The Discovery of the Nucleus

The word atom is no longer being used in its correct context, originally it came from early Greek term 'atomos' to translate to mean 'indivisible'. Thanks to a discovery by British Physicist/Chemist Ernst Rutherford in 1909, this idea began to break down as science started to take a look inside the so-called indivisible atom.

Rutherford began his 'gold foil experiment' in the midst of the JJ Thomson 'Plum Pudding' stage of atomic theories. The experiment involved directing a radioactive source emitting alpha-particles towards gold foil. The gold foil was used because it is very thin so the target is nearly a line of atoms. The area was surrounded by a zinc sulphide screen which will give off a flash of light when hit by an alpha particle. Essentially, the experiment was designed to find out where the alpha particles went after colliding with the nucleus.

The energy levels of the alpha particles were around 6,000,000 eV compared to 0.02 eV of the nearly stationary gold atoms. Because of this the pattern on the detector was fairly unremarkable, most of the alpha particles passed through the gold atoms and whenever an alpha particle struck a gold atom, it simply moved it out of the way. There were however occasional exceptions, 1 in 10,000 alpha particles were deflected by over 90áµ’, these were completely unexplainable with the current impression of the atom.

Using Coulomb's Law (strength of force between two charged particles is inversely proportional to their distance apart squared), Rutherford's team found that the radius between the positive alpha particle and the positive force deflecting it must have been smaller than the radius of the atom in order to achieve the 300N repulsive force required to deflect it by over 90áµ’.

This calculation was extremely significant, it meant that for a neutral gold foil atom, there must be a small, concentrated area of positivity which is surrounded by negativity. Since the electron had already been discovered by Thomson at the end of the 19th century, they must be orbiting the centered positivity, thus the nucleus was born.

This was paradigm changing, the whole concept of the atom was changed and nuclear physics was now well under way, as with many brilliant scientific discoveries it just led to more questions which needed to be answered;

Why don't the electrons and nucleus attract each other and merge? What is keeping the positive subatomic particles together?

These questions needed an answer if the Rutherford 'Nuclear' model of the atom was to be accepted, which kick started nuclear physics and a new dawn for atomic discovery.

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