rutherford scattering experiment bbc bitesize

If you wanted to know more about a dark room that you were unable to go into, you might be able to get an idea of its size and contents by throwing balls into the room and considering their behaviour (the sort of task you might get in an Adventure game made for the computer). you would be able to tell where obstacles were and roughly the size of the room.

Physicists do the same kind of thing when 'lookling at' tiny objects such as atoms. In 1911 Rutherford wanted to find out more about the structure of the atom so he set two of his research students (Geiger and Marsden) the task of bombarding gold atoms with alpha particles, gathering data as to what happened to the 'missiles' and making deductions about the atom's structure from that data. In those days they did not have particle accelerators providing a ready supply of protons or neutrons so they had to use something that could be used as a natural probe - alpha particles were ideal.

for an animation of this experiment

).To achieve this we must use a absorbs all of the alpha particles except those travelling in one direction - unless they are travelling parallel to the sides of the outlet of the collimator they will impact on the side and be absorbed - so only those travelling parallel to each other and in one direction get through!

' - that made angles of greater than 90 with their original trajectory. Rutherford was amazed, he said, 'It was quite the most incredible event that has happened to me in my life! It was almost as incredible as if you fired a 15" shell at a piece of tissue paper and it came back and hit you!'

. This gives an indication of the tedious work involved in taking a checking the results! They would have to be repeated many times to be sure they were not due to anomalies! Would you have been tempted to discount totally inexplicable results and give your professor a set of results he expected? Or would you, like Geiger and Marsden, repeat and repeat and report the strange findings to Rutherford so that a whole new model of the atom could be developed? It is the careful (and honest!) investigation into anomalies that often lead to new theories.

When Rutherford mathematically investigated the results he proposed a model that explained the results that Geiger and Marsden obtained.

The fact that the vast majority of the alpha particles got straight through led Rutherford to propose that the atom was composed primarily of empty space.

The fact that occurred in 1 in 8000 alpha particles indicated that there was a:

(meaning containing lots of mass - he knew the electrons had very little mass and the fact that all of the positive charges were concentrated into a small area meant that the mass was concentrated there too)

(because it repelled the alpha particles) nucleus in the centre of the atom (neutrons had not been discovered at that time - so he made no mention of them!).

So his picture was one of the atom being like the solar system - the sun being the nucleus (taking a very small proportion of the volume of the solar system but being the vast bulk of the mass in it!) and the electrons being like the planets orbiting the 'sun'.

This model was later amended by Bohr (to take into account a couple of points that Rutherford's atom did not fully explain - like the motion of the electrons and the orbital paths that could explain what the Chemists understood of electron behaviour in bonding) to make the model of the atom that you are taught at GCSE but it was still a magnificent advance to our understanding of atomic structure.

The observations made in 1911 by Geiger and Masden carrying out the experiment for Rutherford were a fatal blow to the . J.J. Thomson had put forward this picture of what an atom was like in 1906 and it was accepted scientific theory in 1911. 

It took Rutherford a number of months before he finally decided that the only way his results made sense was if the charge was concentrated in a nucleus - he was the first to put forward such an idea... and let's face it, the idea seemed counter-intuitive - who would think the atom was mainly empty space? He had to be sure his calculations were correct before he put his idea forward to the scientific community.

Once he presented his ideas and explained how he got the results that led to them scientists all over the world repeated his experiment to verify the results and checked his mathematics to see whether his theories were sound.  

We now know that Rutherford was right, but this was a revolutionary suggestion at the time and is a good example of how precise, repeatable experimentation can lead to us changing our thinking about how the Universe works - and lead to better understanding.

If Geiger and Marsden had 'tinkered with' their results to make them do what 'made sense' and was expected we would have not made such a rapid advance in nuclear knowledge! Think of that the next time you are tempted to fabricate a repeat set of readings in class - or to get rid of an anomaly. The 1 in 8000 backscatter could have been seen as an anomaly, but Geiger and Marsden checked the 'anomalies' out - several times to verify they were nothing of the kind. If they had ignored them Rutherford would have been presented with a set of result that showed just what he expected to see... they would have been in accordance with current scientific thinking.

for a mathematical treatment of scattering - used to find the size of the nucleus

to see how .

An interactive simulation of this from Colorado University

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Rutherford Scattering ( AQA GCSE Physics: Combined Science )

Revision note.

Physics Project Lead

Rutherford Scattering

Alpha scattering.

• Physicist, Ernest Rutherford was instructing two of his students, Hans Geiger and Ernest Marsden to carry out the experiment
• They were directing a beam of alpha particles (He 2+ ions) at a thin gold foil
• They expected the alpha particles to travel through the gold foil, and maybe change direction a small amount
• Most of the alpha particles passed straight through the foil
• Some of the alpha particles changed direction but continued through the foil
• A few of the alpha particles bounced back off the gold foil
• The bouncing back could not be explained by the Plum Pudding model, so a new model had to be created

When alpha particles are fired at thin gold foil, most of them go straight through, some are deflected and a very small number bounce straight back

The Nuclear Model

• Ernest Rutherford made different conclusions from the findings of the experiment
• The table below describes the findings and conclusions of A, B and C from the image above:

Alpha Scattering Findings and Conclusions Table

• Rutherford proposed the nuclear model of the atom
• Nearly all of the mass of the atom is concentrated in the centre of the atom (in the nucleus)
• The nucleus is positively charged
• Negatively charged electrons orbit the nucleus at a distance
• The nuclear model could explain experimental observations better than the Plum Pudding model

The Nuclear model replaced the Plum Pudding model as it could better explain the observations of Rutherford’s Scattering Experiment

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Rutherford's Alpha Scattering Experiment

Subject: Physics

Age range: 14-16

Resource type: Lesson (complete)

Last updated

24 January 2024

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Atomic diagrams and the scattering diagram are enhanced with animations. A starter is included to test and recap learning on atomic symbols and isotopes. The presentation has a link to a suitable Youtube clip. The end of the presentation has questions (& answers) on the analysis of Rutherford’s experiment.

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