Introduction
Atom has a complex structure and hence many scientists tried to give a successful model of the atom and that’s why there has been a variety of atomic models throughout the history of atomic physics that refers mainly to a period from the beginning of the 19th century to the first half of 20th century. The first model of the atom was given by an English scientist, John Dalton called Dalton’s atomic model. He came up with an idea that
all matter is composed of very small things that are indivisible and indestructible also.
He called these particles atoms. But in 1897, the electron was discovered by J.J. Thomson, and after that discovery, people realized that atoms are made up of even smaller particles and Dalton’s atomic model was no more valid. After a few years later in 1904 J. J. Thomson proposed another model of atom called the “plum pudding model”. He stated that
an atom is considered as a sphere of positive charge and electrons are embedded in that sphere.
But his model was not universally accepted and he himself was never able to develop a complete and stable model of his concept.
Failure Of Thomson’s Plum Pudding Model
After seeing the lack of confidence of Thompson in his given model, Rutherford felt that let’s test the plum pudding model given by Thompson. He thought “if Thomson’s model is correct then if an alpha particle is to collide with an atom, it would just fly straight through and its path will be deflected by at most a fraction of a degree“.
So, finally, he challenged Thomson’s “plum pudding model“ with the help of physicist Hans Geiger and a 20 years old undergraduate student Ernest Marsden in 1911 by the study of the scattering of alpha particles from the sheets of metals.
Rutherford’s Scattering Experiment
In Ernest Rutherford’s laboratory, at the University of Manchester, Geiger and Marsden carried out experiments to study the scattering of alpha particles by thin metal foils. They bombarded the foils with high-energy alpha particles and observed the number of scattered alpha particles as a function of angle. Based on the Thomson model of the atom, all of the alpha particles should have been found within a small fraction of a degree from the beam, but Geiger and Marsden found a few scattered alphas at angles greater than 90^0 from the beam, which is physically impossible unless they are scattering off something more massive than themselves. This led Rutherford to deduce that the positive charge in an atom is concentrated into a small compact nucleus.
Rutherford was surprised to see the results and he is often quoted
It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back to hit you.
Geiger and Marsden used a narrow beam of energetic alpha particles and directed the beam on a thin gold foil of thickness about 4\rm \ \mu m. Scattered alpha particles were recorded on a zinc sulfide screen placed at some distance around the gold foil.
They observed that most of the alpha-particles, around 98 – 99\% are deflected at a very small angle near 0^0 and some of them were deflected off at different angles from 0^0 – 180^0 and very few of them are reflected back along their path.
From the result of that experiment, Rutherford made some conclusions.
His first conclusion was;
- Most of the space within the atoms is empty.
Why did he say that? Because more than 98\% of alpha particles passed through the gold foil without any deflection. It means those alpha-particles didn’t find anything to interact and we can easily say that those spaces within the atoms are empty.
The second conclusion was;
- The positive charge of the atom occupies very little space inside the atom.
This second conclusion came after it was observed that very few alpha particles were deflected by large angles or bounced back and it means that there must have some positively charged region that is responsible for the large deflection.
His third conclusion was;
- All the positive charges and mass of the gold atom were concentrated in a very small volume within the atom.
Because a very small fraction of \alpha-particles were deflected by 180^0. Since alpha particles are heavy charged particles and it was observed that a very few of them were deflected by the central volume of charge, which was clearly indicated that all the positive charges and mass of the gold atom are concentrated in a very small volume within the atom. That positively charged region is now called the nucleus.