Compare all the proposed models of an atom given in this chapter.

1).Thomson proposed the model of an atom to be similar to that of a watermelon. In which the positive charge in atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon.

On the base of this model; Thomson proposed that:

i)An atom consists of a positively charged sphere and the electrons are embedded in it.

ii)The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

2.)Rutherford designed a model in which fast moving alpha (a)-particles were made to fall on a thin gold foil. On the basis of above experiment Rutherford concluded that:

i)Most of the space inside the atom is empty because most of the (a)-particles passed through the gold foil without getting deflected.

ii)Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

iii)A very small fraction of (a)-particles were deflected by 180⁰, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume with in the atom.

3.)Neils Bohr. Since charged bodies moving in circular motion emit radiations. This will lead to loss of energy of the moving electron and ultimately giving unstable model of atom. To explain stability of atom and atomic spectra, Bohr suggested that electrons are moving round the nucleus in orbits which have fixed energy shells. There is a loss or gain in energy of electron when it moves from one orbit to the other.