A Short Note on Scattering Experiment

The collision of a particle with another particle causes a shift in the direction of a particle, this is known as scattering. A collision can occur between two particles that may or may not repel one another, such as two positive, two negative ions, and does not have to include actual physical contact between the particles, as defined in physics. The collision was taken into account and an experiment called Rutherford’s alpha-particle scattering experiment was performed by Ernest Rutherford, to find out what lies in an atom.

Rutherford’s Scattering Experiment 

Ernest Rutherford was curious to know how the electrons are arranged within an atom and what lies in an atom. Rutherford laid out an experiment for this, which later helped him in concluding what lies in an atom.

In the experiment laid down by Rutherford, high-speed alpha particles were made to collide with a thin gold foil.

As the experiment requires a thin sheet of metal, he chose gold foil for this experiment, which is the most malleable metal. The thickness of this gold foil used in this experiment was 1000 atoms.

The fast-moving -particles have a mass of 4u, and contain a lot of energy and momentum as they are traveling at a high speed. Thus, some alpha particles passed the atom in a straight line, while some deflected from their initial path a little, while a few returned from their initial path.

Rutherford’s Model

To test the plum pudding model, Ernest Rutherford and his students Ernest Marsden and Hans Geiger, 1905 conducted an experiment in which they directed a beam of alpha particles in a vacuum, where thin gold leaves were suspended.

Any alpha particle deflection would be due to collisions with the gold foil rather than collisions with air particles, which is why the vacuum is so critical.

Gold was used in this case because it was the only metal that possessed good malleable properties. It did not crack despite being rolled out very thin.

Due to the thinness of the gold foil, alpha particles could pass through it without any deviation. Several other observations were made during this experiment, which is as follows

A significant number of alpha particles could pass straight through the gold foil             undeviated

•     As they travelled through the gold foil, the alpha particles were deflected by significant angles (> 4°), and they deviated in a very small range. And only a few alpha particles bounced straight back off the gold foil

A large number of particles passed through the gold foil undeviated, so Rutherford assumed that the atoms have mostly space.

A small number of alpha particles were deflected by large angles (> 4°) so there is close attention to the positive charge found in the atom. So, the positive alpha particles were deflected by positive charges present in a concentrated space in an atom.

At last, a few alpha particles bounced back, i.e. deviated by an angle of 180 degrees from their path. This means the chance of head-on collision is very small.

Rutherford made the model of an atom according to his experiment which had the following features

•   There is a positively charged centre in the atom, called the nucleus, which consists of all the mass of atoms
•   The electrons spin around the nucleus in well-defined orbits
•   Nucleus has a very small size in comparison to the atom in which it is present.

The alpha particle scattering experiment shows the presence of a nucleus in the atom, which is concentrated in a very small volume.

It also gives the following important facts about the nucleus of an atom

•   The nucleus of an atom has a positive charge, which repels the alpha particles
•   The nucleus of an atom is dense and contains positively charged particles, which are responsible for the deflection of alpha particles by an angle of 180 degrees

Conclusion 

The alpha particle scattering experiment explains to us that most of the α-particles passed through the gold foil without getting deflected therefore most of the space present in an atom is empty. A very less number of particles were deflected from their path by a significant angle, because of the positive charge of the atom observed in a very concentrated space. A very small number of alpha particles were deflected by very large angles, because of which, it can be concluded that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.