At the beginning of the 20th century not much was known about atoms; except, that they contained negatively charged electron’s, and there must be some positive charge to counter-balance this negative charge because matter is electrically neutral.
So, the question is how is this positive and negative charge distributed throughout the atom?
The most popular Theory at the time was our friend Thompson’s Plum Pudding Model
In 1911 Ernest Rutherford, a student of Thompson’s, created an experiment in order to test this theory.
Rutherford’s Gold Foil Experiment
What Rutherford did with this experiment was fire positively charged alpha particles at a piece of gold foil, and then measure the deflection of the particles as it came out the other side. From this experiment he could draw conclusions about the structure of the foil.
In order to understand this imagine shooting bullets at a mound of snow. We expect the bullets to go through the snow with some deflection and lose of energy because the bullet hit something. If we know the initial and final trajectories of the bullet we can make some conclusions about the consistency of the snow pile, or in Rutherford’s case the consistency of the atom.
Rutherford expected all of the particles to be deflected just a little bit; however, what actually happened is:
most of the particles passed through the foil with no deflection
occasionally the particles where deflected with huge angles, and
rarely the particles bounced back directly!!!
These results surprised Rutherford because they are counterintuitive. Imagine firing shots at a piece of snow, only to have most go through the snow unaffected, and some bouncing back at you!!!
The only explanation for this is :
Rutherford concluded that all of the positive charge and essentially all of the mass of the atom is concentrated in an infinitesimally small fraction of the total volume of the atom, which he called the nucleus!!!
Lets think about Rutherford’s conclusion about the nucleus and why it makes sense.
If the atom is mostly empty space with its mass being a small fraction of the total volume, this would explain why most of the alpha particles experienced no deflection, or in other words they did not hit anything because there was nothing to hit.
If the positive charge of the atom is concentrated in this really small fraction of the total volume, this would explain why some of the particles where deflected at large angles.
This is so because like charges repel each other.
So when the positively charged alpha particle [He^2+] got close to the positively charged nucleus they repelled each other sending the alpha particle flying at a large angle.
This also explains why rarely the alpha particles would fire back from the gold foil because when the positively charged alpha particle collided with the tiny positively charged nucleus the charges repelled and the alpha particle was sent flying directly back in the direction from whence it came.
Rutherford’s Atomic Model
“described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the sun.” (Encyclopedia Britannica)
Rutherford’s proposal for the structure of the atom is still accepted today; however, his theory concerning the motion of the electron’s around the nucleus has since been disproven and expanded upon.