Question

3.

(a) In your own words, explain Rutherford’s reasoning that led him to the conclusion based on his experiments that the positive charge in an atom occupies a small space in the center of the atom?

(b)] Rutherford was not fully satisfied with the atomic model he presented. Discuss some of the problems he might have seen with the model and with its ability to describe the physical phenomena he was aware of.

Answer 1

a)In Rutherford's alpha scattering experiment,the fast moving alpha particles were made to fall on a thin gold foil.Alpha particles are doubly charged He ions

There are three main observations of this experiment

1. Most of the alpha particles passed straight through the gold foil

2.Some of it were deflected by the foil by small angles.

3.Few of it gets reflected back.

These observations shows that the space occupied by positive charge is very small.Since only very small fraction of alpha particles were deflected by large angles indicating that all positive charge and mass of gold atom were concentrated in a very small volume within the atom.

From this Rutherford calculated that radius of nucleus is about 10^5 times less than the atomic radius.

b)Rutherford's model proposed that electron revolve in circular orbit around positive charged nucleus.When charged particle like electron revolve around positive charged nucleus it needs to be accelerated .But according to Electromagnetic theory, whenever a charged particle such as electron is accelerated around charged center,there will be continous radiation of energy.This energy loss would slow down the speed of electron and eventually electron fall into the nucleus.This results in the collapse of atom.But this does not happen.Hence,he could not explain the stability of atom.

Rutherford was also not able to specify the orbits and number of electrons in each orbit.