Why did Thomson’s model of the atom fail?

The correct answer is Option (4) → Both (2) and (3)

In Thomson’s model, the positive charge of the atom is spread uniformly through its whole volume. The electrons are embedded in it like seeds in a watermelon.

Option (1) It could not explain why the atom as a whole is electrically neutral. — Incorrect. Neutrality was no difficulty for the model. The spread-out positive charge was taken to balance the negative charge of the embedded electrons.

Option (2) It could not account for the scattering of α-particles through large angles. — Incorrect. This statement is true. But it is only part of the answer.

Option (3) Its assumption that the positive charge is spread uniformly through the atom was shown to be wrong by later studies. — Incorrect. This statement holds as well. Choosing it alone leaves out the other true statement.

Option (4) Both (2) and (3) — Correct. Both statements hold:

• To turn an α-particle backwards, a large repulsive force is needed. A positive charge spread thinly over the whole atom cannot provide it, so (2) holds.

• The scattering experiments showed that the positive charge and most of the mass are concentrated in a tiny nucleus. This is very different from the uniform spread in Thomson’s model, so (3) holds.

NCERT (2026-27):
Introduction – “The first model of atom was proposed by J. J. Thomson in 1898. According to this model, the positive charge of the atom is uniformly distributed throughout the volume of the atom and the negatively charged electrons are embedded in it like seeds in a watermelon. This model was picturesquely called plum pudding model of the atom. However subsequent studies on atoms, as described in this chapter, showed that the distribution of the electrons and positive charges are very different from that proposed in this model.”

Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom – “… Rutherford argued that, to deflect the α-particle backwards, it must experience a large repulsive force. This force could be provided if the greater part of the mass of the atom and its positive charge were concentrated tightly at its centre.”