The Rutherford's experiment
This fantastic experiment was the first nuclear physics investigation ever! Alpha particles interacted directly with the nuclei (nuclei is the plural form of nucleus) of gold atoms.
It brought about a dramatic conclusion: the atom is mostly empty! In other words, the electrons very far away from the nucleus. It works as if the atom was a football stadium and the nucleus was the ball in the midfield. The animation below illustrates the experiment:
Alpha particles (two protons and two neutrons bound together) are "shot" at a very thin gold leave.
The animation above shows a odel of the atomic structure of the gold leaf; the size of the atoms are not in scale.( Atoms in reality would be thousands of times bigger than the nucleus -represented as a blue dot on the middle of the atom.
Surprisingly, some particles go straight across the sheet of metal, some deviate and others bounce back!!
This result proves that the atom is largely "empty", because most of the alpha particles went straight across the gold leaf, without even being deflected.
To find out the amount of particles that has been deflected at any given angle, a moving detector is used.
Such detector is shown on the animation below:
After measuring the amount of particles scattered at each angle, Rutherford could calculate the approximate size of the atom with relation to the nucleus. He calculated the trajectories using the equations for movement around a central force; the same which are used to calculate the orbits of planets.In this case, instead of attraction there is repulsion, as nuclei and alpha particles are both positive.
This was the first ever experiment in nuclear physics and it opened a new horizon for experimental physicists.
Scattering experiments are still the most important tool to probe the structure of matter.