Why did Thompson's plum pudding model fail?

Thompson's plum pudding model failed because it could not explain the results of Rutherford's gold foil experiment.

J.J. Thompson's plum pudding model was a significant step forward in the understanding of atomic structure. Proposed in 1904, it suggested that atoms were composed of negatively charged electrons embedded in a 'pudding' of positive charge, much like plums in a pudding. This model was based on the knowledge of the time, which included the discovery of the electron and the understanding that atoms were neutral, meaning they had an equal amount of positive and negative charge.

However, the plum pudding model was unable to explain the results of an experiment conducted by Ernest Rutherford and his team in 1909. In this experiment, known as the gold foil experiment, alpha particles were fired at a thin sheet of gold. According to the plum pudding model, the alpha particles should have passed straight through the 'pudding' of positive charge. However, what Rutherford observed was that some alpha particles were deflected at large angles, with a few even bouncing back. This suggested that the positive charge, and most of the mass of the atom, was concentrated in a small central nucleus, contradicting the plum pudding model.

Rutherford's nuclear model of the atom, proposed in 1911, replaced the plum pudding model. This model suggested that the atom consisted of a tiny, dense, positively charged nucleus surrounded by a cloud of negatively charged electrons. This model could explain the results of the gold foil experiment, as the deflection of the alpha particles could be attributed to the positive charge in the nucleus.

In conclusion, Thompson's plum pudding model failed because it could not account for the results of Rutherford's gold foil experiment. This experiment showed that the positive charge and most of the mass of the atom was concentrated in a small central nucleus, contradicting the idea of a 'pudding' of positive charge. This led to the development of Rutherford's nuclear model of the atom, which is still accepted today.