaced.

Before the electron was discovered, atoms were thought to be tiny spheres that could not be divided. In 1897 J.J. Thomson discovered the electron. That led to the plum pudding model: a ball of positive charge with negative electrons embedded in it. In Scatter Shot you start by testing this model, and every alpha particle you fire goes straight through or is deflected only very slightly, which is exactly what the plum pudding model predicts.

The alpha particle scattering experiment

Geiger and Marsden, working with Rutherford, fired alpha particles (small, positively charged particles) at very thin gold foil and recorded where they went. In the game you slide the alpha source along its rails, fire, and watch each track hit the detector screen. The results, and what each one shows, are the core of the exam question:

• Most alpha particles went straight through, so most of the atom is empty space.
• Some were deflected, so the centre of the atom must be charged. Alpha particles are positive and like charges repel, so the nucleus is positive.
• A very few bounced back, so the mass must be concentrated in a tiny nucleus at the centre.

The results led to the conclusion that the mass of an atom is concentrated at the centre (the nucleus) and that the nucleus is charged. This nuclear model replaced the plum pudding model, because a ball of spread-out charge and mass could not explain the deflections and bounce-backs. In the game you rebuild the model yourself: drag the slider to squeeze the positive charge and mass into the centre of each atom and watch the predicted pattern change until it matches the evidence.

Plum pudding model vs nuclear model

• Plum pudding model: positive charge and mass spread evenly through the whole atom, with electrons embedded in it. No nucleus and no empty space.
• Nuclear model: the positive charge and almost all of the mass are in a tiny nucleus at the centre, with electrons outside it. Most of the atom is empty space.

A common mistake is drawing the plum pudding model with electrons orbiting a nucleus. It has no nucleus at all. Another is saying the nucleus is negative: the positive alpha particles were pushed away, so it must be positive.

Bohr, protons and neutrons

The model kept developing after 1911:

• Niels Bohr (1913) adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. His theoretical calculations agreed with experimental observations.
• Later experiments showed that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each with the same amount of positive charge. These were named protons.
• James Chadwick (1932) provided the evidence for neutrons in the nucleus, about 20 years after the nucleus became an accepted scientific idea.

The last stage of Scatter Shot turns this into a shooting gallery: model discs drift through the chamber and you shoot them in the order they were developed, while avoiding discs that show wrong models.

How to play Scatter Shot

Move the mouse, drag, or use the arrow keys to slide the alpha gun, then click, tap or press Space to fire. There are five stages: test the plum pudding model, run the real experiment and find the hidden nuclei (a red marker shows where an alpha particle was pushed off course), rebuild the model, land trick shots in targets on the detector screen, and put the models in time order. A lab notebook asks you to explain each result the way the exam wants it. Wrong answers cost points and show a short explanation with a link to the lesson. Beat your best score and climb from Lab Assistant to Nobel Contender.

Exam tip: for any ‘why did the model change?’ question, give the result, say what it shows, then say why the old model could not explain it.