Explaining Rutherford Scattering

Rutherford Scattering is common place in physics classrooms of schools and colleges all over the world, his very simple experiment shaped our whole views on science, he effectively discovered the nucleus, without this ground work the majority of modern day physics and chemistry would not exist and whole branches of science would not have been founded. Ernest Rutherford’s theory has since been evolved by other scientists and shapes the way we look at science today, he is hailed as the father of nuclear and atomic physics as we know it and won the Nobel Prize for Chemistry in 1908 for his work.

Ernest Rutherford’s experiment was failry simple, he took an alpha emitting source and fired the alpha particles at a layer of gold foil which was only a few atoms across and by placing a detector at the other side of the foil he discovered that most of them passed straight through, but not all of them. Rutherford set about finding out what happens in more detail and set up the following experiment:

Within a sealed vacuum Rutherford set up the alpha emitter and gold foil as described above, and the walls of the vacuum were lined with Zinc Sulphide which reacted with the alpha particles on contact and such, shows where each particle ended up after passing through the foil. The vacuum was in place so the alpha particles could travel freely (as they are stopped by a few centimeters of air as they collide with the particles in it) He then bombarded the gold foil with these Alpha particles, which are made up of two neutrons and two protons (effectively a helium nuclei) and the chemical lining recorded where the alpha particles ended up.

Now, before we see what happened its important to look at what was the theory of the atom before this milestone experiment. The idea that was universally accepted at the time was the plum pudding model, it was proposed by the discoverer of the electron, J.J.Thompson who incidentally went on to be Rutherford’s professor at Trinity College, Cambridge. He said that the atom was like a blob, like a plum pudding with a big spongy section which had plums inside, the sponge was a positively charged mass and the plums where little dots of negative charge (like electrons) and these charges balanced out. In the model the electrons were free to move wherever they liked within this cloud of positive-ness.

Now this being the case, you would expect alpha particles to bounce straight off a wall of gold which the atoms where far larger (turns out to have a proton number of 79), but this did not happen. A lot of the positively charged alpha particles went straight through the gold foil, some felt some force which deflected them to a slight angle and one in eight thousand alpha’s bounced back of the foil right back towards the source. The experiment was likened to firing an artillery cannon at a sheet of tissue paper and the shell bouncing off. With this Rutherford had shown that the majority of the atom was actually empty space. and he could account for the rest of it as-well.

Because he noticed some particles had been deflected at an angle he decided because the particles were positively charged, they must be feeling some repulsion from the nucleus and therefore that most also have a positive charge, he was correct and the force which caused the alpha particles to move at an angle from their path was electrostatic repulsion. As previously mentioned one in 8000 were deflected completely, this was where the alpha was on a direct collision course with the gold nucleus, funnily enough the particles never quite touch, the electrostatic repulsion between the two positive charges means the objects came with in fractions of a millimetre and then the alpha was repelled away, roughly in the direction it came from.

Rutherford had changed the model of the atom. He had found what he called the orbital theory of atoms, and it has remained relatively the same ever since, with a few changes added as one year later after work by a team of German scientists lead by Niels Bohr and the model evolved into the Rutherford-Bohr model. This was a break through in science, Rutherford showed how an atom was a positive nucleus surrounded by electrons, basics which were built up into the complex branches of chemistry, biology and physics today.