Question

Why is the JJ Thomson's model known as the 'plum pudding model'? What were its loopholes?

Answer 1

Dear Student

Thomson model:

According to this model, an atom can be considered as a sphere of uniformly distributed positive charge in which there are electrons distributed symmetrically.

The electrons must be held by the positive charges by electrostatic forces. The mutual repulsions between the electrons are balanced by the force of attraction towards the centre of the sphere.

In an atom with a single electron like the hydrogen atom, the electron must be situated at the centre of the positive sphere.

In an atom with two electrons, like the helium atom, the two electrons must be symmetrically situated on opposite sides of the centre at a distance equal to half the radius of the positive sphere.

In the three-electron system, the electrons should be at the corners of a symmetrically placed equilateral triangle, the side of which is equal to the radius of the sphere.

Proceeding in this manner, Thomson could explain the arrangement of electrons ranging from 1 to 100 inside the positive sphere.

This model was also called 'the plum pudding model'. The electrons are like plums in a pudding (negative charge). The seeds in a watermelon fruit can be given as a parallel comparison.

Drawbacks of above model:

a) Later it was found that the atom has concentrated mass and the positively charged nucleus is at the centre while the negatively charged electrons move in the shells around the nucleus.

b) According to this model, hydrogen can give rise to only one spectral line, contrary to the observed fact of several lines.

c) This model could not explain the large angle scattering of alpha particles by thin metal foils.

Regards