What is the Rutherford Gold Foil Experiment??®
Answers
The scientists bombarded a thin gold foil of thickness approximately 8.6 x 10-6 cm with a beam of alpha particles in vacuum. They used gold since it is highly malleable, producing sheets that can be only a few atoms thick, thereby ensuring smooth passage of the alpha particles. A circular screen coated with zinc sulphide surrounded the foil. Since the positively charged alpha particles possess mass and move very fast, it was hypothesized that they would penetrate the thin gold foil and land themselves on the screen, producing fluorescence in the part they struck.
In line with the plum pudding model, since the positive charge of atoms was evenly distributed and too small as compared to that of the alpha particles, the deflection of the particulate matter, if any, was predicted to be less than a small fraction of a degree.
Observation:
Though most of the alpha particles behaved as expected, there was a noticeable fraction of particles that got scattered by angles greater than 90 degrees. In fact, there were about 1 in every 2000 particles that got scattered by a full 180 degree, that is, they simply retraced their path after hitting the gold foil.
Conclusion
The unexpected outcome could have only one explanation – a highly concentrated positive charge at the center of an atom that caused an electrostatic repulsion of the particles strong enough to bounce them back to their source. The particles that got deflected by huge angles passed close to the said concentrated mass. Most of the particles passed undeviated as there was no obstruction to their path, proving that the majority of an atom is empty.
In addition to the above, Rutherford drew the conclusion that since the dense alpha particles could be deflected by the central core, it shows that almost the entire mass of the atom is concentrated there. Rutherford named it the “nucleus” after performing the experiment in various gases. He also used materials other than gold for the foil, though the gold foil version gained the most popularity.
He further went on to reject the plum pudding model and developed a new atomic structure called the planetary model in which a vastly empty atom holds a tiny nucleus at the center surrounded by a cloud of electrons. His version holds good even today.
RUTHERFORD'S GOLD FOIL EXPERIMENT
Rutherford and his students performed some scattering experiments in which they bombarded thin foils of heavy metals like gold, silver, platinum or copper with a beam of fast moving alpha particles.
OBSERVATIONS
1. Most of the alpha particles passed through the gold foil undeflected.
2. A small fraction of the alpha particles was deflected by small angles.
3. A very few alpha particles bounced back, i.e., were deflected by nearly 180 degree.
CONCLUSIONS
1. Most of the space in the atom is empty because most of the alpha particles passed through the foil undeflected.
2. A few positively charged alpha particles were deflected which shows that the positive charge has to be concentrated only in a very small volume that repelled and deflected the positively charged alpha particles.
3. Rutherford through calculations also showed that the volume occupied by the nucleus is negligibly smaller than the total volume of the atom.
RUTHERFORD'S ATOMIC MODEL
According to this model
1. The positive charge and most of the mass of the atom was densely concentrated in extremely small region, called the nucleus by Rutherford.
2. The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths, called orbits.
3. Electrons and the nucleus are held together by electrostatic forces of attraction.
DRAWBACKS OF RUTHERFORD MODEL
1. Rutherford proposed that electrons revolve at a high speed in circular orbits around the positively charged nucleus. When a charged particle i. e. electron revolves around positively charge nucleus, it needs to be accelerated so as to keep it moving in circular orbits. However, according to electromagnetic theory, whenever a charged particle such as an electron is accelerated around another charged center ( nucleus ) which are under force of attraction, there will be continuous radiation of energy. This loss of energy would slow down the speed of the electron. This would reduce the radius of the electron–orbit. Eventually the electron would fall into the nucleus. The result would be that the atom would collapse. But this does not happen. Thus Rutherford’s atom could not explain the stability of the atom. Failure of Rutherford’s model i.e. reduction of radius of orbit is shown below.
2. Rutherford proposed that electrons revolve around the nucleus in the fixed orbits. However, he did not specify the orbits and the number of electrons in each orbit.